early orthoceratoid cephalopods from the argentine ... - BioOne

J. Paleont., 81(6), 2007, pp. 1266–1283 Copyright 䉷 2007, The Paleontological Society 0022-3360/07/0081-1266$03.00

EARLY ORTHOCERATOID CEPHALOPODS FROM THE ARGENTINE PRECORDILLERA (LOWER–MIDDLE ORDOVICIAN) ¨ RN KRO ¨ GER,1 MATILDE S. BERESI,2 BJO

AND

ED LANDING3

Museum fu¨r Naturkunde, Institut fu¨r Palaöntologie, Invalidenstraße 43, D-10115 Berlin, Germany, 具[email protected]典; 2 Cricyt-Ianigla, Departamento de Paleontologıá, Avda. Ruiz leal s/n, 5500 Mendoza, Argentina, 具[email protected]典; and 3 New York State Museum, State Education Department, Albany, New York 12230, 具[email protected]典 1

ABSTRACT—The Early and Middle Ordovician Orthocerida and Lituitida of Precordilleran Argentina are described, and their systematics and paleogeographic significance are revised. These cephalopods show a strong affinity to coeval faunas of North China, suggesting a location of the Precordillera at middle latitudes in the Southern Hemisphere east of the North China block and relatively close to the Gondwanan margin during the early Middle Ordovician. The descriptive terminology of characters of the septal necks, the position and shape of the siphuncule, and the shape of the connecting ring is improved. The distribution of these characters support an emendation of the Baltoceratidae, Sactorthoceratidae, and Proteoceratidae. Braulioceras n. gen. (Sactorthoceratidae) and Palorthoceras n. gen. (Orthoceratidae) are erected. The new species Braulioceras sanjuanense, Eosomichelinoceras baldisii, Gangshanoceras villicumense, and Rhynchorthoceras minor are proposed. Palorthoceras n. gen. from the Lower Ordovician Oepikodus evae Zone represents the earliest known orthocerid.

INTRODUCTION

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are the stem group of coleoids, the dominant group of modern cephalopods. Of the 139 modern cephalopod genera, only the Recent Nautilus does not belong to the coleoids (Sweeney and Roper, 1998). Coleoids are characterized by a single pair of gills and a small number of arms and radular teeth, while all cochleate coleoids have a spherical protoconch. These characters unite the coleoids with the ammonoids, bactritoids, and orthoceridans as taxa of the Neocephalopoda (Engeser, 1996). The earliest known neocephalopods are known from the early Tulean of the Arenigian Series of Wales (Evans, 2005). However, these early representatives remain very rare until the late Arenigian when the first orthoconic shells with a central siphuncle appear which are referred to the Orthoceratidae McCoy, 1844. By the earliest Middle Ordovician, a complex and diverse fauna of Orthocerida and closely related taxa of the Lituitida was established. However, the details of the evolutionary history of this group during the critical Early-Middle Ordovician boundary interval in the Great Ordovician Radiation are poorly known. Over the last few decades, MSB collected a suite of nautiloids from the Lower–Middle Ordovician of the Argentine Precordillera. This report concentrates on the revision and comparison of the Orthocerida and Lituitida of this collection in order to provide new data on the initial radiation of the Neocephalopoda. This revision also provided new information on the paleogeographical relationships of the Precordillera in the Early–Middle Ordovician. These data are of particular interest because the paleogeographic location of the Precordillera during the Early Paleozoic is highly disputed. The Precordilleran rocks comprise an enigmatic, allochthonous succession that represents an Early Paleozoic microcontinent called the Precordillera or Cuyania terrane. Two conflicting hypotheses explain the origin, history, and final collision of the microcontinent with the Gondwana margin. The first is the widely accepted Laurentian microcontinent model of Astini et al. (1995) and Thomas and Astini (1996). By this model, the Precordillera represent a terrane of Laurentian origin that collided with the western Argentina margin of Gondwana during the Ordovician (Thomas and Astini, 1996, 2003). However, the Cambrian–Ordovician history and movement of the terrane from its separation from Laurentia until its collision with Gondwana is not known (e.g., Astini et al., 1995; Fanning et al., 2004). The second hypothesis considers the Precordillera to be autochthonous and a fragment of Gondwana (Baldis et al., 1989; Acenõlaza and Toselli, 1999; Acenõlaza et al., 2002, Finney et al., 2003a). Recent RTHOCERIDAN NAUTILOIDS

U-Pb geochronology of detrital zircons has indicated a Gondwanan provenance for Lower Cambrian and Upper Ordovician sandstones of Precordilleran Argentina (Finney et al., 2003b), and supports the autochthonous Gondwanan model for the Precordillera terrane. The Lower-Middle Ordovician sedimentary rocks of the Argentine Precordillera represent a subsiding carbonate platform with a diverse and relatively complete fossil record (Albanesi et al., 1999). Numerous reports of different aspects of the complex faunal history of this succession have been published (e.g., Beresi and Rigby, 1993; Benedetto, 1998; Ottone et al., 1999; Carrera, 2001; Albanesi and Ortega, 2002). The rich nautiloid fauna of the Precordilleran carbonates was early noted by A. W. Stelzner (1885). However, the Ordovician cephalopods of Precordilleran Argentina remain poorly known, and were only remarked on in a general review of South American nautiloid faunas (Acenõlaza et al., 1977; Acenõlaza and Beresi, 2002). Interestingly, their review emphasized that the Precordilleran cephalopod fauna is unique to this region and differs considerably from better known, coeval, Early-Middle Ordovician faunas from Baltoscandia and Laurentia. Indeed, the Precordilleran cephalopod faunas contrast with associated biotic groups which are dominated by cosmopolitan taxa. In this report, we provide the first detailed overview of the Precordilleran nautiloids. GEOLOGICAL SETTING AND AGE OF THE FAUNA

The eastern and central belts of Precordilleran Argentina are characterized by a thick succession of Lower-Middle Ordovician platform carbonates. The stratigraphic synthesis of Precordilleran Lower Paleozoic sediments is extraordinarily good, and has involved biostratigraphic and sedimentary data and incorporated magmatic events. The uppermost unit of these carbonates is the fossil-rich San Juan Formation which conformably overlies the La Silla Formation. The conodont biostratigraphy of the San Juan Formation was established by Serpagli (1974), Lehnert (1995) to span the upper Tremadocian (Paltodus deltifer Zone) to the lower Darriwilian (Eoplacognathus suecicus Zone). The top of the San Juan Formation is diachronous, and extends into the Oepikodus evae/Oepikodus ‘‘communis’’ zones in the northern sections (e.g., Guandacol River) to the higher Eoplacognatus variabilis Zone in southern sections (e.g., Villicu´m Range) (Sarmiento, 1985; Hu¨nicken and Ortega, 1987; Albanesi et al., 1999; Albanesi and Ortega, 2002). The base of the formation is characterized by a bioturbated bioclastic wackestone with a mollusk-dominated macrofauna (gastropods, nautiloids), that is interbedded with thinto medium-bedded bio-intraclastic grainstones to packstones and intraclast rudstones. The middle part of the San Juan Formation is a thick transgressive unit that includes a bioturbated bioclastic

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¨ GER ET AL.—ORDOVICIAN ORTHOCERATOID CEPHALOPODS KRO

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and fossiliferous wackestone with a rich fauna (brachiopods, trilobites, pelmatozoans, gastropods, ostracodes, nautiloids, lithistid sponges, receptaculitids, bryozoans, conodonts, green algae, and Girvanella-type fossils). The upper boundary of the San Juan Formation is represented by a gray interval of an often nodular wackestone and mudstone. (For a detailed documentation of the sedimentology see Beresi, 1992; Keller, 1994; Canãs, 1995.) Two reef horizons have been identified. The lower reef horizon (upper Tremadocian) occurs near the base of the formation in the northern Precordillera (Canãs and Carrera, 1993). The upper reef horizon occurs in the lower Middle Ordovician (approximately Baltoniodus triangularis–Paroistodus originalis Zone) (Keller and Flu¨gel, 1996). The San Juan Formation was deposited on an open carbonate shelf, and was bounded to the west by continental slope and oceanic basin deposits (Beresi, 1986). The diverse marine fauna and the lack of features indicative of restricted marine conditions suggest low-energy, subtidal conditions on an open platform during the entire interval of San Juan Formation deposition. The calcareous algae and cyanobacteria indicate deposition within the photic zone. The diachronous facies change that defines the top of the formation implies local epeirogenic subsidence as a mechanism responsible for producing accommodation space, rather than simple eustatic rise, during San Juan deposition. The nautiloid associations occur mainly in the middle (O. evae Zone) and upper parts (E. suecicus Zone) of the San Juan Formation. Nautiloid conchs represent approximately 5% of the total macrofauna in the lower part of the sequence and up to 20% in the upper levels. The nautiloids are commonly associated with gastropods, brachiopods, trilobites, bivalves, and diverse algae. The nautiloids examined in this report were collected at four outcrops (Fig. 1, 2). LOCALITIES

Quebrada Talacasto section, central Precordillera, San Juan Province.⎯The Talacasto section (Fig. 2) is a roadcut on National Road 40, approximately 70 km northwest of San Juan near the village of Iglesias. The succession spans the Lower-Middle Ordovician carbonate sequences of the La Silla and San Juan Formations and the uppermost Ordovician–Silurian siliciclastics of the La Chilca Formation. The Talacasto section is a classical outcrop in the Argentine Precordillera (Kayser, 1876; Stelzner, 1885). Stelzner (1885) noted the rich cephalopod fauna at Talacasto, and the entire succession at Talacasto, and particularly the San Juan Formation, has been subjected to numerous paleontological and sedimentological studies (e.g., Beresi, 1981; Herrera and Benedetto, 1991; Sańchez et al., 1996). K-bentonites from the section have been described by Huff et al. (1995). At Talacasto Creek, the San Juan Formation is 350 m thick. It consists of fossiliferous wackestone–intraclastic packstone in the lower and middle part, and nodular limestones in the upper member. In the middle part of the sequence, the conodont Oepikodus evae (Lindstro¨m, 1955) occurs, whereas the top of the succession is significantly younger (Eoplacognathus suecicus Zone). The medium- to thick-bedded mud-wackestones of the Oepikodus evae Zone yield a rich benthic macrofauna with occasional nautiloids. However, the main fossiliferous interval constitutes the nodular limestones of the upper member, which has common sponge biostromes in the Talacasto area (Beresi, 1986). A thin-bedded, platy, black mudstone-wackstone that alternates with marlstone represents the top of the formation. A major unconformity separates the San Juan Formation from younger deposits (uppermost Ashgillian of the La Chilca Formation). Quebrada Gustavo section, eastern Precordillera, San Juan Province.⎯The Gustavo Creek section occurs in the Villicu´m Range of the eastern Precordillera (Peralta and Beresi, 1999). The

FIGURE 1—Regional overview of outcrop area and localities of the San Juan Formation in the Precordillera, western Argentina.

upper member of the San Juan Formation and the boundary interval with the overlaying upper Darriwilian siliciclastics are exposed at Gustavo Creek. The upper member is composed of argillaceous lime-mudstones, fossiliferous wackestones, K-bentonites, and calcarenites. At the top of the upper member, Eoplacognathus suecicus Zone (middle Darriwilian) conodonts occur (Sarmiento, 1985). The 9 m thick upper member is characterized by a trilobite- and brachiopod-dominated association (assemblage A.I of Peralta and Beresi, 1999) at the base, followed by a lithistid sponge-dominated association (assemblage A.II of Peralta and Beresi, 1999). Five meters above the base of the upper member, the fauna is predominantly composed of longiconic orthocones (endoceridans, ellesmeroceridans, and orthoceridans) (assemblage A.III of Peralta and Beresi, 1999). Above this fossiliferous interval is 6 m of nearly macrofossil-free argillaceous mudstone with interbedded, thin K-bentonites. The upper member is capped by a distinctive, ca. 0.5 m thick grainstone bed that contains abundant crinoid ossicles and orthocone nautiloids (predominantly endoceridans with conches up to 0.6 m long). Don Braulio section, eastern Precordillera, San Juan Province.⎯Don Braulio Creek is a classic locality for Ordovician strata on the eastern slope of the Villicu´m Range. The San Juan

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FIGURE 2—Schematic overview, correlation, and nautiloid occurrences of the localities in the San Juan Formation, Precordilleran Argentina.

Formation is 380 m thick at this locality. The formation is composed of a lower nodular grey wackestone lithofacies, a middle, thick-bedded packstone-wackestone lithofacies, and upper fossiliferous wackestones and mudstones which abruptly change at their top into bioclastic (crinoidal) grainstones and packstones in which nautiloids are concentrated. Ash beds intercalated with the uppermost beds (7 m) have been correlated with K-bentonites at other localities in the Precordillera (Huff et al., 1995). The top of the San Juan Formation is an omission surface overlain by 12 m of a rhythmically bedded, limestone-marlstone sequence of the Gualcamayo Formation (Eoplacognathus suecicus Zone) (Baldis and Beresi, 1981). In the middle San Juan Formation, the Oepikodus evae Zone has been rocognized, and the top of the formation lies in the Eoplacognathus suecicus Zone (Sarmiento, 1985). The Oepikodus evae Zone contains a nautiloid fauna characterized by larger endocerids. The nautiloid diversity is highest

at the top of the San Juan Formation, while the lowest Gualcamayo Formation yields mainly small orthocerids and few brevicones. Cerro Viejo, Huaco area, eastern Precordillera, San Juan Province.⎯The Cerro Viejo de Huaco is located in the Central Precordillera between 30⬚11⬘40⬙ and 30⬚15⬘30⬙S, and 68⬚34⬘30⬙ and 68⬚35⬘20⬙W. The San Juan Formation at Cerro Viejo is 260 m thick, and is overlain conformably by the Los Azules Shale (middle Darriwilian–Upper Ordovician). Yellowish K-bentonite layers occur at the top of the carbonate sequence and in the lower part of the graptolitic, dark Los Azules Shale. Hu¨nicken and Ortega (1987) referred the uppermost San Juan Formation to the Middle Ordovician Eoplacognathus suecicus Zone. However, new collections indicate correlation with the lowest Middle Ordovician Lenodus variabilis Zone (Ottone et al., 1999). The San Juan Formation in the Cerro Viejo is composed of lower nodular wackestones

¨ GER ET AL.—ORDOVICIAN ORTHOCERATOID CEPHALOPODS KRO with chert horizons, middle intraclast packstones-grainstones, and upper marly wackestones and mudstones. The macrofauna is composed of articulate brachiopods, trilobites, gastropods, bryozoans, sponges, calcareous algae such as Nuia, and nautiloids. Orthoceras sp. and Cyrtoceras sp. were cited from this locality by Borrello and Gareca (1951). PALEOBIOGEOGRAPHY OF PRECORDILLERAN CEPHALOPODS

Endoceridans followed by orthoceridans are the most common nautiloids of the San Juan Formation. However, orthoceridan diversity is clearly higher than that of the endoceridans, and they are the preferred cephalopods for paleogeographic comparisons. The paleogeographical pattern of orthoceridan nautiloids during the Paleozoic is highly provincial (e.g., Foerste, 1929; Gnoli, 2002). The utility of orthoceridans in paleogeographical investigations has not been qualitatively or quantitatively tested. However, it is evident that orthoceridans, despite their supposed planktic lifestyle in parts of their ontogeny (Westermann, 1999), were sensitive to biogeographical barriers such as cold currents like other cephalopods (Crick, 1990). The embryonic shell of orthoceridans is small and often shows a constriction at a certain stage—features that strongly suggest the development of paralarvae (Ristedt, 1968; Kro¨ger, 2006). The development of paralarvae in orthoceridans strongly enhanced the probability of widespread geographical distribution, a probability that was constrained by oceanic currents. Similarly, the geographical distribution of Recent coleoids with planktic paralarvae is strongly controlled by water currents (e.g., Rocha et al., 1999; Gonzalez et al., 2005). By analogy with coleoids, orthoceridans should have potential for paleogeographical investigations. Few paleogeographical syntheses provide data on the possible positions and movement of the Precordilleran terrane. The results of these investigations are useful in the Early Ordovician but are highly equivocal for the Middle Ordovician. Pliomerid and prosopiscid trilobites from the Precordillera demonstrate a periGondwana affinity in the Middle Ordovician, but a biogeographically very ambiguous picture is generally provided by these trilobites (Edgecombe et al., 1999). Fortey and Cocks (2003) mention the strongly Laurentian aspect of Precordilleran Early Ordovician brachiopods. Tychsen and Harper (2004) demonstrated that Precordilleran orthid brachiopods show a strong Laurentian aspect during the late Early Ordovician, but commonly include widespread to cosmopolitan taxa during the Middle Ordovician, and thus provide little paleogeographical information. Similarly, Precordilleran trilobites show clear Laurentian affinities in the Early Ordovician, but include a unique mixture of faunal signatures in the Middle Ordovician with warm-water Baltic, Avalonian, and West Gondwanan taxa appearing (Fortey and Cocks, 2003). Orthid brachiopods (Tychsen and Harper, 2004) and conodonts (Zhen and Percival, 2003) of the Precordillera are dominated by widespread and cosmopolitan taxa, and this makes detailed paleogeographical reconstructions difficult. It is notable that Zhen and Percival (2003) grouped the Precordilleran conodonts together with those of South China within a ‘‘Temperate Domain’’ grouping. However, the time span of this ‘‘domain’’ comprises the entire Ordovician, and thus is hardly useful in tracing the Early–Middle Ordovician paleogeographic movements of the Precordillera. Terminal Lower–lower Middle Ordovician conodonts from the San Juan Formation (Serpagli, 1974) have been shown to be dominated by an association of taxa known from the Baltic platform and from Laurentian continental slope successions in western Nevada, the Marathon Basin of Texas, and the Taconic allochthons in eastern New York, Quebec, and western Newfoundland (see Landing and Ludvigsen, 1984; Landing, 1976). These conodont data are comparable with ‘‘temperate’’ and unrestricted marine environments. Li and Servais (2002) placed the

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Early-Middle Ordovician acritarchs of Argentina in their periGondwana province, but gave no information from which region their Argentina data were obtained. The only known Precordilleran Early Ordovician orthocerid, Palorthoceras kayseri n. gen. and sp., is also known from the Great Basin of the western United States. This is consistent with brachiopod data that indicate strong Laurentian affinities of the Precordilleran faunas during this time interval (Benedetto, 1998). However, the Middle Ordovician orthoceridans known from the Precordillera are strongly non-Laurentian in paleogeographic affinity. Of the six Darriwilian orthoceridan genera and species, only Cochlioceras avus Eichwald, 1860, is known from Laurentia. However, C. avus is cosmopolitan, and is known also from Siberia, Baltica, and North China. One Precordilleran genus, Rhynchorthoceras Remele`, 1882, is known from Baltica and North China. Of the remaining taxa, four orthoceridan species and one genus are endemic. The remaining two Middle Ordovician Precordilleran orthoceridan genera known from the Precordillera (Eosomichelinoceras J.-Y. Chen, 1974; Gangshanoceras Zou, 1988) were previously known only from China. It is significant that Braulioceras n. gen., an endemic Precordilleran genus, shows strong affinities to Sactorthoceras coreanicum Kobayashi, 1927, from the North China block. The known orthoceridans of the Puna Basin (Cecioni, 1965) of northwest Argentina differ completely at the species level. The Puna Basin has yielded only the cosmopolitan genus Cochlioceras Eichwald, 1860 and the endemic genus Belloceras Cecioni, 1965. Thus, the Middle Ordovician orthoceridans of the Precordillera show a clear affinity to coeval faunas of North China. This apparent affinity with North China may only represent an artifact of current knowledge because Middle Ordovician Gondwanan cephalopod faunas are poorly known. A Precordilleran-North China connection may in reality represent a peri-Gondwana faunal province, a speculation that requires more data. Nevertheless, the strong affinities of Precordilleran and North Chinese orthoceridan faunas put a spotlight on the unresolved question of the longitudinal paleo-positions of the Precordillera and North China. A review of the available literature reveals an ambiguous picture. In the widely accepted models of Niocaill et al. (1997), Fortey and Cocks (2003), and Cocks and Torsvik (2004), the Precordillera drifted away from low-latitude Laurentia, which is distant from high-latitude West Gondwana and Avalonia, and toward the Argentine margin of Gondwana. In these models the closest neighbors to the Precordilleran terrane are Laurentia to the northeast, Baltica to the east, and West Gondwana to the south. The position of North China is not given, but an entirely Northern Hemisphere position is supposed. This model follows the paleomagnetic reconstruction of Niocaill et al. (1997), which provides no specific data for the North China block. Tychsen and Harper (2004) locate the North China block in the Southern Hemisphere somewhere between Baltica and the eastern margin of Gonwana, but no specific data are given for the Precordillera. Li and Servais (2002) place the North China block on the northeastern edge of Gondwana close to Australia and in the northern hemisphere. In the paleomagnetic reconstructions of Huang et al. (2000) and Yang et al. (2002), the North China block separated from the eastern margin of Gondwana in the earliest Ordovician and moved eastward in the middle latitudes of the southern hemisphere toward the Argentinian margin of Gondwana. In the latter models, North China drifted relatively close to the Appalachian margin of Laurentia in the Middle Ordovician and lay just west of the supposed trajectory of the Precordillera (Astini et al., 1995; Thomas and Astini, 1996). Thus, the models of Huang et al. (2002), Yang et al. (2002), and Li and Servais (2002) are in good agreement with the distribution of Middle Ordovician orthoceridans. Moreover, the reconstruction of ocean currents by Barnes (2004), which supposed an

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FIGURE 3—Main characters of the septal neck, connecting ring, and shape of the siphuncle of some Early-Middle Ordovician orthoconic nautiloids. Orthoconic nautiloids with thin concave siphuncular segments are classified within the Protocycloceratidae of the Ellesmeroceratida. See text for discussion.

eastward-directed peri-Gondwana current, supports the faunal pattern of Middle Ordovician orthoceridans. CONCH TERMINOLOGY

The terminology used herein to describe nautiloid conchs is derived from a series of recent reports (Kro¨ger and Mapes, 2005, 2007; Kro¨ger, 2006; Kro¨ger and Isakar, 2006) that describe a particular fauna or association of orthoceridan cephalopods. These papers are part of a general revision of orthoceridan cephalopods. This revision is part of a planned cladistic analysis of Paleozoic nautiloids. The crucial requirement of this approach is a robust determination and designation of characters. In order to establish an unequivocal morphological terminology, it is necessary to discuss some character designations below. Position of the siphuncle.⎯The conventional terminology for designation of the siphuncle position relies on the position of the soft body axis (Sweet, 1964). Therefore, the position of the siphuncle within the phragmocone is designated as ventral, central, or dorsal. This practice is possible only when the soft body of an ectocochleate cephalopod is oriented with the anterior-posterior axis parallel to the growth axis. The venter can be determined only by reference to the position of the hyponomic sinus at the aperture. However, the hyponomic sinus is not known or is undeveloped in many conchs. Thus, many cyrtoconic forms have an unknown aperture, and the ventral side is simply equated with the side of the shell where the siphuncle is located. To avoid this type of unsubstantiated conclusion, it is proposed herein that the position of the siphuncle be designated with reference to the conch curvature if the position of the hyponomic sinus is not known. This is possible even in orthoconic shells, because no completely straight shell exists. Thus, the siphuncle can be on the convex side of the growth axis of the shell, on the concave side, and in the center. The terms ‘‘prosiphonal’’ and ‘‘antisiphonal’’ designate the position of an element of the conch with reference to the siphuncle. An element is antisiphonal, when it is positioned at the opposite side of the conch with reference to the siphuncle. Septal neck shape.⎯An achoantic septal neck designates a condition in which septal necks are absent—that is, septal perforations have the same diameter as the siphuncular tube (Fig. 3). Loxochoanitic septal necks point obliquely inward and backward from a relatively large septal perforation toward the narrower siphuncle (curvature of the septal neck ⬍90⬚). Suborthochoanitic septal necks are short and recurved; the curvature of the septal neck is ⬎90⬚ and ⬍180⬚. Orthochoanitic septal necks are oriented parallel to the growth axis (curvature of the septal neck 90⬚), and are shorter than half the length of the siphuncular segment. Cyrtochoanitic septal necks are recurved; they slope into an expanded siphuncular tube (curvature of the septal neck ⬎180⬚). Hemichoanitic septal necks point toward the growth axis as orthochoanitic septal necks, but are longer and comprise more than half the length of a siphuncular segment. Holochoanitic septal

necks are longer than the distance between two successive chambers, and are always parallel to the growth axis. Siphuncular tube shape.⎯The siphuncular tube consists of segments that span two adjacent septa. The tube wall is the connecting ring. The thickness of the connecting ring can vary significantly along its length in different taxa. Its thickness is usually greatest midway between the septa. A connecting ring that is thickest at midlength may have a concave inner wall and/or a convex outer wall. The shape of the siphuncular tube reflects the shape of the siphuncle that occupied and produced the siphuncular tube. Indeed, the shape of the siphuncle reflects the pressure regimes of the siphuncle and chamber at the time of production of the siphuncular tube. The contact between the siphuncle and connecting ring reflects this condition. Therefore the inner surface of the connecting ring is considered to be the main character that determined the shape of the siphuncular tube. The shape of the tube is described as ‘‘concave’’ when the minimum cross section diameter of the siphuncle is between two adjacent septa. It is ‘‘tubular’’ when the cross section diameter of the siphuncular segment is nearly constant. It is ‘‘convex’’ when the siphuncular tube is expanded within the chambers. Connecting ring shape.⎯The connecting ring is described as ‘‘straight’’ when the outer and inner surfaces of the ring are parallel along the entire length of the segment; the ring is ‘‘annular’’ when the thickness of the connecting ring is greatest near midlength of the segment. The connecting ring can be ‘‘wedge shaped’’ with its greatest thickness at its adapical or adoral end. MATERIAL

The material comprises 41 specimens collected by MSB during numerous field trips in the San Juan Precordillera in the last few decades. The material is reposited in the paleontological collection of the CRICYT-IANIGLA, Mendoza, Argentina (repository numbers, PI-IANIGLA: No), and the INGEO, Facultad de Ciencias Exactas, Fı´sicas y Naturales de la Universidad Nacional de San Juan (INGEO-PI: No). SYSTEMATIC PALEONTOLOGY

Order ORTHOCERIDA Kuhn, 1940 Family BALTOCERATIDAE Kobayashi, 1935, emend. Emended diagnosis.⎯Orthocones with nearly straight sutures and septal spacing with orthoceridan aspect. Siphuncle marginal or located between center and margin in some advanced forms. Siphuncular tube generally wide, in some forms narrow; tubular or slightly expanded between septa; no diaphragms present. Connecting ring thin compared with ellesmeroceridans, and Sactorthoceratidae. Septal necks suborthochoanitic or orthochoanitic. Apex spheroidal with constriction. Endosiphuncular deposits consist of tubular calcareous rods typically depressed circular in section. Cameral deposits typically mural, episeptal, restricted to adapical half of phragmocone. Comparison.⎯The Orthoceratidae also have a thin connecting ring, but differ from the Baltoceratidae in having a central or nearly central siphuncle, in having strongly surpressed endosiphuncular deposits, and in a generally narrower siphuncular diameter. The Protocycloceratidae differ from the Baltoceratidae in having endosiphuncular diaphragms and concave siphuncular segments. The Sactorthoceratidae differ from the Baltoceratidae in having thicker connecting rings, and a narrower septal spacing. Genera included.⎯Bactroceras Holm, 1899; Cochlioceras Eichwald, 1860; Cyrtobaltoceras Flower, 1964; Eosomichelinoceras, J.-Y. Chen, 1974; Hedstreomoceras Foerste, 1930; Metabaltoceras Flower, 1964; Microbaltoceras Flower, 1964; ?Ogygoceras Ulrich et al., 1944; Rangeroceras Hook and Flower, 1977; Rhabdiferoceras Flower, 1964; Sinocochlioceras Qi, 1980; Veneficoceras Hook and Flower, 1977. Occurrence.⎯Upper Lower–Middle Ordovician; North China, Korea, Siberia, Norway, Laurentian USA, Argentina.

¨ GER ET AL.—ORDOVICIAN ORTHOCERATOID CEPHALOPODS KRO Discussion.⎯The diagnosis of the family largely follows Frey (1995). Herein, we include an apex type that is known in Hedstroemoceras (Balashov, 1957) and Bactroceras (Dzik, 1982) and a connecting ring structure that is known in Cochlioceras (Mutvei, 2002) and Hedstroemoceras (Kro¨ger and Mutvei, 2005). These specific characters are similar to those in known orthoceridans. The Baltoceratidae share with the Orthoceratidae a spherical apex that lacks a cicatrix, and a thin connecting ring that consists of an inner calcified-perforate and outer spherulitic-prismatic layer. Consequently, the Baltoceratidae must be assigned to the Orthocerida. In addition, we restrict the Baltoceratidae to forms with a siphuncle that is tubular or slightly expanded within the chambers. This is a consequence of the emendation of the Ellesmerocerida by Kro¨ger and Mutvei (2005). Kro¨ger and Mutvei (2005) assigned nautiloids that display a siphuncle with a concave outline of siphuncular segments to the Ellesmerocerida. Therefore, the straight conchs of Amsleroceras Hook and Flower, 1977; Cyptendoceras Ulrich and Foerste, 1936; Pachendoceras Ulrich and Foerste, 1936; Rioceras Flower, 1964; and Robsonoceras Ulrich and Foerste, 1936 fall within the Ellesmerocerida as originally proposed by Flower (1964). Dzik’s (1984) synonymization of Rioceras with Cochlioceras Eichwald, 1860, and the classification of Pachendoceras and Robsonoceras within the Baltoceratidea overlooked the important differences of the siphuncular shape of these genera. The structure and thickness of the connecting ring of Cartersoceras Flower, 1964 and Murrayoceras Foerste, 1926, which were assigned to the Baltoceratidae by Flower (1964) and subsequent authors, differ considerably from that of other Baltoceratidae, where they are much thicker and layered (compare Flower, 1964, pl. 26, figs. 8–11). Similarly thick, layered connecting rings are known in the Sactorthoceratidae. Cartersoceras, Murrayoceras, and Wolungoceras Kobayashi, 1931 share a narrow septal spacing and expanded siphuncular segments with other Sactorthoceratidae (compare Fig. 3). Therefore, we assign Cartersoceras, Murrayoceras, and Wolungoceras to the Sactorthoceratidae. When Flower (1964) described the Baltoceratidae, he emphasized the occurrence of an endosiphuncular rod that he considered a unique feature of the family. However, the rods occur in straight nautiloids with large siphuncles in the upper Lower-Middle Ordovician that have very different septal necks, connecting rings, and siphuncular tube shapes. Beneath Cochlioceras, the endosiphuncular rod occurs in Cyptendoceras which has a connecting ring typical of Ellesmeroceratida and is present in Murrayoceras, which displays the connecting ring of typical Sactorthoceratidae. Therefore, we consider the endosiphuncular rods as a synapomorphy of the Baltoceratidae and the Sactorthoceratidae, and a feature that was already developed in the Ellesmeroceratidae. The discrimination between the Orthoceratidae and Baltoceratidae is problematical at first glance. Genera like Eosomichelinoceras, Nilssonoceras Kro¨ger, 2004; and Kinnekulloceras Kro¨ger, 2004 are very similar. The latter two, which are assigned to the Orthoceratidae, differ from the baltoceratid Eosomichelinoceras only in having annulosiphuncular deposits and a siphuncle that is closer to the center than to the conch margin. It seems subjective to separate these genera into two families. However, annulosiphuncular deposits are characteristic of many younger orthoceratid genera (e.g., Geisonoceras Hyatt, 1884), while baltocerid endosiphuncular deposits seem to be restricted to endosiphuncular rods. Moreover, the strongly excentric position of the siphuncle in Eosomichelinoceras is a feature characteristic of the Baltoceratidae, while the siphuncle position of orthocerids is generally close to the center. Genus COCHLIOCERAS Eichwald, 1860, emend. Type species.⎯Orthoceras avus Eichwald, 1860, from the Orthoceratite Limestone of Ropsha near St. Petersburg, Russia. Emended diagnosis.⎯Straight, smooth baltoceratids with large, marginal, tubular or slightly expanded siphuncle (more than

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0.25% of shell diameter). Connecting ring thin compared with Protocycloceras Hyatt (in Zittel and Eastman, 1900) or Sactorthoceras Kobayashi, 1934. Septal spacing of orthoceridan aspect (i.e., two to three septa in a length comparable to conch diameter). Septal necks orthochoanitic. Endosiphuncular deposits in apical parts of nearly mature specimens include an irregular lining and rod on side of siphuncle facing conch margin. Cameral deposits mural, episeptal. Comparison.⎯Hedstroemoceras differs from Cochlioceras in having clearly expanded siphuncular segments and a narrower siphuncle. Bactroceras differs from Cochlioceras in having a much narrower marginal siphuncle. Cartersoceras and Murrayoceras differ from Cochlioceras in having thick connecting rings. Occurrence.⎯Upper Lower–Middle Ordovician; Baltoscandia, Siberia, North China, western Argentina, and Laurentian North America.

Discussion.⎯Cochlioceras, by its original description and as emended by Balashov (1955), only included orthocones with a large marginal siphuncle and short septal necks. However, several species exist in which the siphuncle is slightly or considerably displaced from the conch margin. Although several species have been assigned to Cochlioceras in which the siphuncle is not in contact with the shell margin (such as Cochlioceras roemeri Dzik, 1984), an explicit emendation of the genus that allows inclusion of forms with a siphuncle slightly displaced from the conch margin has not existed until this report. Thus, we have changed the diagnosis in order to make up this inconsistency. COCHLIOCERAS AVUS Eichwald, 1860 Figure 4.4, 4.5 Cochlioceras avus EICHWALD, 1860, p. 1251, pl. 48, fig. 4a, 4b; BALASHOV, 1953, p. 207; 1955, pl. 6, fig. 7; CHANG, 1957, p. 38, 54; BALASHOV AND ZHURAVLEVA, 1962, pl. 6, fig. 7; SALADZIUS, 1966, p. 36, table 1, pl. 3, fig. 8a, b; DZIK, 1984, p. 17, pl. 1, fig. 1, text-figs. 1, e, h, 2.10; KING, 1999, p. 140, text-fig. 2. Endoceras avus (EICHWALD, 1860). FOORD, 1888, p. 146. Cochlioceras sinense CHANG, 1957, p. 55, pl. 2, fig. 2a, b. Cochlioceras sp. ACENÕLAZA AND BERESI, 2002, p. 115, pl. 2, fig. L.

Diagnosis (after Balashov, 1955).⎯Cochlioceras species with conchs with apical angle of approximately 3⬚, conch slightly compressed, ornamented with fine transverse striae (6–8/mm). Sutures straight. Three to four septa in a length comparable to conch diameter. Siphuncle tubular or slightly expanded, marginal, with diameter approximately one-third of conch diameter. Septal necks orthochoanitic. Comparison.⎯Cochlioceras avus differs from all other species of the genus in having a siphuncle that is in contact with the conch. Description.⎯Single specimen from San Juan Formation is 46 mm long, with maximum cross section diameter of 10 mm; diameter at the adapical end is 7.2 mm. Conch is circular in cross section. Outer shell not preserved. Specimen has 19 chambers, resulting in approximately four chambers per cross section diameter. Suture lines straight. Curvature of septa shallow. Septal necks ortochoanitic. Siphuncle marginal, measures approximately 0.4 of cross section diameter. Sipuncular segments tubular or very slightly expanded within the chambers. Material examined.⎯One specimen IANIGLA-PI No 920 V6, lower Darriwilian, Don Braulio Creek, Villicu´m Range, Precordillera, Argentina. Occurrence.⎯Eoplacognathus variabilis–Eoplacognathus suecicus zones, Darriwilian, Middle Ordovician; Baltoscandia, China, western Argentina.

Discussion.⎯The specimen has a siphuncle that is very wide compared with that of other Cochlioceras avus conchs. However, the intraspecific variation of the siphuncular diameter in C. avus is not known. The only known species of the genus with a very wide diameter is C. yuhangense Zou, 1987, from Zhejiang Province, North China, but it differs considerably from C. avus by its expanded siphuncular segments. Therefore, we consider the Precordilleran specimen to be a true representitative of C. avus. We synonymize C. sinense, with C. avus because the diagnostic characters given by Chang (1957) for C. sinense are based on minimal differences of siphuncular diameter and a narrower septal spacing

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FIGURE 4—Baltoceratidae of the San Juan Formation, Precordillera, Argentina. 1–3, Cochlioceras sp., INGEO-PI 686 Vi11, lower Darriwilian, Gustavo Creek, Villicu´m Range; note the compressed cross section and the unique lobate suture lines; 1, polished section of the siphuncle on prosiphuncular side of the conch, ⫻3; 2, antisiphuncular view, ⫻2; 3, lateral view, ⫻2; 4, 5, Cochlioceras avus Eichwald, 1860, PIANIGLA-PI No 920 V6, lower Darriwilian, Don Braulio Creek, Villicu´m Range; 4, lateral view, ⫻2; 5, polished section of the siphuncle on a plane slightly oblique to the growth axis and cutting the siphuncle obliquely within the prosiphuncular half of the conch, distance of the siphuncle from conch margin reflects plane of section, ⫻2.2; 6–9, Eosomichelinoceras baldisii n. sp., from Eoplacognathus suecicus Zone, upper member, San Juan Formation; Talacasto, and Cerro Viejo; 6, holotype, PI-IANIGLA No 923 Ta 10, median section, note the asymmetrical shape of the siphuncle, ⫻3; 7, detail of same specimen showing the thin connecting ring and a septal neck that slightly tips toward the siphuncle’s center, ⫻10; 8, specimen PI-IANIGLA No 922 CV4, lateral view, ⫻2; 9, same specimen, antisphuncular view, ⫻2.

¨ GER ET AL.—ORDOVICIAN ORTHOCERATOID CEPHALOPODS KRO in a single specimen. The septal spacing in C. sinense is approximately six chambers over a distance comparable to the conch diameter. Although the intraspecific variation of C. avus is unknown, it seems reasonable to synonymize it with C. sinense. COCHLIOCERAS sp. Figure 4.1–4.3 Cochlioceras sp. ACENÕLAZA

AND

BERESI, 2002, p. 115, pl. 2, fig. O.

Description.⎯Single specimen is 25 mm long, with maximum cross section diameter of 16 mm, and 12 mm diameter at adapical end. Conch clearly compressed in cross section with short axis/long axis ratio of 0.8. Outer shell not preserved. Fragment has three chambers, resulting in approximately two chambers over a distance comparable to cross section diameter. Sutures with shallow lateral lobes. Curvature of septa strong. Septal necks orthochoanitic. Siphuncle marginal, diameter approximately 0.4 times cross section diameter. Siphuncular segments tubular or very slightly expanded within chambers. Material examined.⎯One specimen INGEO-PI 686 Vi11, lower Darriwilian, Gustavo Creek, Villicu´m Range, Precordillera, Argentina. Occurrence.⎯Eoplacognathus variabilis–Eoplacognathus suecicus Zone, Darriwilian, Middle Ordovician; western Argentina (San Juan Formation).

Discussion.⎯The specimen differs from conchs of the 10 known species of Cochlioceras because of its strongly compressed cross section and its slightly lobate sutures. However, based on the single fragmentary specimen, the proposal of a new species is not justified. Genus EOSOMICHELINOCERAS Chen, 1974 Type species.⎯Eosomichelinoceras huananense J.-Y. Chen, 1974, from the Middle Ordovician of Southwest China. Proposed diagnosis.⎯Smooth or transversally lirate slender, orthoconic baltoceratids with narrow, tubular siphuncle. Siphuncle is strongly excentric, with distance of more than 0.2 of conch diameter from shell wall, and with thickness 0.2% or less of shell diameter. Connecting ring is thin compared with Protocycloceras and Sactorthoceras. Septal necks are orthochoanitic. Endosiphuncular or cameral deposits are not known. Comparison.⎯Eosomichelinoceras differs from all other baltoceratids in having a narrow, tubular, excentric siphuncle. Hedstroemoceras differs from Eosomichelinoceras in having wider siphuncular segments that are more expanded within the chambers. Wolungoceras differs from Eosomichelinoceras in having a wider, more tubular siphuncle with a thicker connecting ring and a clearly narrower septal spacing. The Baltoscandian orthoceridans Nilssonoceras Kro¨ger, 2004, and Kinnekulloceras Kro¨ger, 2004, differ in having annular endosiphuncular deposits and a more central siphuncle. Bactroceras differs in having a siphuncle that is close to the conch margin or marginal. Occurrence.⎯Upper Lower–Middle Ordovician; southwestern China, Tibet, North China, and western Argentina.

Discussion.⎯Referring any species to Eosomichelinoceras is problematical because the genus is defined only very briefly by J.-Y. Chen (1974, p. 142). Moreover, J.-Y. Chen (1974) referred to ‘‘Chen (1964)’’ for original description of the genus and for proposal of the genotype ‘‘Eosomichelinoceras huananense Chen, 1964.’’ However, we have been unable to find a ‘‘Chen (1964)’’ report. Thus, the figures given in J.-Y. Chen (1974, pl. 61, figs. 1–3) are the earliest illustrations of the type. Consequently, subsequent authors refer to the genus as Eosomichelinoceras J.-Y. Chen, 1974 (e.g., T.-E. Chen, 1987). Evans (2005, text-fig. 8) pinpointed in a very detailed description of Bactroceras that all known specimens referred to Bactroceras are characterized by a siphuncular position very close to the conch margin (distance of the siphuncle from conch wall between 5% and 15% of the conch diameter). Only in very early growth stages the position is more variable in Bactroceras. The distance of the siphuncle of Eosomichelinoceras from the conch margin is clearly above 15% of conch diameter, which can serve as a distinction between the two genera. However, there is some possibility that a closer investigation of the type species will reveal annulosiphuncular deposits.

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If so, Nilssonoceras or Kinnekulloceras must be synonymized with Eosomichelinoceras. However, despite these difficulties, a genus such as Eosomichelinoceras is needed in order to classify nautiloids similar to Bactroceras, but with a siphuncle that is considerably removed from the conch margin. EOSOMICHELINOCERAS BALDISII n. sp. Figure 4.6–4.9 Diagnosis.⎯Eosomichelinoceras species with compressed conch with low apical angle (3⬚–5⬚). Sutures with shallow lateral lobe. Three to four septa occur over a length comparable to cross section diameter. Siphuncle is eccentric, located approximately midway between conch wall and center, with diameter ca. 0.2 times conch cross section. Siphuncular segments are slightly expanded toward conch wall. Septal necks are orthochoanitic, tilted slightly toward center of siphuncle. Endosiphuncular and cameral deposits are not known. Description.⎯Conch apical angle varies from 5⬚ in fragment PI-IANIGLA No 922 CV4 (maximum cross section diameter 10 mm) to 1.9⬚ in holotype. Holotype conch has largest cross section diameter (15.5 mm). Conch cross section compressed with short axis/long axis ratio of 0.83 (in holotype). Sutures with shallow lateral lobes. Septal spacing ranges from three (PI-IANIGLA No 921 CV2, 922 CV4) to four septa (holotype) over a distance comparable to cross section diameter. Septal necks orthochoanitic, slightly tilted toward siphuncule center. Siphuncular segments tubular in lateral view, slightly expanded in transverse view toward conch. Siphuncule diameter ca. 0.15 times conch cross section diameter (in holotype). Siphuncle displaced 7 mm from conch wall where holotype conch has 17 mm cross section diameter. Connecting ring thin. Etymology.⎯In honor of the late Bruno Alberto Juan Baldis, a regional geologist at the Universidad Nacional de San Juan, Argentina, and the teacher of MSB. Type.⎯Specimen PI-IANIGLA No 923 Ta 10 from Eoplacognathus suecicus Zone, upper member of the San Juan Formation; Talacasto Creek, Central Precordillera, Argentina. Other material examined.⎯Two specimens from Cerro Viejo, Huaco area, Eoplacognathus variabilis Zone, San Juan Formation at PI-IANIGLA 921, 922. Occurrence.⎯Eoplacognathus variabilis–Eoplacognathus suecicus zones, Darriwilian, Middle Ordovician; western Argentina (San Juan Formation).

Discussion and comparison.⎯Eosomichelinoceras baldisii n. sp. is unique in its genus because the shapes of the septal necks, which are slightly tilted toward the center of the siphuncle and the slightly expanded siphuncular segments. Eosomichelinoceras baldisi n. sp. occupies an intermediate morphological position between Eosomichelinoceras and Hedstroemoceras in displaying partly expanded siphuncular segments. However, the narrow siphuncule and the slender orthoconic shell justify an assignment to Eosomichelinoceras. Eosomichelinoceras baldisi n. sp. from Precordilleran Argentina is the first record of the genus outside of China. Family SACTORTHOCERATIDAE Flower, 1946, emend. Emended diagnosis.⎯Orthocerids with smooth or annulated, orthoconic or slightly cyrtoconic conchs with straight sutures. Relatively close septal spacing of ellesmeroceridan aspect (five or more chambers over a distance comparable to conch cross section diameter). Siphuncle is central or located between conch wall and center, but never marginal. Siphuncle is narrow, tubular, or slightly expanded between the septa. Connecting rings are relatively thicker than in other Orthocerida. Septal necks are suborthochoanitic or orthochoanitic. Endosiphuncular deposits occur in apical parts of nearly mature specimens, and either form an irregular lining or are composed of endosiphuncular rods. Cameral deposits are mural and episeptal in apical portions the conch. Comparison.⎯The connecting rings in the Sactorthoceratidae are thinner than those in the Ellesmeroceratidae, but thicker than those of the Orthoceratidae. The Clinoceratidae differ in having a siphuncle that is never central and in displaying a peculiar longiconic to cyrtoconic fusiform shell shape. The Dawsonoceratidae differ by the regular presence of endosiphuncular deposits, thinner

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connecting rings, and a wider septal spacing. In addition, the Dawsonoceratidae display much less variation in the shape of the siphuncular tube. Dawsonoceratid siphuncular tubes are always slightly expanded, but show a definite constriction at the transition into the septal neck. The Baltoceratidae differ from the Sactorthoceratidae in having thinner connecting rings and generally a more tubular siphuncle. The Apocrinoceratidae differ in having strongly curved conchs and cyrtochoanitic septal necks. The Polymeridae and Rangeroceratidae differ from the Sactorthoceratidae in having characteristic endosiphuncular and cameral deposits (see Evans 2005). Genera included.⎯Braulioceras n. gen.; Cartersoceras Flower, 1964; Centroonoceras Kobayashi, 1934; Glenisteroceras Flower (in Flower and Teichert, 1957); Leptoplatophrenoceras Zou and Chen (in J.-Y. Chen and Zou, 1984); Murrayoceras Foerste, 1926; Sactorthoceras Kobayashi, 1934; Scipioceras Zhuravleva, 1964; Sigmocycloceras Kobayashi, 1934; Wennanoceras J.-Y. Chen, 1976; ?Wolungoceras Kobayashi, 1931. Occurrence.⎯Upper Lower–Middle Ordovician; North China, Korea, Siberia, Norway, Laurentian North America (New York), western Argentina.

Discussion.⎯Flower’s (1946) original definition clearly emphasized suborthochoanitic septal necks as a definitive character of the family. However, Flower (1962, p. 30) later emended the Sactorthoceratidae so that it only included forms that are very similar to the genotype of Sactorthoceras, S. gonioseptum Kobayashi, 1927. Unfortunately, the genotype of Sactorthoceras is very peculiar because its septal necks are geniculate and form a distinct wedge. With this emendation, the family was essentially restricted to a degraded monotypic taxon with limited utility. The original definition of Sactorthoceras (see Kobayashi, 1934) allowed much wider morphologic variation than that of Flower’s (1962) emended family. Kobayashi (1934) included three morphological groups in the genus: 1) a group characterized by S. shimamurai Kobayashi, 1934, with a tubular siphuncle; 2) a group with S. tenuicurvatum Kobayashi, 1934, with a slightly expanded siphuncule; and 3) a group characterized by S. gonioseptum, with a constricted siphuncular tube. Flower’s (1962) emendation restricted the entire family to Sactorthoceras forms comparable to S. gonioseptum. We recommend that Sactorthoceras should be restricted to suborthochoanitic forms of the S. tenuicurvatum and S. gonioseptum groups, and that all of Kobayashi’s (1934) Sactorthoceras species represent a family-level taxon that includes all three groups. Thus, our emendation of the Sactorthoceratidae invokes Flower’s (1946) original diagnosis of the group. Since the erection of the family, several similar Middle Ordovician orthocones have been proposed, mainly from sections in North China. Some of these forms are similar to Protocycloceras, but show a tubular or slightly expanded siphuncle (e.g., Wennanoceras, P. gangshanense Zou, 1988). Other forms are similar to Michelinoceras Foerste, 1932, but show a much narrower septal spacing and thicker connecting rings (e.g., Sactorthoceras shimamurai). The problematical Ellesmeroceras tchungense Balashov, 1962, from the upper Lower Ordovician of the Siberian Platform, has a siphuncle that is located very close to the shell wall and displays orthochoanitic septal necks and a tubular siphuncle similar to that in some Baltoceratidae. However, the very narrow septal spacing clearly distinguishes E. tchungense from the Baltoceratidae, and the species probably represents a new genus of the Sactorthoceratidae. The Whiterockian genus Murrayoceras has a connecting ring that is typical of the Sactortoceratidae (Flower, 1964, pl. 28, figs. 1–6) and has siphuncular segments that are concave in early growth stages and convex in more mature growth stages. Murrayoceras is morphologically intermediate between ellesmeroceridans and typical sactorthoceratids (e.g., Sactorthoceras).

Flower (1964, p. 135) regarded the Apocrinoceratidae as a family represented by slender cyrto- and orthoconic conchs that, unlike the Protocycloceratidae, have thick connecting rings that are expanded within the chambers. This definition overlaps with the Sactorthoceratidae. In fact, Apocrinoceras Teichert and Glenister, 1954 is a cyrtocone with strongly recumbent septal necks, and is more similar to discosoridans than to orthoceridans. If the Apocrinoceratidae has any utility, then it should be restricted to forms similar to Apocrinoceras. Thus, the problematical genus Glenisteroceras should be referred to the Sactorthoceratidae. Available evidence indicates the Sactorthoceratidae are limited to a relatively short time interval within the latest Early Ordovician to Middle Ordovician. The aim of the emendation of the family has been to include a number of forms in this time interval into a higher taxon because they appear to be closely related. The Polymeridae and the Rangeroceratidae are similar to the Sactorthoceratidae in several aspects, such as the narrow septal spacing, the comparatively thick, slightly expanded connecting rings and the expanded siphuncular segments. Therefore, a close phylogenetic relationship with these earliest orthoceratid families can be assumed. Genus BRAULIOCERAS new genus Type species.⎯Braulioceras sanjuanense n. sp. from the Middle Ordovician of the San Juan Formation, Argentine Precordillera. Diagnosis.⎯Smooth orthoconic orthocerids with very close septal spacing (ca. eight septa over a length comparable to conch cross section diameter). Septal necks orthochoanitic. Siphuncle central, slightly expanded between septa and has a beaded appearance. Siphuncular tube ca. 0.1 times conch cross section diameter. Thickness of connecting ring similar to that in ellesmeroceridans. Endosiphuncular or cameral deposits unknown. Comparison.⎯Sactorthoceras differs from Braulioceras in having suborthochoanitic septal necks and a siphuncular tube that is constricted at the septal foramina. Murrayoceras differs by having conchs with a larger apical angle and by showing a change in septal neck shape during ontogeny. Etymology.⎯Named for the topotype locality on Don Braulio Creek in the Villicu´m Range, San Juan Province, Argentina. Occurrence.⎯Eoplacognathus suecicus Zone, Middle Ordovician; western Argentina (San Juan Formation).

Discussion.⎯Although the siphuncular tube of Braulioceras expands within the chambers and is, in general, very similar to Kobayashi’s (1934) ‘‘Group of Sactorthoceras tenuicurvatum,’’ the new genus is significantly different. Sactorthoceras wongiforme Kobayashi, 1934 and S. tenuicurvatum clearly have suborthochoanitic septal necks, and therefore belong to Sactorthoceras sensu stricto. We propose Braulioceras for Sactorthoceratidae with a smooth shell, a slightly expanded siphuncle, and short orthochoanitic septal necks. ‘‘Michelinoceras’’ nanjingense Pan, 1986, from the Middle Ordovician of North China may represent a Braulioceras. However, the shape of the connecting ring of this species is not presently known. BRAULIOCERAS

new genus and species Figure 5.1–5.3

SANJUANENSE

Polygrammoceras sp. ACENÕLAZA

AND

BERESI, 2002, p. 115, pl. 2, fig. E

Diagnosis.⎯Same as for the genus. Description.⎯Holotype 36 mm long, with maximum cross section diameter of 15.9 mm, minimum diameter of 14.9 mm. Conch very slightly cyrtoconic, cross section circular. Sutures are straight. Septal spacing is narrow (ca. eight chambers occur over length comparable to conch cross section diameter). Interseptal distance in holotype is ca. 2 mm. Septal necks are orthochoanitic. Siphuncule is central, ca. 2 mm in diameter. Segments of siphuncular tube are slightly expanded within chambers, each segment nests in the preceding one. Connecting ring thickness is 0.14 mm. Connecting ring consists of opaque mass of calcite. Siphuncle and camera are without deposits.


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FIGURE 5—Sactorthoceratidae and Proteoceratidae of the San Juan Formation, Precordilleran Argentina. 1–3, Braulioceras sanjuanense n. gen. and sp., holotype, PI-IANIGLA No 925, Eoplacognathus suecicus Zone, San Juan Formation, Middle Ordovician; Don Braulio Creek; 1, lateral view, note narrow septal spacing, the straight sutures, and the slightly bent conch, ⫻1.9; 2, median section, note the thick connecting ring and the expanded siphuncular segments, ⫻6.5; 3, median section, note the orthochoanitic septal necks, ⫻12; 4–9, Gangshanonceras villicumense n. sp.; 4, specimen PI-IANIGLA No 928 Vi5, from Eoplacognathus variabilis Zone, Darriwilian, San Juan Formation, Middle Ordovician; Gustavo Creek, Villicu´m Range, lateral view, ⫻2; 5, same specimen, median section, ⫻2; 6, holotype PI-UNSI No 691 Vi82, Darriwilian, Gustavo Creek, note adult septal crowding, ⫻1.7; 7, Same specimen, detail of the siphuncular tube with orthochoanitic septal necks, tubular adult siphuncle and thin connecting ring, ⫻2.7; 8, same specimen detail of the adult siphuncle and septal neck, ⫻14; 9, specimen PI-UNSI No 692 Vi74, from Darriwilian, Gustavo Creek, showing episeptal cameral deposits, ⫻3.3; 10, Gangshanoceras sp., PI-IANIGLA No 937 Vi6, from Eoplacognathus suecicus Zone, upper member, San Juan Formation; Talacasto, Villicu´m Range, ⫻1.2.

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Etymology.⎯Named for occurrence of the species in the San Juan Formation of western Argentina. Type.⎯Holotype PI-IANIGLA No 925 Vi42, the only known specimen. Occurrence.⎯Eoplacognathus suecicus Zone, San Juan Formation, Middle Ordovician; Don Braulio Creek, Precordilleran Argentina.

Family PROTEOCERATIDAE Flower, 1962, emend. Emended diagnosis.⎯Orthocerids known from smooth or annulated, faintly cyrtoconic longicones with straight sutures. Siphuncle central or located between conch wall and center (never marginal) on convex side of shell. Siphuncule narrow, tubular, or slightly expanded between septa. Septal necks cyrtochoanitic or suborthochoanitic in juvenile stages and orthochoanitic in adult. Apex is spherical with constriction, and lacks a cicatrix. Parietal or annular endosiphuncular deposits and cameral deposits known. Comparison.⎯The Proteoceratidae differ from the Orthoceratidae in having suborthochoanitic septal necks in juvenile stages and orthochoanitic necks in mature stages. The connecting ring morphology is identical in both of these families. Genera included.⎯Archigeisonoceras T.-E. Chen, 1984; Baykonuroceras Barskov, 1972; Ephippiorthoceras Foerste, 1925; Euorthoceras Foerste, 1893; Gangshanoceras Zou, 1988; Gorbyoceras Shimizu and Obata, 1935; Isorthoceras Flower, 1962; Liulinoceras Zou and Shen (in J.-Y. Chen and Zou, 1984); Mesnaquaceras Flower, 1955; Metephippiorthoceras Zhuravleva, 1957; Monomuchites Wilson, 1961; Orthonybyoceras Shimizu and Obata, 1936; Paraproteoceras Chen (in J.-Y. Chen et al., 1981); Proteoceras Flower, 1955; Pseudeskimoceras Shimizu and Obata, 1936; Pseudoliolinoceras Zou and Shen (in J.-Y. Chen and Zou, 1984); Stereospyroceras Flower, 1955; Tofangoceras Kobayashi, 1927; Tofangocerina Kobayashi, 1936; Treptoceras Flower, 1942; Ulmioceras Zhuravleva, 1990. Occurrence.⎯Upper Lower Ordovician–Upper Silurian; cosmopolitan.

Discussion.⎯The emended diagnosis of the Proteoceratidae largely follows Flower (1962). However, the diagnostic features of the family are broadened to include taxa with conchs with suborthochoanitic septal necks. This emendation is justified by a consideration of the minimal differences between the septal neck shape of such Middle Ordovician genera as Gangshanoceras, with suborthochoanitic septal necks, and Liulinoceras, with short cyrtochoanitic septal necks. This emendation requires the assignment of Archigeisonoceras and Gangshanoceras to the Proteoceratidae. Consequently, Archigeisonoceras is the earliest representative of the Proteoceratidae. The apex and connecting ring characters of Archigeisonoceras are similar to those of other known Orthocerida (Kro¨ger, 2006). The occurrence of suborthochoanitic juvenile septal necks in early representatives of the family means that a reference of the Proteoceratidae to the Orthocerida is justified. Sweet (1964) and subsequent authors tentatively classified the Proteoceratidae in the Pseudorthoceratinae because of their juvenile cyrtochoanitic septal necks. This association of genera with conchs with suborthochoanitc septal necks means that the emended family is a homogenous clade with clear orthoceridan affinities. Genus GANGSHANOCERAS Zou, 1988 Type species.⎯Gangshanoceras jurongense Zou, 1988, from the Dawan Formation in Jiangsu, north China. Other species.⎯Gangshanoceras densum Zou, 1988; G. guichinense Ying, 1989; G. villicumense n. sp.; G. wannanense Ying, 1989. Diagnosis.⎯Proteoceratids with smooth, slender cyrtoconic conchs with circular cross section. Expansion rate of conch progressively decreases from juvenile to adult growth stages. Siphuncle is excentric and on convex side of growth axis in juvenile conchs, but more central in mature growth stages. Siphuncular segments are slightly expanded in chambers. Septal spacing is close (more than five septa occur in length comparable to conch

cross section diameter). Septal necks are suborthochoanitic in early growth stages, orthochoanitic in mature growth stages. Parietal endosiphuncular deposits are developed (diagnosis after Zou, 1988). Comparison.⎯Archigeisonoceras differs from Gangshanoceras in having annular endosiphuncular deposits, and in lacking a shift in siphuncle position during ontogeny. Liulinoceras and Pseudoliulinoceras differ from Gangshanoceras in having a central siphuncle and cyrtochoanitic septal necks in early growth stages. Occurrence.⎯Upper Lower–Middle Ordovician; North China and Precordilleran Argentina. The recovery of Gangshanoceras in the Precordillera is its first record outside of China.

GANGSHANOCERAS VILLICUMENSE n. sp. Figure 5.4–5.9 Diagnosis.⎯Gangshanoceras species with conchs with circular cross section, ornamented with fine transverse lirae; expansion rate is ca. 6⬚. Interseptal distance small (ca. five to six septa occur over a length comparable to conch cross section diameter). Septal curvature is broad. Siphuncle is excentric in juvenile stages, central in mature stages. Siphuncle diameter 0.15 times conch cross section diameter. Siphuncle slightly expanded within chambers in juvenile growth stages, tubular in mature stages. Septal necks suborthochoanitic in juvenile growth stages, orthochoanitic in adult stages. Endosiphuncular deposits unknown. Cameral deposits episeptally developed. Description.⎯Conch faintly ornamented with transverse striae, approximately 20–25 per 10 mm (specimen PI-IANIGLA No 928 Vi5). Apical angle of conch highly variable, maximum 9⬚ and minimum 2⬚ (mean 6⬚, n 13). Close but highly variable septal spacing (mean septal distance 0.18 times cross section diameter, maximum 0.32 times, minimum 0.08 times, n 20). Siphuncle diameter varies between 0.23 times and 0.12 times cross section diameter (mean 0.15, n 20). Siphuncle expanded within chambers in juvenile growth stages, tubular in mature stages. Septal necks suborthochoanitic if cross section diameter is less than 13–14 mm, orthochoanitic in larger fragments. Adult septal crowding in specimen holotype at 16 mm cross section diameter, at specimen PI-IANIGLA No 930 Vi100 at 26 mm, at specimen INGEO-PI No 691 Vi82 at 21 mm. Etymology.⎯Named for the Sierra de Villicu´m in western Argentina. Type.⎯Holotype INGEO-PI No 691 Vi82, Eoplacognathus variabilis Zone, Darriwilian, San Juan Formation, Middle Ordovician; Gustavo Creek, Villicu´m Range, Argentina. Other material examined.⎯Twenty specimens from the San Juan Formation of Precordilleran Argentina in the PI-IANIGLA collection. Occurrence.⎯Eoplacognathus variabilis Zone, Darriwilian, San Juan Formation, Ordovician; Don Braulio Creek and Gustavo Creek in the Villicu´m Range, Precordilleran Argentine.

Discussion and comparison.⎯The species is the most common orthocerid in the San Juan Formation. The variation in septal spacing, siphuncular diameter, and adult size of the available specimens is relatively high. However, the material does not allow separation of the specimens into two or more species. The extreme values may represent separate species or simply a relatively variable species. Future collecting may resolve this problem. Gangshanoceras villicumense n. sp. is unique within the genus in having a nearly tubular siphuncle with low excentricity. The excentricity of its siphuncle is less than in all Chinese species of Gangshanoceras. GANGSHANOCERAS sp. Figure 5.10 Description.⎯Specimen 97 mm long, maximum cross section diameter of 18 mm, diameter at adapical end ca. 14 mm. Conch circular in cross section, outer shell not preserved. Three to four chambers present in a length comparable to conch cross section diameter. Sutures straight. Broad curvature of septa, septal necks orthochoanitic throughout specimen. Siphuncle tubular, central, diameter ca. 0.12 times conch cross section diameter. Material examined.⎯Specimen PI-IANIGLA 937 Ta6, Eoplacognathus variabilis–Eoplacognathus suecicus zones, Darriwilian, Talacasto, Precordilleran Argentina.

¨ GER ET AL.—ORDOVICIAN ORTHOCERATOID CEPHALOPODS KRO Occurrence.⎯Eoplacognathus variabilis–Eoplacognathus suecicus Zones, Darriwilian, Middle Ordovician; western Argentina (San Juan Formation).

Discussion.⎯This specimen represents an undescribed species of Gangshanoceras. It differs from previously described species of Gangshanoceras in its combination of a central siphuncle with low expansion rate and wide septal spacing. However, the erection of a new species is not possible because a differential diagnosis would require more material. Family ORTHOCERATIDAE McCoy, 1844 Diagnosis.⎯Orthocerids with orthoconic conchs with slender, central or subcentral, tubular or moderately expanded siphuncle. Septal necks suborthochoanitic or orthochoanitic, respectively. Shell smooth or annulated, with more-or-less well developed longitudinal and transverse ribs, ridges, or combination of these features. Bowl-shaped apical shell small, straight, without cicatrix, without initial constriction, smooth or ornamented with transversal and longitudinal elements, but without annulations. Endosiphuncular and cameral deposits occur only in most apical chambers of quasi-mature specimens (diagnosis after Kro¨ger and Isakar, 2006). PALORTHOCERAS new genus Type species.⎯Palorthoceras kayseri n. sp. from the upper Lower Ordovician of the San Juan Formation, Precordilleran Argentina. Included species.⎯Palorthoceras kayseri n. gen. and sp., Michelinoceras buttsi Flower, 1962. Diagnosis.⎯Orthoceratids with smooth very slender orthoconic conchs with relatively widely spaced septa (ca. three septa along a length comparable to cross section diameter). Siphuncle is central, slightly expanded between septa; septal necks are suborthochoanitic. Endosiphuncular deposits occur as annuli at septal perforations or as parietal endosiphuncular deposits; episeptal and hyposeptal deposits known. Cameral deposits are episeptal. Endosiphuncular deposits occur as irregular lining, annuli, or parietal deposits. Etymology.⎯Palorthoceras, an old, ancient (Latin palæo) form compared with Orthoceras. Occurrence.⎯Upper Lower–lower Middle Ordovician; western Argentina (San Juan Formation) and western United States (Florida Mountains Formation, Wahwah Formation).

Discussion and comparison.⎯The genus is known from specimens that represent the oldest known orthoceratids, and occur in the Florida Mountains Formation of New Mexico and West Texas, and the coeval Wahwah Limestone in the Ibex area, western Utah (Oepikodus evae–Paroistodus originalis Zone-equivalents), and in the lower San Juan Formation, Precordilleran Argentina (Oepikodus evae Zone). There seems to be a quite large variation in the development of the endosiphuncular deposits within the genus; more material may enhance the understanding of the intra- and interspecific variation of this character. Orthoceras Bruguie`re, 1789 differs from Palorthoceras in having a characteristic fine longitudinal and transverse ornamentation on the conch, a wider septal spacing, and orthochoanitic septal necks. Michelinoceras differs in having a wider septal spacing and a narrower, strictly tubular siphuncle. Finally, Pleurorthoceras Flower, 1962 differs from Palorthoceras in having an excentric, tubular siphuncle that is narrower and constricted at the septal foramina. PALORTHOCERAS KAYSERI new genus and species Figure 6.1, 6.2, 6.5 Orthoceras sp. KAYSER, 1876, p. 14, pl. 5, fig. 5; CECIONI, 1953, p. 1; ACENÕLAZA AND BERESI, 2002, p. 8. Michelinoceras sp. HOOK AND FLOWER, 1977, p. 44, 45, pl. 4, figs. 4–8; pl. 11, figs. 6–8, pl; 17, figs. 4–7; pl. 19, figs. 4–6. non Michelinoceras sp. HOOK AND FLOWER, 1977, p. 45, pl. 17, figs. 1–3, 8–13.

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Diagnosis.⎯Species of Palorthoceras n. gen. with conch with a very low apical angle (ca. 1⬚), ca. four chambers occur over a length comparable to conch cross section diameter, siphuncle diameter is 0.17 times cross section diameter of conch. Siphuncle is slightly expanded within chambers. Endosiphuncular and cameral deposits are unknown. Description.⎯Holotype lacks outer shell. Holotype 43 mm long, maximum cross section diameter 12 mm, minimum diameter 11 mm, apical angle ca. 1⬚. Shell with circular cross section. Sutures straight with 3 mm separation, interseptal distance of 0.25 times conch cross section diameter. Siphuncle central, 2 mm diameter, very slightly expanded within chambers. Connecting ring thin. Septal necks suborthochoanitic. Etymology.⎯Species named in honor of Emanuel Kayser, the first paleontologist to describe fossils from Precordilleran Argentina (see Kayser 1876). Type.⎯Holotype PI-IANIGLA No 940 Ta 5, Oepikodus evae Zone, San Juan Formation, upper Lower Ordovician; Quebrada de Talacasto section, central Precordillera, San Juan Province. Other material examined.⎯The species is only known from the holotype. Occurrence.⎯Oepikodus evae Zone, upper Lower Ordovician; San Juan Formation; Precordilleran Argentina and western United States (Florida Mountains Formation).

Discussion and comparison.⎯Palorthoceras kayseri is one of the oldest orthocerids. It is approximately coeval with the early orthocerid Michelinoceras primum Flower, 1962, from Oepikodus evae Zone-equivalent strata in the Fillmore Formation of the Ibex area, western Utah. Some of the specimens that have been described as Michelinoceras sp. from the Florida Mountains Formation of New Mexico (Hook and Flower, 1977) are assigned to the species as they are very similar to the fragment from the Precordillera. Palorthoceras buttsi (Flower, 1962) differs from P. kayseri in having annular endosiphuncular deposits, a wider interseptal distance, and a wider siphuncle. Genus ORTHOCERAS Bruguie`re, 1789 Type species.⎯Orthoceratites regularis Schlotheim, 1820, Orthoceratite Limestone from Tallinn, Estonia. Diagnosis.⎯‘‘Straight, orthoconic shells with longitudinal impressions of the living chamber. Exterior sculptured with transverse lines of growth forming a banding somewhat similar to that in Geisonoceras, but the bands are composed of densely crowded minute longitudinal ribs, which are especially well shown on a slightly weathered surface. Apertural angle is small. Air chambers and siphuncle are of medium size; siphuncle is central or subcentral.’’ Siphuncle is tubular, septal necks are orthochoanitic (diagnosis from Troedsson, 1931, p. 12). Comparison.⎯Palorthoceras conchs differ from those of Orthoceras in having slightly expanded siphuncular segments and suborthochoanitic septal necks. Michelinoceras conchs have a wider septal spacing and a smooth exterior surface. Finally, Pleurorthoceras differs from Orthoceras in having an excentric siphuncle with characteristic constrictions at septal foramina and in having suborthochoanitic septal necks. Occurrence.⎯Eoplacognathus suecicus–Pygodus anserinus zones, Middle Ordovician; Baltoscandia, China?, Argentina?

Discussion.⎯The paleogeographic distribution of Orthoceras remains undetermined because the genus is frequently used as ‘‘wastebasket’’ for poorly known, simple, straight orthocones with a central siphuncle and orthochoanitic septal necks. Moreover, the designation Michelinoceras is used interchangeably with Orthoceras for these nautiloids. Chinese Middle Ordovician forms such as M. chaoi Chang, 1957, M. beianense Ying, 1989, M. paraelongatum Chang, 1962, and M. sinoceraforme Lai, 1960 may represent species of Orthoceras, but no description of the external conch ornamentation has been provided for these species. Orthoceras is not known from North America. ORTHOCERAS? sp. Figure 5.3, 5.4, 5.6 Description.⎯Specimen PI-IANIGLA No 941 Vi70 35 mm long, maximum cross section diameter 11.5 mm, diameter at adapical end ca. 8 mm.

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FIGURE 6—Orthoceratidae of the San Juan Formation, Precordilleran Argentina. 1, 2, 5, Palorthoceras kayseri n. gen. and sp., holotype PI-IANIGLA No 940 Ta 5, from the Oepikodus evae Zone, San Juan Formation, upper Lower Ordovician; Quebrada de Talacasto section, central Precordillera; 1, Median section showing details of the thin connecting ring and suborthocoanitic septal necks, ⫻11; 2, median section showing details of the septal neck, ⫻20; 5, same specimen as in 1 and 2, median section of the entire fragment, ⫻2.7. 3, 4, 6, Orthoceras? sp., PI-IANIGLA No 942 Vi70, from Darriwilian, Gustavo Creek, Villicu´m Range, medial section showing detail of the septal neck and connecting ring, ⫻4; 4, same specimen, medial section of entire fragment, ⫻4.7; 6, PI-IANIGLA No 942 Vi73, from Darriwilian, Gustavo Creek, Villicu´m Range, median section of the entire fragment, ⫻2.5.

Conch is circular in cross section; outer shell is not preserved. Fragment displays ten chambers, with ca. three chambers over a length comparable to cross section diameter. Suture lines are straight. Septal curvature is broad; septal necks are orthochoanitic; siphuncle is tubular, central, with diameter ca. 0.18 times conch cross section diameter. Specimen PI-IANIGLA No 942 Vi73 ca. 25 mm long, maximum cross section diameter 15 mm, diameter at adapical end ca. 11.5 mm. Conch is circular in cross section with outer shell not preserved. Fragment displays four chambers, with ca. 2.5 chambers over a length comparable to conch cross section diameter. Suture lines are straight. Septal curvature is broad; septal necks are orthochoanitic; siphuncle is tubular, central with diameter ca. 0.15 times conch cross section diameter. Material examined.⎯Two specimens PI-IANIGLA No 941 Vi70, 942 Vi73, Darriwilian, Gustavo Creek, Villicu´m Range, Precordilleran Argentina. Occurrence.⎯Eoplacognathus variabilis–Eoplacognathus suecicus zones, Darriwilian, Middle Ordovician; western Argentina (San Juan Formation).

Discussion.⎯The tubular, central siphuncle, orthochoanitic septal necks, chamber spacing, lack of endosiphuncular deposits, and the general conch shape of the specimens all allow an assignment to Orthoceras. However, a confident generic assignment of the specimens is impossible because the external conch ornamentation, the adult body chamber, and the apical characters are

not preserved. For these reasons, only a tentative assignment to Orthoceras? is proposed. Order LITUITIDA Starobogatov, 1974 Diagnosis.⎯Cephalopods with longicone orthoconic conchs with cyrtoconic or coiled apical part. Siphuncle is on convex side of shell in earliest ontogenetic stages, on concave side in later stages. Siphuncle is narrow; septal necks are long, orthochoanitic or hemichoanitic; connecting ring is often disaggregated and covered with cameral deposits that extend to cover the septal neck and inner parts of the connecting ring. Endosiphuncular and endocameral deposits are adnate. Cameral deposits often show longitudinal lamellae or sheets that longitudinally divide chambers. Apex is subspherical without cicatrix. Narrow and deep hyponomical sinus occurs (diagnosis largely after Dzik, 1984). Discussion.⎯Starobogatov (1974) defined the lituitidans as a cephalopod taxon of the same rank as the Orthoceratida. However, Dzik (1984, p. 131–141) first emphasized the peculiar features of this group. The fragile, often disaggregated connecting ring, the adnate deposits with vertical lamellae, the long orthochoanitic


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FIGURE 7—Rhynchorthoceras minor n. sp. from the San Juan Formation, Paroistodus originalis–Eoplacognathus variabilis zones, Middle Ordovician, Precordilleran Argentina. 1, INGEO-PI No 693 Vi53, from Gustavo Creek section, Villicu´m Range, lateral view, ⫻2.6; 2, INGEO-PI No 694 Vi53, view of convex side of the conch, note the traces of transverse striation, ⫻3.4; 3, holotype PI-IANIGLA No 943 Vi10, from from Don Braulio section, lateral view, ⫻2.6; 4, INGEO-PI No 695 Vi53, from Gustavo Creek section, Villicu´m Range, median section, ⫻6.4; 5, same specimen, detail of the siphuncle showing the long septal necks, ⫻13; 6, INGEO-PI No 695 Vi53, from Gustavo Creek section, Villicu´m Range, cross section, showing the longitudinal lamella on convex side of the conch, ⫻6.4; 7, PI-IANIGLA No 947 Vi107, from Don Braulio section, median section, showing detail of the septal necks without cameral deposits, ⫻4.7; 8, same specimen, median section, showing different development of cameral deposits in the chambers, ⫻3; 9, same specimen, median section, showing more apical chambers with cameral deposits cover septal necks, ⫻7; 10, PI-IANIGLA No 943 Vi10, from Don Braulio section, median section, specimen with relatively low expansion rate and wide septal spacing, ⫻5.

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septal necks, and the frequent occurrence of juvenile coiling distinguish the group from the Orthocerida and justify the ordinal level of the lituitidans. The Orthocerida and the Lituitida appear in the Early Paleozoic, and are the earliest cephalopods with a spherical apex and lack of a cicatrix on the conch. The two orders compose the Neocephalopoda (Engeser, 1996). The earliest Lituitida are known from the lower Middle Ordovician (Volkhovian) of Baltoscandia (Dzik, 1984) and from coeval Paroistodus originalis–Eoplacognathus variabilis zones in China (Qi, 1980) and, now, Precordilleran Argentina. Family SINOCERATIDAE Shimizu and Obata, 1935 Diagnosis.⎯Lituitidans without a coiled apical shell; apex cyrtochoanitic; siphuncle is subcentral; hyponomic sinus is weakly developed (after Dzik, 1984). Discussion.⎯Dzik (1984) noted that the distinction between lituitidans and orthoceridans may seem to be arbitrary in some taxa, particularly if the Sinoceratidae are considered. However, the presence of typical cameral and endosiphuncular deposits even in the earliest Sinoceratidae seems to represent a distinctive criterion to distinguish the two groups. Rhynchorthoceras Remele´, 1882, an early representative of the Sinoceratidae, shows the characteristic lituitidan vertical lamella, and a probable syn vivo resorption of the connecting ring is documented (Sweet, 1958, p. 121, 122, 139). The Precordilleran lituitidans show these features very clearly (see Fig. 7.6, 7.9). Genus RHYNCHORTHOCERAS Remele´, 1882, emend. Type species.⎯Lituites breynii Boll, 1857, from Orthoceratite Limestone of erratics, northern Germany. Emended diagnosis.⎯Lutuitidans with orthoconic longicone conchs with slight curvature at apex. Siphuncle is tubular or slightly expanded within chambers, large (diameter one-sixth of conch diameter), subcentral and displaced toward convex side of shell. Septal necks are orthochoanitic, cameral deposits cover septal necks in some specimens. Cameral deposits with single vertical lamella on concave side of shell. Comparison.⎯Ancistroceras Boll, 1857 differs from Rhynchorthoceras in having coiled juvenile growth stages. The shell shape of Sinoceras Shimizu and Obata, 1935 is generally more elongate and slender in nearly growth stages, and Sinoceras has longer septal necks. Occurrence.⎯Middle Ordovician of Baltoscandia, North China, and Precordillera Argentina. The genus is not known from Laurentian North America.

Discussion.⎯In Baltoscandia, Rhynchorthoceras is dominantly represented by species with transversely ornamented conchs. The original diagnosis of the genus therefore included this. However, smooth species, such as Orthoceras conicum Hisinger, 1837, occur which are similar to the genotype of Rhynchorthoceras in all aspects except for the ornamentation. Therefore, Dzik (1984) referred O. conicum to Rhynchorthoceras. We agree with this suggestion, and have modified the generic diagnosis in order to correct the inconsistency. In the Chinese literature on Ordovician cephalopods, Rhynchorthoceras and Ancistroceras are used interchangeably. Thus, the criterion of a coiled apex is not applied to species assigned to Ancistroceras by Qi (1980), J.-Y. Chen and Zou (1984), and Xu and Lai (1987). It also seems that conchs with a high expansion rate are more likely to be assigned to Ancistroceras in these reports. Consequently, many of the species assigned to Ancistroceras in these reports may actually represent Rhynchorthoceras species. The entire group that includes Ancistroceras, Rhynchorthoceras, and Sinoceras needs revision in order to provide an unequivocal classification. RHYNCHORTHOCERAS MINOR n. sp. Figure 7 Clinoceratidae indet. ACENÕLAZA AND BERESI, 2002, p. 113, pl. 1, fig. I. Family Orthoceratidae gen. et sp. indet. ACENÕLAZA AND BERESI, 2002, p. 115, pl. 2, fig. I.

Diagnosis.⎯Rhynchorthoceras species showing a combination of narrow septal spacing (ca. 0.19 mm), compressed cross section, and apical angle of 13⬚. Conch is cyrtocone in early growth stages, nearly straight in later growth stages, weakly transversely striated. Septal neck length is 0.4 times chamber height. Siphuncle tubular with diameter ca. 0.14 times conch cross section diameter. Description.⎯Outer shell is weakly transversely ornamented (INGEO-PI No 695 Vi53) with ca. 20 striae/10 mm. Striae with shallow lobe on convex side of conch. Largest specimen with 13 mm cross-cross section diameter (PIIANIGLA No 943 Vi10). Initial 5 mm of conch is strongly cyrtocone with siphuncle on convex side, later growth stages are very slightly cyrtoconic in opposite direction with siphuncle on concave side. Conch expansion is initially strong but subsequently decreases. Apical angle 9⬚–16⬚ (mean 13⬚, n 7). Conch cross section is compressed with short axis/long axis ratio of 0.8 (INGEO-PI No 695 Vi53). Septal spacing is narrow (mean 0.19 times cross section diameter, maximum 0.38 times, minimum 0.16 times). Siphuncle is tubular, diameter 0.14 times of conch cross section diameter. Septal necks are orthochoanitic with length ca. 0.4 times chamber height (INGEO-PI No 695 Vi53). Vertical lamella are present in INGEO-PI No 695 Vi53. Etymology.⎯From Latin minoro ⫽ decreasing; all known specimens are small compared with other Rhynchorthoceras conchs. Type.⎯Holotype PI-IANIGLA No 943 Vi10 from Paroistodus originalis– Eoplacognathus variabilis zones, Middle Ordovician; Don Braulio section, Villicu´m Range, San Juan Province. Other material examined.⎯Eleven additional specimens are in the PI-IANIGLA and INGEO-PI collections from the Eoplacognathus variabilis Zone, Darriwilian, Middle Ordovician, Gustavo Creek and Cerro Viejo, Huaco area, Gustavo Creek, Villicu´m Range eastern Precordillera and Cerro Viejo, Huaco area, San Juan Province, Argentina. Occurrence.⎯Paroistodus originalis–Eoplacognathus variabilis Zones, Middle Ordovician; western Argentina (San Juan Formation).

Discussion and comparison.⎯Rhynchorthoceras minor conchs are unique in the genus by their combination of narrow septal spacing, compressed cross section, and relatively low apical angle. Rhynchorthoceras minor also differs from many species of the genus in having an only slightly ornamented shell. Rhynchorthoceras conicum (Hisinger, 1837) is smooth, but differs in having a circular cross section and a more slender conch. Rhynchorthoceras subcurvatum (Qi, 1980) differs in having a siphuncle that is expanded within the chambers, and R. densum (Qi, 1980) differs in having a narrower siphuncle and a larger apical angle. No adult specimen of Rhynchorthoceras minor has been found. However, larger specimens show a reduced expansion rate of the conch, and suggest a relatively small adult size of the species with a conch reaching approximately 20 mm in cross section diameter. This is smaller than in any of the previously described species of Rhynchorthoceras. Rhynchorthoceras densum and R. subcurvatum occur in the Dawan Formation of Anhui, North China, in strata approximately coeval with the occurrence of R. minor in the San Juan Formation (i.e., Paroistodus originalis–Eoplacognathus variabilis Zones). These three species represent the earliest known lituitidans. ACKNOWLEDGMENTS

The sections were prepared by J. Sua´rez. This work is part of DFG project KR 2095-2 (BK principal investigator). MSB is indebted to B. Baldis, who accompanied her for the first time to the Don Braulio section, where orthocerid nautiloids were collected during the Ph.D. field work. She is grateful to S. and N. Peralta, who accompanied her to the Gustavo Creek (Villicu´m Range) and Cerro Viejo (Huaco area), Argentine Precordillera collecting nautiloids. The systematic synthesis benefited from the general help and support of D. Korn, Museum fu¨r Naturkunde, Berlin. We are grateful for the careful reviews of H. Mutvei, Stockholm, and an anonymous reviewer. This work is a contribution to IGCP Project 503: Ordovician Paleogeography and Paleoclimate. REFERENCES

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