a new ichnospecies of nereites from carboniferous tidal-flat ... - BioOne

J. Paleont., 74(1), 2000, pp. 149–157 Copyright q 2000, The Paleontological Society 0022-3360/00/0074-0149$03.00

A NEW ICHNOSPECIES OF NEREITES FROM CARBONIFEROUS TIDAL-FLAT FACIES OF EASTERN KANSAS, USA: IMPLICATIONS FOR THE NEREITES–NEONEREITES DEBATE M. GABRIELA MANGANO,1,3 LUIS A. BUATOIS,1,3 CHRISTOPHER G. MAPLES,1,4

AND

RONALD R. WEST2

Kansas Geological Survey, 1930 Constant Ave, The University of Kansas, Campus West, Lawrence, 66047, and 2 Department of Geology, Kansas State University, Manhattan, 66506, ,[email protected]. 3 Present address: Instituto Superior de Correlacioń Geolo´gica, Casilla de correo 1 (CC), 4000 San Miguel de Tucumań, Argentina, ,[email protected]. 4 Present address: Department of Geological Sciences, Indiana University, Bloomington, 47405, ,[email protected]. 1

ABSTRACT—Predominantly horizontal, gently curved to slightly sinuous traces constituting uniserial rows of imbricated, subspherical sediment pads occur in Pennsylvanian tidal-flat facies of eastern Kansas. These traces exhibit a complex, actively filled internal structure. The presence of a median tunnel enveloped by overlapping pads of reworked sediment indicates that these biogenic structures should be included in the ichnogenus Nereites MacLeay in Murchison, 1839. A new ichnospecies, N. imbricata, is erected. Externally, Nereites imbricata differs from the other Nereites ichnospecies by the large, tightly packed, imbricated pads that commonly result in an annulated appearance on bedding-planes. Internally, obliquely arranged, arcuate laminae envelope the median tunnel and tend to follow the outline of the external semispherical pads. Additionally, the behavioral pattern reflected by N. imbricata is less specialized than that of the other Nereites ichnospecies. Eione monoliformis Tate, 1859 resembles N. imbricata in general appearence, but lack the diagnostic Nereites internal structure, and is invariably preserved as positive epireliefs. Occurrence of Nereites imbricata as both median tunnels surrounded by reworked sediment (Nereites preservation) and uniserial rows of imbricated sediment pads (Neonereites preservation) supports the notion that Neonereites Seilacher, 1960 is a preservational variant of Nereites. The ichnogenus Nereites is an eurybathic form and is a common component of Paleozoic shallow-marine facies.

INTRODUCTION

comprising spherical to subspherical sediment pads arranged in uniserial chains are relatively common in Paleozoic shallow-marine sequences. Several ichnogeneric names have been used to designate these traces, including Eione Tate, 1859, Petromonile Casey, 1961, Margaritichnus Bandel, 1973, and Neonereites Seilacher, 1960, as well as such informal names as ‘‘worm castings’’ (Shrock, 1935), ‘‘segmented trails’’ (Teichert, 1941), and ‘‘constricted burrows’’ (Archer, 1984). Uniserial chains of sediment pads have recently been found in Virgilian tidal-flat facies of the Stull Shale Member of the Kanwaka Shale Formation of eastern Kansas. Although apparently massive in cross-sectional view, several partially weathered, bedding-plane specimens display an internal axial tunnel, suggesting that the traces are affiliated with the ichnogenus Nereites MacLeay in Murchison, 1839. A new ichnospecies, N. imbricata, is defined herein. The relationship of Nereites with other forms, particularly Neonereites Seilacher, 1960, and Scalarituba Weller, 1899, is controversial. Some authors have argued that these three ichnotaxa represent preservational variants of the same form (e.g., Seilacher and Meischner, 1965; Chamberlain, 1971; D’Alessandro and Bromley, 1987; Rindsberg, 1994; Uchman, 1995), whereas others have retained them as separate ichnogenera (e.g., Hakes, 1976; Benton, 1982; Fillion and Pickerill, 1990; Pickerill, 1991; Crimes and McCall, 1995; Orr et al. 1996). The purpose of this contribution is, therefore, to describe a new ichnospecies of Nereites from the Stull Shale Member, to discuss its implications in the current debate on the ichnotaxonomic status of Nereites, and to compare Nereites imbricata with similar ichnotaxa known in Paleozoic strata, particularly Carboniferous.

T

RACE FOSSILS

LOCATION AND GEOLOGIC SETTING

Specimens described herein are from the Upper Pennsylvanian (Virgilian) Stull Shale Member of the Kanwaka Shale Formation (Shawnee Group) exposed at Waverly (SW¼, NW¼, sec. 19, T19, R16E), eastern Kansas (Fig. 1). The outcrop is a stream

cut with well-exposed bedding-plane surfaces. In the terminology of cyclothems, the Stull Shale Member is regarded as an outside shale (nearshore/terrestrial) (Heckel, 1977, 1994). Carbonate-siliciclastic cycles cropping out in Kansas represent the transgressive-regressive deposits of an eperic sea that covered the North American Midcontinent during the late Paleozoic. At Waverly, 5.8 m of the upper part of the Stull Shale Member is exposed. At the base, the succession consists of interbedded mudstone and very fine-grained rippled sandstone. Channelized very fine-grained sandstone with trough cross-lamination and inclined heterolithic stratification characterize the middle part of the section. Orthomyalinid (bivalve) packstone, wackestone and shale make up the upper interval (Fig. 2). The trace fossils described in this paper occur on the soles of thin-bedded, rippled sandstone beds in the lower part of the Waverly succession (Fig. 2). These sandstone beds are included in a 60-cm-thick heterolithic unit. This unit is characterized by discontinuous light gray (yellowish orange when weathered), very fine-grained sandstone beds separated by thin mudstone partings. Mud chips and shell debris are relatively common at the base of the sandstone beds. Symmetrical, asymmetrical, interference, and flat-topped ripples, as well as sand volcanoes, load casts, and flute marks are the most common physical sedimentary structures. Bedding types include lenticular bedding with connected flat lenses, wavy bedding, and wavy flaser bedding. Desiccation cracks and low-angle cross-lamination are locally present. This unit exhibits a high-diversity invertebrate ichnofauna that, together with Nereites, includes the ichnogenera Arenicolites, Asteriacites, Asterosoma, Chondrites, Conichnus, Cruziana, Curvolithus, Diplocraterion, Lockeia, Olivellites, Palaeophycus, Parahaentzschelinia, Pentichnus, Phycodes, Phycosiphon, Planolites, Protovirgularia, Rhizocorallium, Rosselia, Rusophycus, Skolithos, Teichichnus, Trichophycus. Specimens of other ichnospecies of Nereites (N. cambrensis, N. jacksoni, and N. missouriensis) are also present in these tidal-flat deposits. This heterolithic unit is interpreted as intertidal mixed- to sand-flat deposits. Alternation of tidal currents with suspension fallout during slack-water periods is indicated by the existence

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FIGURE 1—Map showing distribution of the Shawnee Group in Kansas and position of the Waverly trace fossil locality.

of lenticular, wavy, and wavy flaser bedding. Trace fossils are preserved for the most part on bedding surfaces, with minor disruption of the original sedimentary fabric. Preservation of trace fossils was enhanced by the alternation of sand and mud layers. High diversity of biogenic structures suggests that the tidal flat was formed along an open coast with normal-marine conditions. SYSTEMATIC ICHNOLOGY

Ichnogenus NEREITES MacLeay in Murchison, 1839 Type ichnospecies.—Nereites cambrensis Murchison, 1839. Diagnosis.—Selectively preserved, curved, winding to regularly meandering or spiralling, unbranched, predominantly horizontal trails, consisting of a median backfilled tunnel enveloped by an even to lobate zone of reworked sediment (after Uchman, 1995 and Orr and Pickerill, 1995). Discussion.—The taxonomy of the ichnogenus Nereites and its relationship with other ichnotaxa, such as Neonereites Seilacher, 1960, and Scalarituba Weller, 1899, have been discussed extensively in the literature, and consensus has not yet been achieved. Some authors have argued that these three ichnotaxa represent preservational variants of the same form (e.g., Seilacher and Meischner, 1965; Chamberlain, 1971; D’Alessandro and Bromley, 1987; Rindsberg, 1994; Uchman, 1995), while others have retained them as separate ichnogenera (e.g., Hakes, 1976; Fillion and Pickerill, 1990; Pickerill, 1991; Crimes and McCall, 1995; Orr and Pickerill, 1995; Orr et al., 1996). Chamberlain (1971) demonstrated that single specimens could be preserved as hypichnial, biserially arranged lobes or pustules (Neonereites biserialis view) or as epichnial median furrows with

reworked lobes on both sides (Nereites-Phyllodocites view). Orr et al. (1996, Fig. 6) illustrated a similar situation. Chamberlain and Clark (1973) also documented transition from Scalarituba to Neonereites. Recently, Uchman (1995) discussed this problem in detail, stressing the importance of a central tunnel enveloped by a zone of reworked sediment as a diagnostic feature (sensu Fu¨rsich, 1974) of the ichnogenus Nereites. He concluded that the type of preservation should not be regarded as an ichnotaxobase at the ichnogeneric level and therefore suggested that Scalarituba should best be considered junior synonym of Nereites. Neonereites should not be regarded as a separate ichnotaxon because it is the preservational expresion of several different ichnotaxa, including Nereites and probably Hormosiroidea Scha¨ffer, 1928 (D’Alessandro, 1980; Uchman, 1995). Additionally, Uchman (1995) regarded the course (meandering vs. nonmeandering), width of the central tunnel and the envelope zone, nature of the envelope (including shape of lobes), type of preservation, and selective preservation of morphological elements as accessory features (sensu Fu¨rsich, 1974) that could be considered for ichnospecific assignments. Conversely, Pickerill (1991) preferred to retain the three forms as separate ichnogenera on the basis that 1) preservation of the diagnostic features of Nereites, Neonereites, and Scalarituba in a single specimen is extremely rare; 2) not all ichnospecies of Neonereites can easily be accommodated within Nereites or Scalarituba; 3) reconstruction of the internal morphology of the trace is difficult; and 4) inclusion of Neonereites within Nereites or Scalarituba would result in ambiguities. Pickerill (1991) also compared this situation with the intergradation of one ichnotaxon into another,

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FIGURE 2—Stratigraphic section of the Stull Shale Member at Waverly, showing a detail of the sandstone-dominated heterolithic unit containing Nereites imbricata.

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as in the case of compound specimens of Thalassinoides, Ophiomorpha, and Gyrolithes, noting that these ichnogenera are nevertheless still retained as valid. However, both situations are not strictly equivalent because, while compound specimens reflect changes in behavior (Pickerill, 1994), in the case of Nereites-Neonereites-Scalarituba morphological variation is just an artifact of position within the substrate and taphonomic overprint rather than a significant change in behavior. The latter case is more akin to the Treptichnus-Plangtichnus or the Scolicia-Subphyllochorda problem, in which the two ichnogenera in each pair represent different toponomic expressions of the same fabricational case, and therefore have been considered synonymous (Buatois and Mańgano, 1993; Uchman, 1995). Toponomic and taphonomic aspects are essential to trace fossils yielding significant paleoecologic, sedimentologic, and stratigraphic information, but should not play a role in ichnotaxonomy. Crimes and McCall (1995) indicated that Nereites and Neonereites are quite distinct and are rarely preserved together; therefore, they should be considered as separate ichnogenera. However, several examples in which Nereites and Neonereites (or Scalarituba) occur at the same locality are known (e.g., Chamberlain, 1971; Chamberlain and Clark, 1973; Hakes, 1976; Vialov, 1979; Pickerill, 1981; Benton, 1982; Pickerill and Harland, 1988; Orr and Pickerill, 1995; Uchman, 1995 and references therein; Orr et al., 1996). In specimens from the Stull Shale Member, transition from Neonereites uniserialis-like to Nereites-like preservation (Fig. 3.1, 3.4) supports the notion that Neonereites is a preservational variant of Nereites. Herein we therefore adopt Uchman’s (1995) proposal on Nereites because it focuses on behavior rather than preservational circumstances and better reflects our understanding of its mode of construction. Several ichnogeneric names have been used to designate uniserial rows of subspherical sediment pads in Carboniferous shallow-water rocks, including Eione, Petromonile, and Margaritichnus. In most cases, however, a detailed evaluation of selective preservation of morphologic features is lacking, causing ichnotaxonomic confusion. The ichnogenus Eione was proposed by Tate (1859) for horizontal traces consisting of uniserial sediment pads from Carboniferous rocks of England, which he interpreted as fossil worms. Maples and Suttner (1990) subsequently used this name for apparently similar structures. However, as noted by Benton (1982) and reiterated by Rindsberg (1994), the name Eione is preoccupied by the gastropod genus Eione Rafinesque, 1814, its senior homonym. Thus, a new ichnogeneric name is required for these distinctive structures. Although the traces described in this paper resemble E. monoliformis Tate, 1859, in external morphology, some significant differences should be noted. First, sectioning of topotype material of E. monoliformis failed to reveal any internal structure (J. Pollard, personal commun., 1997; J. Buckman, personal commun., 1997), while the Waverly traces have a well-developed median tunnel flanked by obliquely arranged, arcuate laminae of reworked sediment. Second, the type specimens of E. monoliformis are invariably preserved as positive epireliefs, and the Stull Shale traces are preserved as positive hyporeliefs. Therefore,

specimens from the Carboniferous of Kansas clearly represent a different ichnotaxon. Casey (1961) introduced the ichnogenus Petromonile for horizontal traces resembling a string of beads that occur in the Lower Greensand of England. In so doing, he referred to specimens originally described by Bensted (1862) as sponge spicules of Siphonia benstedii. Rindsberg (1994) later used Petromonile for subhorizontal traces with imbricated sediment pads. The specimens described and illustrated by Bensted (1862, pl. 18) occur in Cretaceous strata of the valley of Medway in England. According to the original illustrations of Bensted (1862) and the subsequent description by Casey (1961), these uniserial pads form part of larger branched structures that actually resemble Thalassinoides-like crustacean galleries. Although the taxonomic status of Petromonile is pending revision of its type specimens, it certainly differs from the unbranched uniserial chains of sediment pads typically described from the Carboniferous. Bandel (1967, p. 6) proposed the name Cylindrichnus for balllike structures, ‘‘commonly aligned like a string of pearls.’’ He interpreted the ball-like structures as fecal pellets of a large wormlike animal. As this name was preoccupied by Cylindrichnus Toots (in Howard, 1966), it was subsequently replaced by Margaritichnus by Bandel (1973). Margaritichnus gained popularity and was later used by Hakes (1976), Narbonne (1984), Houck and Lockley (1986), Lockley et al. (1987), and Seilacher (1990). Hakes (1976) re-examined the type specimens of Margaritichnus reptilis and suggested that they were actually discrete structures formed by vertical movements of anemone-like organisms. Subsequently, Pemberton et al. (1988) reassigned Margaritichnus within the group of plug-shaped burrows. Our re-examination of the type specimens of Margaritichnus and the additional specimens collected by Hakes (1976) supports the conclusions of Hakes (1976) and Pemberton et al. (1988). The ball-like structures of Margaritichnus only occasionally coalesce in apparent chains of a few individuals of different size (e.g., Bandel, 1967, fig. 3.2; Hakes, 1976, fig. 8.1e). In contrast to the genetically related sediment pads described in this paper, aligned ball-like structures of Margaritichnus record penecontemporaneous or successive chance settlings of anemone-like animals, rather than an individual structure resulting from the feeding activity of a single tracemaker. Devera (1989) suggested that Eione, Scalarituba, Nereites and Neonereites may represent different preservational expressions of the same trace. However, specimens currently ascribed to Eione are preserved as positive epireliefs, and Neonereites is typically preserved in positive hyporelief. More important, Eione seems to lack any internal structure (J. Pollard, written commun., 1997; J. Buckman, written commun., 1997). Nereites represents combined locomotion and feeding activities, and therefore is considered a grazing trace (Pascichnion) (Seilacher, 1983, 1986; Orr, 1995). Nereites is most likely produced by a wormlike sediment-feeder, probably an enteropneust, that sorts out the coarse sediment with its protosoma and stows it in the lobes around the median tunnel. The food-rich finer particles are passed through the gut and stuffed into the median

→ FIGURE 3—Nereites imbricata from the Stull Shale Member at Waverly, 288535. 1, General view of sandstone sole with several specimens of N. imbricata, 30.28; 2, detailed view of the specimen illustrated on the lower left of 1. Note imbrication of subspherical sediment pads and typical Neonereites preservation, 31.13; 3, detailed view of the specimen illustrated on the lower right of 1. 31.23; 4, holotype. Detailed view of the specimen illustrated at the center of 1. Note transition between Nereites and Neonereites preservation. Internal structure of N. imbricata, showing the axial tunnel, surrounded by transverse to arcuate laminae. On the left, internal laminae tend to follow the outline of the external pads, 31.07; 5, heavily weathered specimen with median furrow showing the position of the tunnel and poorly preserved lateral zone of reworked sediment. 31.13. Shallower-tier, bilobate traces of Cruziana problematica are crosscut by Nereites imbricata in 1, 2, 4, and 5.

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FIGURE 4—Reconstruction of Nereites imbricata. Bottom surface view.

tunnel (Seilacher, 1986). According to this constructional model, Nereites is a complex radial- and backfilled structure that reflects a selective deposit-feeder strategy. Although the archetype of the deep-marine Nereites ichnofacies (Seilacher, 1967), this ichnogenus represents an eurybathic form. In the Paleozoic, Nereites is a common component of both shallow- and deep-marine deposits (e.g., Conkin and Conkin, 1968; Hakes, 1976; Seilacher, 1983; Fillion and Pickerill, 1990; Rindsberg, 1994; Orr, 1995; Orr and Pickerill 1995; Orr et al., 1996), meanwhile it is almost exclusively a deep-marine form in the Mesozoic and Cenozoic (McCann and Pickerill, 1988; Crimes and McCall, 1995; Uchman, 1995). Nereites ranges in age from the Late Precambrian-Early Cambrian (Acenõlaza and Durand, 1973; Crimes and Germs, 1982) to the Miocene (Uchman, 1995) or possibly Quaternary (Ekdale and Lewis, 1991). NEREITES

new ichnospecies Figures 3–4

IMBRICATA

Eione MANGANO, BUATOIS, MAPLES,

AND

WEST, 1996, p. 133

Diagnosis.—Straight, curved to slightly sinuous traces having a median tunnel enveloped by obliquely arranged, arcuate laminae, externally resulting in a uniserial row of imbricated and tightly packed, subspherical to annulate, large sediment pads. Description.—Predominantly horizontal, unbranched, curved to slightly sinuous traces composed of imbricated subspherical sediment pads arranged in uniserial rows (Fig. 3.1–3.4). Shape and length (measured parallel to the trace axis) of pads are highly variable among specimens and reflects the degree of packing of the reworked enveloping sediment. Pads are 12.8–17.2 mm wide and 3.6–14.9 mm in length. In some specimens, particularly where vertical undulation is involved, nested pads do not display the characteristic subspherical shape. These burrows commonly are relatively short (27.8–37.8 mm) and highly convex, and are formed by tightly packed pads, resulting in an annulated appearance. Length of the burrows is up to 119.6 mm. In a few specimens, the internal structure is clearly visible. In

bidimensional view, a thin median tunnel, 1.2–2.9 mm wide, is flanked by transverse to arcuate laminae (0.5–1.2 mm wide) (Fig. 3.4). In three-dimensional view, internal lamination seems to envelope the axial tunnel and tends to follow the outline of the external subspherical pads (Fig. 3.4). Laminae are not clearly arranged in distinctive lateral lobe-like structures. The median tunnel is filled with fine-grained sand similar to the host rock. No meniscate structure was detected in the median tunnel fill. In weathered specimens, a median depression represents the axial tunnel (Fig. 3.5). Preserved as positive semirelief or full relief structures on the base of symmetrical and asymmetrical rippled sandstones, 1.2–2.2 cm thick. Etymology.—Latin imbricatus, imbricated. Type.—Holotype, University of Kansas Museum of Invertebrate Paleontology, slab 288535. Other material examined.—Six slabs (288503, 288517, 288536, 288537, 288538) with 11 specimens exhibiting different preservational variants. Slabs are housed at the Museum of Invertebrate Paleontology of the University of Kansas. Occurrence.—Upper Pennsylvanian (Virgilian); Stull Shale Member of the Kanwaka Shale Formation, Kansas. Discussion.—The median tunnel is clearly observable in partially weathered bedding plane specimens with exposed internal structure (Fig. 3.4, 3.5). Interestingly, the tunnel is hardly visible in polished sections probably because of poor lithologic contrast and absence of linings. Although apparently structureless, the median tunnel probably holds thick packs of sandy sediment interfingered with thin clay laminae. The sandy appearence of the trace is probably the product of a geopetal effect rather than of real sediment homogeneity (cf. Seilacher, 1986, fig. 3.1). Following Goldring et al. (1997), the predominant toponomic expression of Nereites imbricata is Neonereites Seilacher, 1960. Nereites imbricata seems to represent a relatively shallow tier, crosscutting shallower structures, such as Cruziana problematica and Asteriacites lumbricalis. Reviews and descriptions of several ichnospecies of Nereites were presented by Benton (1982), Crimes and McCall (1995), and particularly Orr and Pickerill (1995) and Uchman (1995). However, the taxonomic status of many of its ichnospecies is still far from clear. Orr and Pickerill (1995) regarded relative range of size, ratio of the dimensions of the long and short axes of the lateral lobes, lobe density, and shape of lateral lobes as ichnotaxobases at the ichnospecific level. Although a detailed review of Nereites is beyond the scope of this paper, some taxonomic comments are pertinent here. There is general consensus that N. macleayii Murchison, 1839, N. cambrensis Murchison, 1839, N. jacksoni Emmons 1844, and N. pugnus Emmons 1844 are distinctive ichnospecies (Benton, 1982; Orr and Pickerill, 1995). Nereites (Scalarituba) missouriensis (Weller, 1899) and that Nereites (Helminthoida) irregularis (Schafhaütl, 1851) should be added to the list (Uchman, 1995). Uchman (1995) suggested that accessory preservational features could be used to differentiate among different Nereites ichnospecies. However, he proposed that ichnospecies formerly included under Neonereites (N. uniserialis, N. biserialis and N. multiserialis) would be better considered informally as ichnosubspecies or preservational variants of Nereites missouriensis. It seems more cautious, however, that in those cases in which internal structure is not observable, the ichnospecific assignment be uncertain. For example, specimens with external sculpture of Neonereites multiserialis should be assigned to Nereites isp., preservational variation Neonereites multiserialis. Nereites murotoensis Katto, 1960, and N. tosaensis Katto, 1960, probably represent Protovirgularia, most likely P. longespicata (De Stefani, 1885). Re-examination of the type specimens of N. saltensis Acenõlaza and Durand, 1973, suggests that

MANGANO ET AL.—NEW CARBONIFEROUS NEREITES ICHNOSPECIES FROM KANSAS this is most likely a valid ichnotaxon. Although internal structure is poorly preserved, Nereites saltensis displays the diagnostic features of Nereites, namely a median furrow and surrounded reworked sediment. Nereites saltensis is characterized by its irregular meandering course with angular kinks that reflect failures in the execution of the meandering program (Acenõlaza and Durand, 1973, fig. 2A; Seilacher, 1986, fig. 3.5b). As pointed out by Crimes and McCall (1995), the most important collection of Nereites is probably that of Delgado (1910), subsequently briefly reviewed by Perdigao (1961). Delgado (1910) proposed nine ichnospecies of Nereites, namely N. barroisi, N. marcoui, N. roemeri, N. choffatti, N. liebei, N. cabrali, N. barrandei, N. lorioili, and N. castroi. Although no definite taxonomic evaluation can be made without direct re-study of this collection, some tentative comments are offered. Nereites barroisi, N. marcoui, N. roemeri, N. liebei, N. barrandei, N. lorioili, and N. castroi seem to be indistinguishable from N. cambrensis. Nereites cabrali is probably a junior synonym of N. macleayii. Finally, lobes in N. choffatti are very similar to those in N. macleayii, but the illustrated specimen has a spiral geometry, which contrasts with the typical meandering course of N. macleayii (Orr and Pickerill, 1995). Nereites delpeyi Bourrouilh, 1973, was originally described only in an unpublished thesis and still awaits formal definition (Orr et al., 1996). Orr (1994) and Orr et al. (1996) provide description of topotype specimens of this ichnospecies. The ovate to lanceolate lobes of Nereites fengxianensis Cui, Yu, Mei and Meng, 1996 suggests that this ichnotaxon is probably a junior synonym of N. cambrensis. Nereites jacki was proposed by Pek et al. (1978). Examination of their figured material (Pek et al., 1978, Fig. 1.3) and brief description indicates that these specimens can be included in Nereites, but do not warrant a new ichnospecies. Nereites jacki is best regarded as a nomen dubium. Nereites imbricata differs from the other ichnospecies of Nereites by the characteristic external morphology of conspicuous uniserial imbricate sediment pads (resembling Eione) and the poorly specialized, nonmeandering course. Nereites imbricata winds in horizontal planes and undulates in vertical planes, producing tightly packed overlapping pads that commonly result in an annulated appearance. Internally, Nereites imbricata departs from the other Nereites ichnospecies in the presence of rather overlapped, obliquely arranged lamination that envelope the axial tunnel. Additionally, the enveloping-sediment-width: axialtunnel-width ratio is significantly larger than in the other Nereites ichnospecies, having conspicuous sediment pads. Meanwhile the architectural elements of Nereites imbricata suggest the proposed general mode of construction of Nereites previously outlined, details in the internal structure may indicate some departures from the archetypal constructional model (cf. Chamberlain, 1971; Seilacher, 1986). Chamberlain (1971, p. 228–229, fig.5A–I) proposed two possible modes of development of Nereites ( 5 Scalarituba and Neonereites). Both modes involve essentially the alternate movement of the producer from one side to the other of the median tunnel producing pockets (‘‘lobes’’) of manipulated sediment. Selected grains are ingested taking an internal route to the back of the animal, and packed as fecal material in the median tunnel. In most ichnospecies of Nereites, particularly in epichnial preservation (see, for example, N. cambrensis, N. macleayii, N. jacksoni, and N. pugnus in Orr and Pickerill, 1995), lateral, slightly offset lobes give evidence of this general mode of construction. The internal structure of N. imbricata shows continuous, obliquely arranged, arcuate enveloping laminae surrounding the median tunnel. This suggests that, instead of alternate bilateral or multilateral probings, the producer of N. imbricata deviated slightly from the median tun-

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nel and made a downward and forward probing followed by essentially concentric exploration of the sediment. Accordingly, each individual subspherical pad resembles a multilayered (‘‘onion’’) structure. The concentric structure of the enveloping sediment may be compared with type 5 of obligatory concentric lamination of Goldring (1996), and the spiralling of a narrow burrow proposed for the construction of Asterosoma by Chamberlain (1971). ACKNOWLEDGEMENTS

MGM and LAB would like to thank the Argentinian Research Council (CONICET) for financial support and the Kansas Geological Survey for technical and logistical facilities. MGM also thanks Sigma Delta Epsilon, The Paleontological Society, and the Mid-America Paleontological Society for providing financial support for her research on tidal flat ichnofaunas. Earlier drafts of this manuscript were critically read by J. Buckman, J. Pollard, and A. Rindsberg, who supplied valuable comments. J. Pollard and J. Buckman also provided unpublished information on topotype material of Eione. A. Uchman and R. Pickerill are particularly thanked for their thoroughful reviews. We also thank L. Brozius for editing the manuscript, J. Charlton for photographic work, M. Schoneweis and D. Ruiz Holgado for the drawings, and the late A. Kamb and A. Hart for loaning specimens housed in the Museum of Invertebrate Paleontology of the University of Kansas. REFERENCES

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