New multituberculate mammal from the Early ...

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New multituberculate mammal from the Early Cretaceous of eastern North America1

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Richard L. Cifelli, Cynthia L. Gordon, and Thomas R. Lipka

Abstract: Multituberculates, though among the most commonly encountered mammalian fossils of the Mesozoic, are poorly known from the North American Early Cretaceous, with only one taxon named to date. Herein we describe Argillomys marylandensis, gen. et sp. nov., from the Early Cretaceous of Maryland, based on an isolated M2. Argillomys represents the second mammal known from the Arundel Clay facies of the Patuxent Formation (Lower Cretaceous: Aptian). Though distinctive in its combination of characters (e.g., enamel ornamentation consisting of ribs and grooves only, cusp formula 2:4, presence of distinct cusp on anterobuccal ridge, enlargement of second cusp on buccal row, central position of ultimate cusp in lingual row, great relative length), the broader affinities of Argillomys cannot be established because of non-representation of the antemolar dentition. Based on lack of apomorphies commonly seen among Cimolodonta (e.g., three or more cusps present in buccal row, fusion of cusps in lingual row, cusps strongly pyramidal and separated by narrow grooves), we provisionally regard Argillomys as a multituberculate of “plagiaulacidan” grade. Intriguingly, it is comparable in certain respects to some unnamed Paulchoffatiidae, a family otherwise known from the Late Jurassic – Early Cretaceous of the Iberian Peninsula. Résumé : Bien que figurant parmi les fossiles de mammifères du Mésozoïque les plus couramment observés, les multituberculés ne sont pas bien documentés dans les sédiments du Crétacé précoce de l’Amérique du Nord, un seul taxon ayant été nommé a` ce jour. Nous décrivons Argillomys marylandensis, gen. et sp. nov., du Crétacé précoce du Maryland, a` la lumière d’une M2 isolée. Argillomys est le deuxième mammifère connu du faciès a` argiles d’Arundel de la Formation de Patuxent (Crétacé inférieur : Aptien). Bien qu’Argillomys se distingue par sa combinaison de caractères (p. ex. ornementation de l’émail composée uniquement de côtes et de rainures, formule 2:4 des cuspides, présence d’une cuspide distincte sur la crête antérobuccale, deuxième cuspide plus grande sur la rangée buccale, position centrale de la dernière cuspide dans la rangée linguale, grande longueur relative), ses affinités plus élargies ne peuvent être établies en raison de la non-représentation de la dentition antémolaire. Étant donné l’absence d’apomorphies couramment observées chez les Cimolodonta (p. ex. trois cuspides ou plus dans la rangée buccale, fusion de cuspides dans la rangée linguale, cuspides fortement pyramidales et séparées par d’étroites rainures), nous considérons provisoirement Argillomys comme étant un multituberculé de degré « plagiaulacidé ». Fait intéressant, il est semblable, a` certains égards, a` certains Paulchoffatiidés, une famille par ailleurs connue du Jurassique tardif – Crétacé précoce de la péninsule ibérique. [Traduit par la Rédaction]

Introduction Multituberculates are incomparably the best known of Mesozoic mammals, yet major gaps remain in their fossil record. The most notorious of these gaps spans the Early Cretaceous of North America. A moderately diverse assemblage (nine species, grouped into four genera and three or more families) of taxa, mostly represented by reasonably informative specimens (such as dentulous jaw fragments), is known from the Upper Jurassic Morrison Formation (Simpson 1927, 1929; Engelmann and Callison 1998). Beginning at the Albian– Cenomanian (Early–Late Cretaceous) boundary, multituberculate faunas are now known from a number of time intervals through the ensuing Late Cretaceous, though most of the

known fossils consist of isolated teeth, and only a few preCampanian assemblages are reasonably well sampled (e.g., Fox 1971; Clemens and Kielan-Jaworowska 1979; Clemens et al. 1979; Eaton 1995; Eaton and Cifelli 2001). Such qualifications seem insignificant, however, by comparison to the record of Early Cretaceous multituberculates from North America. Small samples of isolated multituberculate teeth from the Trinity Group (Oklahoma and Texas) and Cloverly Formation (Wyoming and Montana), both Aptian–Albian in age, are under study (C.L.G. and R.L.C., unpublished data). To date, however, the published record for North American multituberculates from the approximately 44-million-year duration of the Early Cretaceous (time scale of Palmer and Geissman 1999) consists of a single described tooth (Cifelli 1997) and

Received 30 March 2012. Accepted 14 June 2012. Published at www.nrcresearchpress.com/cjes on 18 April 2013. Paper handled by Associate Editor Craig S. Scott. R.L. Cifelli and T.R. Lipka. Sam Noble Museum, 2401 Chautauqua Ave. Norman, OK 73072, USA. C.L. Gordon. Department of Biology, University of Oklahoma, Norman, OK 73019, USA. Corresponding author: Richard L. Cifelli (e-mail: [email protected]). 1

This article is one of a selection of papers published in this Special Issue in honour of Richard C. Fox.

Can. J. Earth Sci. 50: 315–323 (2013)

doi:10.1139/e2012-051

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brief mention elsewhere (Patterson 1956; Gordon et al. 2000; Cifelli and Gordon 2005; Carrano and Oreska 2011). Unsurprisingly, significant problems in multituberculate systematics can be traced, at least in part, to this glaring deficiency in the fossil record. Archaic taxa (Jurassic, North America and Europe, but ranging into the Albian–Cenomanian of Utah; see Eaton and Cifelli 2001) are generally relegated to the paraphyletic suborder “Plagiaulacida” (see Kielan-Jaworowska and Hurum 2001; Kielan-Jaworowska et al. 2004). A morphological hiatus separates “plagiaulacidans” from most of the typical and far better known Late Cretaceous – Paleogene multituberculates, generally considered to represent a monophyletic group, Cimolodonta (e.g., Stucky and McKenna 1993; Simmons 1993; Kielan-Jaworowska and Hurum 2001; Weil and Krause 2008; but see Fox 2005 for contrasting view). Affinities among major clades of multituberculates, not to mention the integrity of Cimolodonta and their relationship(s) to the various sorts of “plagiaulacidans”, remain highly fluid (Simmons 1993; Rougier et al. 1997; Kielan-Jaworowska and Hurum 2001; Kielan-Jaworowska et al. 2004). Herein we describe a new multituberculate from the Early Cretaceous of the eastern United States. The importance of this taxon currently lies mainly in its geographic and temporal occurrence: it is represented by a single tooth that, by itself, provides scant information bearing on multituberculate radiations of the Early Cretaceous. The specimen is from the Arundel Clay facies of the Patuxent Formation (Potomac Group), Prince George’s County, Maryland. Biochronology based on palynomorphs suggests that this unit is Aptian, (Doyle 1992), probably late Aptian, in age (Stanford et al. 2007, 2011). Terrestrial vertebrates, mostly dinosaurs and mostly represented by isolated bones and (or) teeth, have long been known from the Arundel Clay (Marsh 1888; Gilmore 1921). Early discoveries occurred within the context of commercial mining operations for sedimentary iron ore (e.g., Kranz 1996). A sustained field program by one of us (T.R.L.) resulted in the recovery of many new fossils from a single site in the Arundel Clay. The site, formerly mined commercially for clay, is now known as Dinosaur Park, managed by the Prince George’s County Department of Parks and Recreation. Previously described fossils from the site, USNM locality 41614, were obtained by surface prospecting (Lipka 1998; Cifelli et al. 1999; Rose et al. 2001; Lipka et al. 2006) and include those of the triconodontid mammal Arundelconodon hottoni Cifelli et al., 1999. The specimen described herein is the single mammalian fossil obtained through screenwashing and associated concentration techniques, aimed at recovery of vertebrate microfossils. The fossil-yielding zone at the site is reasonably productive insofar as fragmentary bones and teeth of large vertebrates are concerned. Informative microfossils are abundant at USNM locality 41614 (T.R.L. and R.L.C., work in progress). However, processing of washed concentrate is impeded considerably by the fact that the rock also happens to be a prolific source of concretions, carbonized wood, and other unwanted residues. Rock matrix was reduced through a two-phase screenwashing process (standard water wash followed by kerosene displacement method); further concentrated using the heavy liquid ZnBr2; and picked under a binocular dissecting microscope, using established procedures (see Cifelli et al. 1996 and references therein). Field data for USNM locality 41614 are on file at the Department of Pale-

Can. J. Earth Sci., Vol. 50, 2013 Fig. 1. Argillomys marylandensis, left M2 (USNM 497739, holotype); occlusal stereopair.

obiology, United States National Museum (currently National Museum of Natural History). Taxonomic usage and dental terminology follow Eaton (1995), Kielan-Jaworowska and Hurum (2001), and Eaton and Cifelli (2001). The M2 of multituberculates includes two primary cusp rows; we follow Hahn and Hahn (2004) in referring to these as “buccal” and “lingual” (when cusp formula is referred to, the cusp numbers are given in that order, separated by a colon). The anterobuccal part of M2 is variably developed in multituberculates; among taxa considered herein, a small shelf and ridge (here termed “anterobuccal ridge”) are generally present, though an entire, third row of cusps may be present in advanced forms (note Clemens and Kielan-Jaworowska 1979). Measurement abbreviations: AP, anteroposterior length; LB, linguobuccal width. Institutional abbreviations: DINO, Dinosaur National Monument, Jensen, Utah, USA; IPFUB, Institut für Paläontologie der Freie Universität Berlin, Germany; OMNH, Oklahoma Museum of Natural History, Norman, Oklahoma, USA; USNM, United States National Museum (currently National Museum of Natural History), Smithsonian Institution, Washington, DC, USA.

Systematic paleontology Class Mammalia Linnaeus, 1758 Order Multituberculata Cope, 1884 ?Suborder “Plagiaulacida” Ameghino, 1889 Family uncertain Argillomys marylandensis, gen. et sp. nov. (Fig. 1) ZOOBANK LSID.: urn:lsid:zoobank.org:act:63737680-0649-4300B119-7D026AE3E0F0 INCLUDED SPECIES:

Monospecific.

DIAGNOSIS: M2 with enamel ornamentation, as in Plagiaulacidae, Eobaataridae, and some members of the “Paracimexomys group” (but not Paracimexomys Archibald, 1982 sensu stricto), differing from most of those taxa in lacking pitting and being restricted to ribs and grooves. Distinct from most comparable taxa in presence of a distinct cusp on anterobuccal ridge and four cusps in lingual row. Resemble most Plagiaulacidae and Eobaataridae, but not most Cimolodonta, in the following: cusps of lingual row well individualized (not forming a ridge); cusp bases well separated, with valley floors clearly

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visible (not developed as narrow, vertically walled grooves); anterior crown margin sigmoidal (not straight). Resembles “Paracimexomys group” and most other Cimolodonta, differing from Eobaataridae and Plagiaulacidae, in separation of anterior rim from first cusp of buccal row. Differs from most comparable taxa in relative enlargement of the second (rather than first) cusp of the buccal row, in great length relative to width (except Ameribaatar Eaton and Cifelli, 2001) and central placement of last cusp in lingual row (except Monobaatar Kielan-Jaworowska et al., 1987, Arginbaatar Kielan-Jaworowska et al., 1987, and Paulchoffatiidae).

COMMENT: Because knowledge of Argillomys marylandensis is restricted to a single M2, intraspecific variation cannot be assessed, and affinities of the taxon are largely speculative. Whatever its relationships, however, the combination of characteristics seen in USNM 497739 is clearly distinctive. We refer this specimen to a new genus and species to reflect this distinctness, to promote comprehensiveness and clarity in future comparisons, and to reflect known diversity in a poorly represented part of the fossil record.

USNM 497739, left M2 (AP ⫽ 2.63, LB ⫽ 1.87, following Eaton 1995, fig. 3), lightly worn, and complete except for the loss of the roots and minor bits of enamel at the crown base, along the lingual margin of the lingual cusp row and the anterobuccal rim of the anterobuccal ridge.

Taxa We compare Argillomys with a variety of Multituberculata, including various “Plagiaulacida”, members of the “Paracimexomys group” (which are generally conceived as basal Cimolodonta; Eaton and Cifelli 2001; Kielan-Jaworowska and Hurum 2001; Kielan-Jaworowska et al. 2004), and a few other cimolodontans from the Late Cretaceous of North America. Sampling is not intended to be comprehensive: it is limited to those taxa that are represented by M2 and morphologically comparable to Argillomys marylandensis or that are useful for evaluating the significance of character distributions. Eobaataridae and the “Paracimexomys group” are sampled as comprehensively as possible because of their suspected close relationship to the origin of Cimolodonta (e.g., Simmons 1993; Eaton 1995; Rougier et al. 1997; Kielan-Jaworowska and Hurum 2001). Given the great uncertainties surrounding relationships of many early multituberculates, taxa are simply listed alphabetically by genus (or informal name given in the text that follows). Taxa, pertinent literature, and published figures are as follows (literature citations and published illustrations are given in brackets to prevent confusion with taxon authorities, which are given in parentheses when generic referral has changed since original publication of a species name): Ameribaatar Eaton and Cifelli, 2001 (family incertae sedis): A. zofiae Eaton and Cifelli, 2001 [Eaton and Cifelli 2001, figs. 19e, 19f]. Arginbaatar Kielan-Jaworowska et al., 1987 (Arginbaataridae): A. dimitrievae Kielan-Jaworowska et al., 1987 [Kielan-Jaworowska et al. 1987, fig. 3d, pl. 7, figs. 1, 2 (questionably referred to the species)]. Bernardodon Hahn and Hahn, 1999 (Pinheirodontidae): B. atlanticus Hahn and Hahn, 1999 [Hahn and Hahn 1999, fig. 2c]. Bolodon Owen, 1871 (Plagiaulacidae): B. osborni Simpson, 1928 [Kielan-Jaworowska et al. 1987, fig. 3a; Kielan-Jaworowska and Ensom 1992, pl. 3, figs. 8, 9]. Bryceomys Eaton, 1995 (“Paracimexomys group”: family incertae sedis): B. fumosus Eaton, 1995 [Eaton 1995, fig. 9h], B. hadrosus Eaton, 1995 [Eaton 1995, fig. 9m], B. intermedius Eaton and Cifelli, 2001 [Eaton and Cifelli 2001, fig. 10; Fig. 2a herein]. Cedaromys Eaton and Cifelli, 2001 (“Paracimexomys group”: family incertae sedis): C. bestia (Eaton and Nelson, 1991) [Eaton and Cifelli 2001, fig. 13], C. parvus Eaton and Cifelli, 2001 [Eaton and Cifelli 2001, fig. 16]. Cimexomys Sloan and Van Valen, 1965 (Cimolodonta, family incertae sedis): C. judithae Sahni, 1972 [Montellano et al. 2000, fig. 1]. Cimolodon Marsh, 1889 (Cimolodontidae): C. electus Fox 1971 [Fox 1971, fig. 5a]. Cimolomys Marsh, 1889 (Cimolomyidae): C. gracilis Marsh, 1889 [Clemens 1963, fig. 38; Archibald 1982, fig. 23]. Ctenacodon Marsh, 1879 (Allodontidae): C. laticeps (Marsh, 1881) [Kielan-Jaworowska and Hurum 2001, pl. 1, fig. 4], Ctenacodon sp. (Engelmann and Callison, 1998, fig. 11). Dakotamys Eaton, 1995 (“Paraci-

HOLOTYPE:

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ETYMOLOGY: The genus is named for the argillaceous (Gr.) composition of the Arundel Clay facies ⫹ mys (mus, L.), a commonly used suffix for multituberculate generic nomina. The species is named for the State of Maryland, where the holotype and only known specimen was collected. LOCALITY: USNM locality 41614 (39°04=14⬙N 76°52=08⬙W), former clay pit (now site of Dinosaur Park) near Muirkirk, Prince George’s County, Maryland; Arundel Clay facies of the Patuxent Formation (Aptian, Lower Cretaceous).

TYPE

DESCRIPTION:

USNM 497739 (Fig. 1) is long relative to width (LB:AP ⫽ approximately 0.71), even when allowance is made for the loss of some enamel along its lingual margin. The anterolingual part of the crown projects strongly anteriorly. As a result, the anterior margin of the tooth is distinctly sigmoidal. Cusp formula is 2:4. The anterior cusp is the smallest in the lingual row. A distinct, rounded cingulum ascends anteriorly from this first cusp, then turns abruptly posterobuccally, forming a tight fold and lingually directed valley. The cingulum then turns gently anterobuccally, then more sharply posterobuccally, terminating at a small but distinct cusp on the anterobuccal ridge. The anterobuccal portion of the cingulum is cuspidate. All cusp bases are well separated, with well-defined, flatbottomed valleys (rather than grooves) between them. The four major cusps of the tooth (lingual row cusps 2 and 3; buccal row cusps 1 and 2) are transversely and anteroposteriorly aligned between and within rows, respectively; the second cusp of the buccal row is noticeably the largest. All are subconical, with cusps 2 and 3 of the lingual row being slightly compressed labiolingually. Ribs ascend from near the apex of each cusp; on the anterobuccal face of cusp 1 in the buccal row, where they are best expressed, ribs terminate at respective cuspules on the cingulum. In some places (e.g., lingual and anterobuccal slopes of buccal row cusp 2), the ribs appear to form cusp edges, but the cusps do not have a pyramidal or crescentic form (see definitions in Eaton and Cifelli 2001). The last cusp of the lingual row is somewhat compressed anteroposteriorly and is buccally displaced with respect to the three anteroposteriorly aligned cusps that precede it. The central valley is thus closed posteriorly: as it reaches the last cusp of the lingual row, the central valley forks, forming an inverted Y-shaped pattern.

Comparisons

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mexomys group”: family incertae sedis): D. malcolmi Eaton, 1995 [Eaton 1995, fig. 7m]. Eobaatar Kielan-Jaworowska et al., 1987 (Eobaataridae): E. magnus Kielan-Jaworowska et al., 1987 [Kielan-Jaworowska et al. 1987, fig. 3b, pl. 7, fig. 4; Kielan-Jaworowska and Hurum 2001, pl. 3, fig. 2]. Glirodon Engelmann and Callison, 1999 (family incertae sedis): G. grandis Engelmann and Callison, 1999 [Engelmann and Callison 1999, figs. 20, 21; Kielan-Jaworowska and Hurum 2001, pl. 1, fig. 3; Fig. 2b herein]. Iberica Badiola et al., 2011 (Plagiaulacidae or Eobaataridae): I. hahni Badiola et al., 2011 [Badiola et al. 2011, fig. 3; M2 tentatively referred to the species]. Iberodon Hahn and Hahn, 1999 (Pinheirodontidae): I. quadrituberculatus Hahn and Hahn, 1999 [Hahn and Hahn 1999, fig. 2d]. Loxaulax Simpson, 1928 (Eobaataridae): L. valdensis (Woodward, 1911) [Clemens and Lees 1971, pl. 1c, pl. 4c]. Monobaatar Kielan-Jaworowska et al., 1987 (?Eobaataridae): M. mimicus Kielan-Jaworowska et al., 1987 [Kielan-Jaworowska et al. 1987, fig. 3c, pl. 7, fig. 3 (M2 questionably referred to the species)]. Paracimexomys Archibald, 1982 (“Paracimexomys group”: family incertae sedis): here we follow a narrow definition of the genus, as given by Eaton and Cifelli (2001). Comparisons are based on P. priscus (Lillegraven, 1969) [Archibald 1982, fig. 39b]; cf. Paracimexomys (following usage of Eaton and Cifelli 2001): cf. Paracimexomys cf. robisoni [Eaton 1995, fig. 6f], cf. Paracimexomys cf. perplexus [Eaton and Cifelli 2001, fig. 8]. Parendotherium vel Eobaatar [Eobaataridae, Hahn and Hahn 1992, fig. 10]. Pinheirodon Hahn and Hahn, 1999 (Pinheirodontidae): P. vastus Hahn and Hahn, 1999 [Hahn and Hahn, 1999, fig. 2b]. Paulchoffatiidae. Of the four morphs that have been most thoroughly described and illustrated, we included three for which illustrations of complete teeth were available: “genus A”, “genus B”, and “genus D species 1” of Hahn and Hahn (1998, figs. 19, 20, 21, 23; Fig. 2c herein). Sinobaatar Hu and Wang, 2002 (Eobaataridae): S. fuxinensis Kusuhashi et al., 2009 [Kusuhashi et al. 2009, figs. 12, 14], S. xiei Kusuhashi et al., 2009 [Kusuhashi et al. 2009, figs. 3, 4]. Polymorphisms indicated in the text that follows represent variations in the current sample and are not intended as expressions of the entire range of variation for given taxa. Characters We focused our attention on 12 characteristics, mostly taken from literature, that we were able to observe on M2 of Argillomys marylandensis. As will become clear in the discussion later in the text, ancestral states are sometimes unclear. Many characters are multi-state, and various polymorphisms occur. For these reasons and for clarity in listing the taxa and conditions observed, we have assigned numerical codes to the various states. States are given individually for the separate species listed in the preceding text; where state is given for a genus, reference is intended only for the particular species included in the sample. In addition to Argillomys (Fig. 1), we illustrate a Paracimexomys-group taxon (Bryceomys intermedius, Fig. 2a) and two “plagiaulacidans” (Glirodon grandis and an unnamed paulchoffatiid; Figs. 2b and 2c, respectively), to facilitate comparison of various character states. Enamel ornamentation (1) Kielan-Jaworowska and Hurum (2001) included this character in analyses of relationships within Multituberculata.

Can. J. Earth Sci., Vol. 50, 2013 Fig. 2. A stem cimolodontan (a) and two “plagiaulacidans” (b, c), illustrating some of the character states discussed in the text. (a) Bryceomys intermedius, left M2 (OMNH 26679), Cedar Mountain Formation (Albian–Cenomanian), Utah. (b) Glirodon grandis, right M2 (DINO 10822), Morrison Formation (Kimmeridgian), Utah. (c) Paulchoffatiidae, genus B, left M2 (IPFUB B (m2)-1), Guimarota beds (Kimmeridgian), Portugal (from Hahn and Hahn 1998).

They recognized two states: smooth or ornamented enamel. They indicated that the latter condition is often associated with cusp fusion, regarded herein as a separate character (4, in the text that follows). Ornamentation on M2 is rather variable in nature and extent. Two forms (listed here as separate states, though the distinction is not always clear) may be recognized: ribbing (often with grooves) that descends from cusp apices, and pitting, which is most common around cusp bases and, when present, is associated with ribbing. Polarity for this character is problematic. Kielan-Jaworowska and Ensom (1992) included enamel ornamentation in their diagnosis of Plagiaulacidae. Kielan-Jaworowska and Hurum (2001) posited incipient ornamentation as a synapomorphy of all Multituberculata except the “allodontid line” (Allodontidae, Zofiabaataridae, Glirodon) of “Plagiaulacida”, with greater expression of pits and grooves being a more advanced condition uniting the “plagiaulacid line” (Albionbaataridae, Eobaataridae, Plagiaulacidae) of “Plagiaulacida” and Cimolodonta. This is a reasonable hypothesis, based on available data, though the conditions in certain taxa are perplexing and suggest homoplasy or reversal. For example, strong ornamentation (both ribbing and pitting) is characteristic of most “Paracimexomys group” taxa but is absent in Paracimexomys priscus (Eaton and Cifelli 2001). Ornamentation absent (0; Fig. 2b): Ameribaatar, Arginbaatar, Cimexomys, Ctenacodon, Glirodon, Paracimexomys, Sinobaatar. Published by NRC Research Press

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Ornamentation consisting of ribbing only (1; Figs. 1, 2c): Iberica, Iberodon, Monobaatar, cf. Paracimexomys, Parendotherium vel Eobaatar, Paulchoffatiidae genus A, Paulchoffatiidae genus B, Paulchoffatiidae genus D species 1, Argillomys, Pinheirodon. Ornamentation including both ribbing and pitting (2; Fig. 2a): Bernardodon, Bryceomys, Cedaromys, Cimolodon, Dakotamys, Eobaatar, Loxaulax. Polymorphic (1/2): Bolodon, Cimolomys. Cusp count: buccal row (2) Cusp formula is commonly treated as a single character (e.g., Simmons 1993; Hahn and Hahn 2004); we find it less confusing to consider the counts for the buccal and lingual rows separately (most taxa included in the sample bear a non-cuspidate anterobuccal ridge on M2; presence of cusps on this ridge is treated in character 8). Comparisons among the various groups of “Plagiaulacida” suggest that a count of two cusps on the buccal row is probably primitive for Multituberculata. A third cusp is present in some taxa included in the sample; this is developed as a swelling of the anterior cingulum and hence appears to be non-homologous with buccal cusp 1 of taxa having only two cusps. Two cusps (0; Figs. 1, 2b, 2c): Argillomys, Arginbaatar, Bernardodon, Bolodon, Ctenacodon laticeps, Ctenacodon sp, Glirodon, Iberodon, Loxaulax, Monobaatar, cf. Paracimexomys cf. perplexus, Parendotherium vel Eobaatar, Paulchoffatiidae genus B, Pinheirodon. Three cusps (1; Fig. 2a): Ameribaatar, Bryceomys hadrosus, Cedaromys bestia, Cedaromys parvus, Cimexomys, Cimolodon, Dakotamys, Eobaatar, Iberica?, Paracimexomys priscus, cf. Paracimexomys cf. robisoni, Paulchoffatiidae genus D, Sinobaatar fuxinensis, Sinobaatar xiei. Polymorphic (0/1): Bryceomys fumosus, Bryceomys intermedius, Paulchoffatiidae genus A. Three to four cusps (2): Cimolomys. Cusp count: lingual row (3) Widespread presence of three cusps among “Plagiaulacida” and basal Cimolodonta suggests that this is the primitive count for cusps of the lingual row among Multituberculata. Cimolodontans (particularly taxa not considered herein) commonly have a higher number of cusps, and polymorphism is common; our coding represents an arbitrary convenience for distinguishing among taxa included in the sample. Three cusps (0; Fig. 2b): Ameribaatar, Arginbaatar, Bernardodon, Bolodon, Bryceomys fumosus, Bryceomys hadrosus, Cimexomys, Ctenacodon laticeps, Ctenacodon sp., Eobaatar, Glirodon, Iberica, Monobaatar, Paracimexomys priscus?, cf. Paracimexomys cf. perplexus, Pinheirodon, Sinobaatar fuxinensis. Four cusps (1; Figs. 1, 2a, 2c): Argillomys, Cedaromys bestia, Cedaromys parvus, Dakotamys malcolmi?, Loxaulax, cf. Paracimexomys cf. robisoni, Parendotherium vel Eobaatar, Paulchoffatiidae genus B, Paulchoffatiidae genus D, Sinobaatar xiei. Polymorphic (0/1): Bryceomys intermedius, Cimolomys. Five or more cusps (2): Cimolodon. Polymorphic (1/2): Paulchoffatiidae genus A. Two cusps (3): Iberodon.

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Cusp separation (cusp fusion or coalescence of Kielan-Jaworowska and Hurum 2001) (4) A tendency for cusps of a given row to be fused at their bases (presumed to be an apomorphic condition) is best expressed on lower molars of multituberculates (e.g., Kielan-Jaworowska et al. 1987). On M2, it is recognized mainly in the lingual cusp row. The distinction between separated and fused cusps is not always clear, and it may be worthwhile developing quantitative means of distinguishing the states. Worn teeth are especially difficult to categorize. Fusion of cusps is related to two other characters listed later in the text (cusp shape, character 4; valleys and grooves, character 5). They are treated separately herein, but a close re-examination and, perhaps, synonymy would be called for if the data are to be subjected to algorithmbased cladistic analysis. Cusps of lingual row well individualized (0; Figs. 1, 2b, 2c): Ameribaatar, Argillomys, Arginbaatar, Bernardodon, Ctenacodon, Eobaatar, Glirodon, Iberodon, Monobaatar, ?Parendotherium vel Eobaatar, Paulchoffatiidae genus A, Paulchoffatiidae genus B, Paulchoffatiidae genus D species 1, Sinobaatar. Cusps of lingual row connected by their bases, so that the lingual row forms a cuspidate ridge (1; Fig. 2a): Bolodon, Bryceomys, Cedaromys, ?Cimexomys, Cimolodon, Cimolomys, Dakotamys, Iberica, Loxaulax, Paracimexomys, cf. Paracimexomys. Cusp shape (5) Most “plagiaulacidans” have conical to weakly pyramidal cusps, assumed to represent the primitive condition. Some taxa among the sample have strongly pyramidal or crescentic cusps, as reflected in the coding that follows. Conical to weakly pyramidal cusps (0; Figs. 1, 2b, 2c): Argillomys, Arginbaatar, Bernardodon, ?Bolodon, Ctenacodon, Eobaatar, Glirodon, Iberica, Iberodon, ?Loxaulax, Monobaatar, Parendotherium vel Eobaatar, Paulchoffatiidae genus A, Paulchoffatiidae genus B, Paulchoffatiidae genus D species 1. Strongly pyramidal cusps (1; Fig. 2a): Ameribaatar, Bryceomys, Cedaromys, Cimexomys, Cimolodon, Cimolomys. Polymorphic (0/1): cf. Paracimexomys, Sinobaatar. One or more cusps crescentic (2): Cimexomys, Cimolodon, Cimolomys. Valleys between cusps (6) Most “plagiaulacidans” have well-separated cusps, so that intervening valleys have floors that are clearly visible. In some taxa in the sample, the valleys are developed as narrow grooves between cusps. This state is associated with development of strongly pyramidal cusps and, perhaps, with cusp coalescence. The most distinctive of the grooves lie between cusps of the lingual row. The presumed primitive condition, with well-separated conical cusps, is associated with open valley floors. Cusp coalescence (and generally pyramidal cusp shape) results in the advanced state for this character: narrow grooves. Further coalescence, as in Paracimexomys priscus, results in a ridge lacking discernible grooves (note Archibald 1982, p. 114). For comparative purposes, this is defined as a separate state herein. It appears that certain members of the “Paracimexomys group” are uniquely derived with respect to this character. The three pinheirodontids included in the sample could not readily be categorized in terms of the foregoing character states: in these taxa, cusps of the lingual row appear to be Published by NRC Research Press

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united by a thin, longitudinal crest. We tentatively recognize this unique feature as a distinct state. Valleys with visible floors separate cusps (0; Figs. 1, 2b, 2c): Ameribaatar, Argillomys, Arginbaatar, Ctenacodon, Eobaatar, Glirodon, Loxaulax, Monobaatar, Paulchoffatiidae genus A, Paulchoffatiidae genus B, Paulchoffatiidae genus D species 1. Valleys developed as narrow grooves (1; Fig. 2a): Bolodon, Bryceomys, Cedaromys, Cimexomys, Cimolodon, Cimolomys, Loxaulax, Parendotherium vel Eobaatar, Sinobaatar. Polymorphic (1/2): cf. Paracimexomys. Valleys and grooves weak, irregular, or lacking; cusps of lingual row developed as variable apices on a ridge (2): Dakotamys, Iberica, Paracimexomys. Cusps of lingual row subconical in shape, but transverse grooves between cusps apparently interrupted by thin longitudinal crest (?3): Bernardodon, Iberodon, Pinheirodon. Proportions (7) Most multituberculates in the sample have a sub-square M2, with tooth length only slightly exceeding width. This is presumed to be the primitive condition. In a few cases (as with Argillomys), M2 is noticeably longer than wide. An arbitrary numerical distinction between the two conditions is made for present purposes (general comparison). Careful scrutiny of value distribution would be needed to develop a defensible coding system for the purposes of phylogenetic analysis. M2 sub-square, LB:AP ⱖ0.80 (0; Figs. 2a, 2b): Bolodon (0.96), Bryceomys (0.84), Cedaromys (0.84), Cimexomys (0.94), Cimolodon (0.87), Cimolomys (0.91), Ctenacodon (0.85), Dakotamys (0.84), Eobaatar (0.92), Glirodon (0.94), Iberica (0.92), Loxaulax (0.81), Monobaatar (0.99), Paracimexomys (0.82), cf. Paracimexomys (0.93), Parendotherium vel Eobaatar (0.80), Paulchoffatiidae, genus D species 1 (0.86), Sinobaatar (0.89, 0.93). M2 distinctly longer than wide, LB:AP ⬍0.80 (1; Figs. 1, 2c): Ameribaatar (0.78), Argillomys (0.71), Arginbaatar (0.78), Bernardodon (0.76), Iberodon (0.77), Paulchoffatiidae genus A (0.66), Paulchoffatiidae genus B (0.76), Pinheirodon (0.69). Anterobuccal ridge and (or) cusps (8) Presence of cusps on the anterobuccal ridge, so that M2 has three cusp rows, is a derived condition within Multituberculata (e.g., Rougier et al. 1997). The condition is widespread among Cimolodonta and also occurs in a few “plagiaulacidans”. Anterobuccal ridge with no well-defined cusps present (0; Fig. 2a): Ameribaatar, Arginbaatar, Bernardodon, Bolodon, Bryceomys, Cedaromys, Ctenacodon, Eobaatar, ?Iberica, Iberodon, Loxaulax, Monobaatar, Paulchoffatiidae genus A, Paracimexomys, cf. Paracimexomys, Parendotherium vel Eobaatar, Sinobaatar. Cusp(s) present on anterobuccal ridge (1; Figs. 1, 2b, 2c) (number present): Argillomys (1), Cimexomys (1), Cimolodon (1–2), Cimolomys (2–3), Dakotamys (1) Glirodon (2), Parendotherium vel Eobaatar (1?), Paulchoffatiidae genus B (2?), Paulchoffatiidae genus D species 1 (1). Relationship of anterior rim (cingulum) to first cusp of buccal row (9) We recognize two distinct conditions: in some taxa the anterior rim is confluent with the first cusp of the buccal row;

Can. J. Earth Sci., Vol. 50, 2013

in others the two structures are separate. Kielan-Jaworowska and Hurum (2001) included the first state among diagnostic characters shared by Eobaataridae and the “Paracimexomys group”. Distribution of states among taxa sampled leaves character polarity unclear. Furthermore, the distinction between the two conditions is problematic in some cases. One issue seems to be the addition of an anterior cusp to the buccal row in advanced taxa. This anterior cusp generally lies on the rim (and hence is confluent with it). However, it appears not to be homologous with the “true” first cusp of the buccal row. Anterior rim separate from first cusp of buccal row (0; Figs. 1, 2c): ?Ameribaatar, Argillomys, Arginbaatar, Cimolodon, ?Cimolomys, Dakotamys, Monobaatar, ?Paracimexomys, Paulchoffatiidae genus A, Paulchoffatiidae genus B, Paulchoffatiidae genus D species 1. Anterior rim confluent with first cusp of buccal row (1; Fig. 2b): Bernardodon, Bolodon, Cedaromys, Cimexomys, ?Ctenacodon, Eobaatar, Glirodon, Iberica, Iberodon, Loxaulax, ?cf. Paracimexomys, Parendotherium vel Eobaatar, Pinheirodon. ?Polymorphic (0/1): Bryceomys, ?Sinobaatar. Relative sizes of cusps, buccal row (10) Most taxa in the sample have two cusps in the buccal row (see note on cusp homologies, character 9); they are generally subequal in size, or the anterior cusp is slightly larger. This is assumed to represent the primitive condition. A common (presumably apomorphic) condition among Cimolodonta is distinct enlargement of the anterior (non-cingular) cusp; three of the taxa in the sample (including Argillomys) are unusual in that the second cusp is noticeably the largest (also presumed to be an apomorphic state). First two non-cingular cusps of buccal row subequal, or anterior cusp slightly larger (0; Figs. 2a, 2b): Ameribaatar, Arginbaatar, Bernardodon, Bolodon, Cedaromys, Cimexomys, Ctenacodon, Eobaatar, Glirodon, Iberica, Loxaulax, Monobaatar, Parendotherium vel Eobaatar, Paulchoffatiidae genus A, Sinobaatar. Anterior non-cingular cusp distinctly the largest in the buccal row (1): Cimolodon, Cimolomys, Dakotamys, Iberodon, Paracimexomys, ?Pinheirodon. Second non-cingular cusp distinctly the largest in the buccal row (2; Fig. 1): Argillomys, Paulchoffatiidae genus B, Paulchoffatiidae genus D species 1. Polymorphic (0/1): Bryceomys, cf. Paracimexomys. Anterior margin (11) The anterior margin of M2 varies in outline among multituberculates. For present purposes, we recognize two states but do not consider data in hand sufficient to speculate as to which is ancestral. In Argillomys, as in certain “plagiaulacidans” and some members of the Paracimexomys group, the anterior margin of M2 is sinuous, often with the central valley and lingual side of the tooth projecting anteriorly. In other “plagiaulacidans”, and more commonly among cimolodontans, the anterior margin of M2 is relatively straight. Straight anterior margin (0): Ameribaatar, Bernardodon, Cimexomys, Cimolodon, Cimolomys, ?Ctenacodon, Dakotamys, Eobaatar, Monobaatar, cf. Paracimexomys, Paulchoffatiidae genus D species 1, Sinobaatar. Sinuous anterior margin (1; Figs. 1, 2a, 2c): Argillomys, Arginbaatar, Bolodon, Cedaromys, ?Glirodon, Iberica, Loxaulax, Published by NRC Research Press

Cifelli et al.

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Paracimexomys, Parendotherium vel Eobaatar, Paulchoffatiidae genus B, Pinheirodon. Polymorphic (0/1): Bryceomys, Paulchoffatiidae genus A. Last cusp of lingual row (12) In most multituberculates (both “plagiaulacidans” and cimolodontans), cusps of the lingual row are more or less aligned anteroposteriorly, which we take to be the primitive condition. We provisionally recognize some “plagiaulacidans” as distinct in this respect: the posterior cusp of the lingual row is placed somewhat buccal to a line connecting apices of preceding cusps. In Argillomys (also some Paulchoffatiidae), a third, more extreme condition is seen: the posteriormost cusp of the lingual row is centrally placed at the posterior margin of the tooth and occludes the posterior part of the central valley. Aligned with preceding cusps (0; Figs. 2a, 2b): Bernardodon, Bolodon, Bryceomys, Cedaromys, Cimexomys, Cimolodon, Cimolomys, Ctenacodon, Dakotamys, Eobaatar, Glirodon, Iberica, Iberodon, Loxaulax, Paracimexomys, cf. Paracimexomys, Parendotherium vel Eobaatar, Pinheirodon, Sinobaatar. Placed buccal to a line connecting apices of preceding cusps (1): Ameribaatar, Arginbaatar, Monobaatar. Centrally placed at posterior margin of tooth, occluding central valley (2; Figs. 1, 2c): Argillomys, Paulchoffatiidae genus A, Paulchoffatiidae genus B, Paulchoffatiidae genus D species 1.

Discussion Multituberculates are commonly divided into a paraphyletic “Plagiaulacida” (or “Plagiaulacoidea”) and Cimolodonta, with the latter usually recognized as a monophyletic group (Simmons 1993; for dissenting view, see Hahn and Hahn 1999; Fox 2005). Unfortunately, the most diagnostic characteristics of Cimolodonta lie in the antemolar dentition, particularly the premolar series (Kielan-Jaworowska and Hurum 2001). Given that Argillomys marylandensis is represented only by M2, its affinities at even the broadest taxonomic level within Multituberculata are debatable. As shown by the comparisons presented earlier in the text, patterns among individual molar characteristics are frustratingly difficult to assess and serve to highlight the problems of dealing with isolated teeth. As with so many other dental characters, enamel ornamentation has a perplexing distribution among multituberculates. Like some “plagiaulacidans” (e.g., among Pinheirodontidae, Paulchoffatiidae) Argillomys has ribs and grooves only. We are unable to ascribe significance to the pattern of ornamentation seen in Argillomys, though it is worth noting that the strong pitting commonly (but not universally) encountered among cimolodontans and some eobaatarid “plagiaulacidans” is lacking. The cusp formula of Argillomys deserves some comment, as it represents an unusual combination. The presence of only two cusps in the buccal row of Argillomys is a primitive condition seen in many “plagiaulacidans” (except eobaatarids); three are present in most cimolodontans. In the lingual row, however, Argillomys has four cusps, which is an advanced state seen among some “plagiaulacidans” and early cimolodontans. The presence of a cusp on the anterobuccal ridge, as seen in Argillomys, is also an advanced state lacking

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in most “plagiaulacidans” and members of the Paracimexomys group. With respect to cusp fusion (or coalescence), cusp shape, and the development of valleys and grooves (characters 4 – 6 previously described), Argillomys is more clearly distinct from Cimolodonta (including members of the Paracimexomys group), sharing the presumed primitive conditions generally seen among “plagiaulacidans”. Argillomys presents several unusual apomorphic character states. The first concerns the proportions of the M2: it is comparatively long relative to width. A proportionately long M2 is also seen in Paulchoffatiidae, Pinheirodontidae, Ameribaatar, and Arginbaatar. As well, the second cusp of the buccal row is enlarged in Argillomys: in most other multituberculates (except two of the paulchoffatiids included in the comparative sample), either the cusps of this row are subequal or it is the first that is enlarged. Finally, Argillomys is atypical in that the ultimate cusp of the lingual row is centrally placed and occludes the central valley — a condition otherwise seen among some Paulchoffatiidae. What are the possible affinities of Argillomys, given that diagnostic material (incisors and premolar series) for the taxon is unknown? Pressed to speculate, we adopt the working hypothesis that Argillomys represents a late-surviving multituberculate of “plagiaulacidan” grade. Most significant in this respect, we believe, is the absence of several apomorphic character states generally observed among Cimolodonta: three or more cusps in the buccal row, fusion of cusps in the lingual row, and strongly pyramidal cusps separated by narrow grooves (rather than open valleys). Interestingly, Argillomys is comparable in several respects (e.g., pattern of ornamentation, cusp formula, proportions, presence of a cusp on the anterobuccal ridge, enlargement of second cusp in buccal row, central placement of last cusp in lingual row) to one or more of the unnamed paulchoffatiids included in the sample (note Paulchoffatiidae, genus B, for example; Fig. 2c). Paulchoffatiidae are otherwise known mainly from the Guimarota Coal Mine (see Kielan-Jaworowska et al. 2004 and references therein), Portugal, and are thought to be part of a larger radiation of multituberculates restricted to the Late Jurassic – Early Cretaceous of the Iberian Peninsula and nearby northern Africa (Kielan-Jaworowska and Hurum 2001). Given scantiness of available data, however, we stop short of suggesting a special relationship of Argillomys to Paulchoffatiidae, and instead close by emphasizing the need for a better fossil record — particularly from the North American Early Cretaceous — in developing an understanding the affinities of “plagiaulacidans” and stem Cimolodonta.

Acknowledgements We acknowledge, with thanks, the long-term support of Jeff Magee and Cherokee Sanford Brick Company for T.R.L.’s field program. The scanning electron microscope photos in Fig. 2 were taken by Emily Munding. Partial support for this research was provided by National Science Foundation grant DEB-9870173 (with Research Experiences for Undergraduates supplement) to R.L.C.

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