1. Faunal List, Upper Maastrichtian of Western North America Based on literature (1-6) and museum specimens (see SI 3 for complete references and specimen numbers) . Species in bold are either named or recognized here for the first time.
Iguanidae incertae sedis
Pariguana lancensis POLYGLYPHANODONTIA
Polyglyphanodontia incertae sedis
Obamadon gracilis Polyglyphanodontidae
Polyglyphanodon sternbergi Chamopsiidae
Tripennaculus n. sp.
Laramie chamopsiid SCINCOMORPHA
Scincomorpha incertae sedis
Lonchisaurus trichurus Scincoidea incertae sedis
Exostinus lancensis Anguidae
Platynota incertae sedis
Coniophis precedens Alethinophidia incertae sedis
SQUAMATA INCERTAE SEDIS
Sweetwater County lizard
2. Stratigraphy A. Maastrichtian
The Cretaceous lizards included in this study come from the Hell Creek Formation of Montana, the Lance and Ferris formations of Wyoming, the Frenchman Formation of Saskatchewan, the Scollard Formation of Alberta, the Laramie Formation of Colorado, and the North Horn Formation of Utah. All formations are inferred to be late Maastrichtian in age and contain typical late Maastrichtian faunas (i.e., Lancian Land Vertebrate Age). Age constraint (Table S2) is based on several lines of evidence, including radiometric dating, magnetostratigraphy, palynostratigraphy, and vertebrate biostratigraphy, which is primarily based on mammalian faunas.
i. Frenchman Formation
The entire Frenchman Formation is constrained on the basis of magnetostratigraphy to the final reversed interval of the Cretaceous, C29r (7), which spans the final 300,000 years of the Cretaceous (8) and so the Frenchman lizards are no older than 300,000 years. It is unclear where the Gryde locality (which produced the Frenchman lizards discussed here) lies in section, but it lies 20 meters up the exposed outcrop, and the entire formation is a little more than 50 meters thick in this area (9). This suggests that the Gryde lizards date to significantly less than 300,000 years before the K-Pg boundary.
There are fewer constraints on the age of the Lance Formation in eastern Wyoming because no radiometric dates or magnetostratigraphic studies have been published. Here, the K-Pg claystone occurs just below the basal lignite in the Fort Union Formation, at the very top of the Lance Formation; it displays the signature features of the Chicxulub impact, including an iridium anomaly, a layer of spherules, shock-metamorphosed mineral grains, and the abrupt disappearance of many pollen species, followed by the appearance of a layer rich in fern spores (10). In this area, the Lance Formation is roughly 2,500 feet thick, and UCMP V5620 (Lull 2) lies 2,100 feet above the base of the Lance, and UCMP V5711 (Bushy Tailed Blowout) lies in the top half of the formation (11). Based on some assumptions that are admittedly simplistic, i.e. that the Lance was deposited at a constant rate over the final 1.3 Ma of the Cretaceous (as found for the Hell Creek Formation of North Dakota: Hicks et al., 2002), this would indicate ages of ~200,000 and <650,000 years before the K-Pg boundary for UCMP V5620 and UCMP V5711, respectively. The occurrence of the marsupial Glasbius in both sites, a genus that appears between 900,000 and 600,000 years before the K-T boundary in Montana (12) also supports a latest Maastrichtian age, assuming that the introduction of Glasbius occured more or less simultaneously across the region. Palynostratigraphic studies of this area (13) also show that UCMP V5620 lies well above the first occurrence of pollen species that occur in the Latest Cretaceous in North Dakota. These are Striatopollis tectatus and Liliacidites altmurus. In the Hell Creek Formation, these species appears about 40 meters below the boundary (14), which would correspond 500,000 years before the K-Pg boundary (8). Again, assuming that they appeared simultaneously across the region, then this would support a latest Maastrichtian age for the lizards occurring in the Lull 2 locality.
The same assumptions regarding stratigraphy would suggest an age of ~200,000 years before the K-Pg boundary for the Black Butte assemblage in Sweetwater County, southern Wyoming (UW V-79032) which lies 33 meters below the K-T transition in a 233 meter section (15).
iii. Hell Creek Formation
The Hell Creek of Garfield/McCone Counties in Montana spans the final 1.8 million years of the Cretaceous (12); unfortunately no published stratigraphic data are available for the localities that produced the lizards described here. No stratigraphic data are available for the lizards from Carter County and Dawson County, Montana, but similar ages are assumed here.
iv. Scollard Formation
The Cretaceous portion of the Scollard can be constrained to the final 1.5 million years of the Cretaceous (16).
v. Laramie Formation
The Laramie site UCM 77062 is the oldest site in this study, dating to 2.5 Ma before the K-T (17).
vi. North Horn Formation.
The age of the Cretaceous exposures of the North Horn Formation in Utah remains uncertain, but on the basis of the mammal fauna it is assigned to the Late Maastrichtian (18).
vii. Bighorn Basin Lance
The stratigraphy of the Bighorn Basin sites is not well-known but they contain characteristic ‘Lancian’ mammals (19) suggesting a Late Maastrichtian age; furthermore the occurrence of Glasbius in the Hewitt’s Foresight locality (19) supports this assignment.
viii. Ferris Formation
Finally, the age of the Cretaceous part of the Ferris Formation is not well-constrained, but on the basis of the fauna these assemblages are late Maastrichtian in age (20). The sole lizard reported from the Ferris, Odaxosaurus, occurs high in section but given that it crosses the K-Pg boundary, the precise age of this specimen is not an issue.
It should be kept in mind that the the point here is not to make definitive claims about the precise age of last occurrence for each taxon, but rather to emphasize that the fauna described here is as close as one can possibly come to providing a picture of what the squamate fauna looked like just before the K-T impact, within the inevitable constraints imposed by an imprecise understanding of the chronostratigraphy of these formations, and an extremely patchy fossil record. Given that a number of lizard genera appear to have stratigraphic ranges of 10 Ma or more (SI3), it is unlikely that a significant percentage of the species described here became extinct in the hundreds of thousands of years that separate these localities from the K-T boundary, and the various localities can also be assumed to represent, more or less, a picture of a single fauna rather than a succession of faunas.
The ages of the Palaeocene localities are less of an issue: a survivor is a survivor regardless of whether it occurs a few tens of thousands of years after the K-T boundary, or millions of years later. Therefore, for the purposes of inferring survival and for rarefaction, the analysis of Paleocene species lumped together localities from the Puercan and Torrejonian land vertebrate ages, a period spanning some 4 million years (21), because relatively few Puercan localities are known. Inferences of Puercan and Torrejonian age have previously been made on the basis of the co-occurring mammals. Insofar as this lumping of assemblages might tend to bias the study, treating these various assemblages as a single fauna would inflate Paleocene diversity and make the fauna appear more diverse and less well-sampled than it really is. Given that this would bias the study against the hypothesis we are investigating (i.e. a major drop in diversity from the Maastrichtian to Palaeocene) we argue that this assumption is conservative.
The Bug Creek fauna, however, is problematic in representing a mixed assemblage (22, 23) containing Palaeocene and reworked Maastrichtian species. For examining Paleaocene diversity, we assumed the Bug Creek lizard fossils were Palaeocene if they were (a) from species known to cross the K-Pg boundary based on undisputed Palaeocene fossils occurring elsewhere, or (b) they represented species entirely unknown from the Maastrichtian. Although it is conceivable that some of these specimens could be reworked from the Maastrichtian, in this case, it would only bias the data against the conclusions of this paper by inflating Paleocene diversity, and in this sense our assumptions are again conservative.
Table S2. Stratigraphically highest occurrences for species failing to cross the K-T boundary. Occurrence data: Estes, 1964; Gao and Fox, 1996, this study (SI3).
Institutional abbreviations. AMNH, American Museum of Natural History, New York, New York; UCM, University of Colorado Museum, Boulder, Colorado; UCMP, University of California Museum, Berkeley, California; USNM, Smithsonian Institution National Museum of Natural History, Washington, DC; YPM, Yale Peabody Museum, New Haven, Connecticut; YPM-PU, Yale Peabody Museum, Princeton University collection.
Pariguana new genus
Etymology. The genus name is derived from Greek para, near, and the genus name Iguana.
Diagnosis.As for the type and only species.
Type species.Pariguana lancensis.
Pariguana lancensisnew species
Etymology. The specific name refers to the Lance Formation.
Diagnosis. Small iguanid characterized by the following combination of characters: teeth tall, slender, with tapering crowns and weak accessory cusps; coronoid extends forward on lateral surface of jaw beneath last tooth, Meckelian groove constricted suddenly ahead of anterior inferior alveolar foramen.
Holotype. AMNH 22208, partial mandible.
Locality and Horizon. UCMP V5711, Bushy Tailed Blowout; upper Maastrichtian Lance Formation, Niobrara County, eastern Wyoming.
Description. The type and only known specimen consists of the posterior half of a dentary and the associated parts of the splenial, coronoid, and angular. The dentary is weakly bowed in lateral view and extends posteriorly well beyond the toothrow and below the coronoid. In medial view, the Meckelian groove is broad posteriorly, but exhibits a sharp taper just ahead of the anterior inferior alveolar foramen, where the splenial is pinched between the subdental ridge and the ventral margin of the dentary. This distinctive taper distinguishes Pariguana from the Paleocene iguanid Swainiguanoides milleri (24). The subdental ridge is also flat medially, in contrast to the more rounded subdental ridge of Swainiguanoides. The ventral margin of the dentary wraps beneath the splenial, an iguanian synapomorphy. The splenial has a large, elliptical anterior inferior alveolar foramen, and below and slightly behind it a smaller mylohyoid foramen. A long, slender sliver of the angular extends well forward between the splenial and angular. The coronoid extends beneath the subdental ridge medially, and on the lateral surface of the dentary there is a facet where the coronoid would extended onto the dorsolateral surface of the dentary. This anterolateral process of the dentary is a derived feature seen in a number of crown iguanids.
There are 13 tooth positions preserved, and the complete mandible would probably have had between 20 and 30 teeth. Implantation is pleurodont. Several teeth bear large resorption pits at their bases, and four teeth in the back of the jaw are short and implanted at odd angles, a pathology. Tooth crowns are tall and slender, with tapered tips. The tip is tricuspid, with a broad, blunt central cusp flanked by tiny mesial and distal cusps. Medially, the accessory cusps are separated from the central cusp by grooves. The teeth generally resemble those of Swainiguanoides, but differ in having a narrower crown.
Discussion.Pariguana can be referred to the Iguania on the basis of the constriction of the Meckelian groove by the ventral margin of the dentary. Pariguana exhibits two features that are found in crown Iguanidae but not among stem Iguanidae from the Cretaceous of Mongolia (e.g. Temujinia, Zapsosaurus): anterior extension of the coronoid beneath the toothrow on the lateral surface of the jaw, and posterior extension of the dentary beneath the coronoid process. On this basis, Pariguana appears to represent either the sister taxon of crown Iguanidae, or a member of the crown; in the present analysis Pariguana is the sister taxon to Hoplocercidae. Given the uncertainty over the relationships of crown iguanids, and the limited material available for Pariguana, this conclusion should be considered tentative. Although Pariguana closely resembles the Paleocene Swainiguanoides milleri, it differs in the strongly tapering shape of the Meckelian groove, the flat subdental ridge, and in having more strongly tapered tooth crowns.
The earliest and most primitive known iguanids and acrodonts are Asian, suggesting that the Iguania originated in Asia and later migrated into North America (Gauthier et al., 2012). Until now, the earliest known iguanids were Paleocene, which suggested that this migration occurred in the Palaeocene (Gauthier et al., 2012); the presence of Pariguana in the Maastrichtian of North America shows that this immigration event had occurred prior to the end of the Cretaceous. The end-Cretaceous mass extinction appears to have eliminated the Asian iguanids, but spared the North American lineage, leading to a North American radiation of iguanids in the Palaeogene.
Although Gao and Fox (6) identify several Cretaceous lizards as iguanids, these specimens do not exhibit any derived features that allow confident referral to either Iguania or Iguanidae. In particular, the Maastrichtian fossils they refer to Iguanidae can be referred to Tripennaculus, an unusual polyglyphanodontian with strongly tricuspid teeth (25); the others appear to represent polyglyphanodontians as well. Accordingly, Pariguana represents the earliest definitive evidence of Iguanidae in North America.
Fig. S1.Pariguana lancensis n. gen et sp., AMNH 22208 left dentary. In A, medial view; B, dorsal view; C, ventral view, D, lateral view. Abbreviations: aiaf, anterior inferior alveolar foramen; cf, coronoid facet; mf, mylohyoid foramen; sdr, subdental ridge; vl, ventral lip of dentary.
POLYGLYPHANODONTIA Alifanov 2000
Definition. Polyglyphanodon sternbergi and all species closer to Polyglyphanodon sternbergi than to any extant species. This taxon includes Borioteiioidea as defined by Nydam et al. (26). However, Borioteiioidea is a node-based taxon, whereas Polygyphanodontia is a stem-based taxon, and would therefore include any species which were found to lie along the stem of Borioteiioidea.
POLYGLYPHANODONTIA INCERTAE SEDIS
Obamadon new genus
Etymology. The genus name refers to Barack Hussein Obama and odon (Greek) = tooth, in reference to the tall, straight teeth, and the manner in which Mr. Obama has acted as a role model of good oral hygiene for the world.
Diagnosis. As for the type and only species.
Type species.Obamadon gracilis.
Obamadon gracilis new species
Etymology. From the Latin gracilis, slender.
Diagnosis. Small polyglyphanodontian characterized by the following combination of characters: dentary slender, symphysis weakly developed, tooth implantation subpleurodont, teeth lack basal expansion, tooth crowns with a tall central cusp separated from accessory cusps by deep lingual grooves.
Holotype. UCMP 128873, right dentary.
Referred Material. UW 44954, partial dentary.
Locality and Horizon. Holotype: UCMP V 74116, Baldy Butte; upper Maastrichtian Hell Creek Formation, Garfield County, Montana. Referred specimen: UW 81013, Hewitt’s Foresight One, upper Maastrichtian Lance Formation, Bighorn Basin, Park County, Wyoming.
Description. The holotype consists of a partial dentary with five teeth, missing the posterior end. The dentary is relatively long and slender in lateral view when compared to the deep dentary seen in most polyglyphanodontians. The dorsal margin is straight in lateral view, in contrast it is curved in many other polyglyphanodontians (e.g. Leptochamopsdenticulatus and Socognathus unicuspis).
The dentary bears a large mental foramen at its tip, and at least seven smaller foramina on its lateral surface. The dentary symphysis is typical of polyglyphanodontians in being V-shaped, but the symphysis is small and weakly developed, a primitive feature. The symphysis is weakly angled relative to the jaw, such that the left and right dentaries would have diverged at an angle of approximately 60º.
The Meckelian canal is deep and broad except at the tip of the jaw, where it is constricted by the subdental ridge. The subdental ridge is narrow posteriorly, becomes deep as it approaches the fourth tooth position, and then tapers again towards the tip of the jaw. The subdental ridge projects dorsally to form a low, narrow subdental shelf. The ventral surface of the subdental ridge bears a distinctive slot-and-ridge articulation for the splenial, with a distinct slot on the subdental ridge to receive the dorsal margin of the splenial, and a low ridge medial to the slot that would have inserted into a V-shaped groove on top of the splenial. This configuration is a derived feature of polyglyphanodontians (e.g., Chamops segnis, Socognathus brachyodon).
There are five teeth preserved in the jaw with perhaps 15 or 16 tooth positions. The back of the dentary is missing, but the total tooth count was likely between 20 and 25. Teeth implant in a subpleurodont fashion, extending less than halfway down the medial surface of the mandible. In this respect, Obamadon is comparable to Meniscognathus and Leptochamops, but more primitive than advanced chamopsiids such as Chamops and Socognathus. Teeth are secured to the jaw with a strong basal deposition of cementum. In contrast to most other polyglyphanodontians, there are multiple open tooth positions, suggesting ongoing tooth replacement. This may represent a primitive feature of Obamadon, or it could be that the animal was still actively growing and had not yet received the adult set of teeth. No replacement pits are visible in any of the implanted teeth.
The tooth crowns themselves are columnar. In contrast to chamopsiids, the bases of the crowns are only slightly wider than the tips. In this respect, the teeth resemble those of Tripennaculus eatoni (25). Teeth are strongly tricuspid. There is a tall, conical central cusp, which is very slightly recurved; it is separated from small, low mesial and distal cusps by deep grooves on the lingual surface of the tooth. Here again, the teeth closely resembles Tripennaculus. However, Tripennaculus is distinguished by taller mesial and distal cusps, which bear ridges that extend down the inside of the tooth as distinct blades. Tripennaculus is also distinguished by closely packed, posteriorly curved tooth crowns.
Discussion.Obamadon is identified as a polyglyphanodontian on the basis of the V-shaped dentary symphysis, interlocking slot-and-ridge articulation for the splenial, and subpleurodont tooth implantation. Although this specimen has previously been referred to Leptochamops (27) it is clearly more primitive than Leptochamops and other taxa referred to the Chamopsiidae in the weak development of the dentary symphysis and the lack of barrel-shaped tooth crowns. The current analysis places Obamadon near the base of the Polyglyphanodontia, although it is possible that it represents a basal chamopsiid. The well-developed tricuspid teeth, grooves on the teeth, and lack of barrel-shaped tooth crowns all suggest affinities with Tripennaculus, although the short, straight teeth and small accessory cusps preclude referral to that genus.
Fig. S2.Obamadon gracilis n. gen et sp. (Polyglyphanodontia) left dentary. In A, medial view; B, dorsal view; C, ventral view, D, lateral view. Abbreviations: ds, dentary symphysis; mec, Meckelian fossa; sdr, subdental ridge; sp, splenial contact.
CHAMOPSIIDAE Denton and O’Neill 1995
Definition. Chamops segnis and all species closer to Chamops segnis than to Polyglyphanodon sternbergi or Macrocephalosaurus ferrugenous.
Denton and O’Neill (28) defined Chamopsiinae as the node-based taxon containing Chamops segnis, Prototeius stageri, Leptochamops denticulatus, and Meniscognathus altmani, but the published phylogenetic analysis only included Prototeius and Chamops. Nydam et al. (29) later raised the Chamopsiinae to family rank. They also questioned whether Prototeius is closely related to Chamops, and so the monophyly of the Chamopsiinae as originally defined remains an open question. Nydam et al.’s redefinition of Chamopsiinae includes a list of species, but not a formal definition. The current study did find that the North American taxa exclusive of Polyglyphanodon form a clade, but the affinities of Prototeius are beyond the scope of this analysis, and given that its relationships remain unclear, a new stem-based definition for the name is proposed here that largely conforms to the original intent of Denton and O’Neill while remaining robust against changes in tree topology.
Socognathus Gao and Fox 1996
Revised Diagnosis. Polyglyphanodontian characterized by the following combination of characters: long, straight dentary that is dorsoventrally shallow and labiolingually expanded, massive dentary symphysis, short, robust crowns with reduced mesial and distal cusps.
Type species.Socognathus unicuspis Gao and Fox 1996
Socognathus brachyodon new species
Etymology. Greek brachy, short, and odon, tooth.
Diagnosis.Socognathus with posterior teeth having strongly swollen, weakly tricuspid crowns.
Holotype. YPM PU 16724, dentary (Fig. S2)
Referred Specimens. YPM-PU 21375, dentary.
Locality and Horizon. Polecat Bench, Sec. 31, T57N, R98W; Late Maastrichtian Lance Formation, Bighorn Basin, Park County, Wyoming.
Description. The holotype of Socognathus brachyodon (YPM-PU 16724) consists of a virtually complete dentary with all teeth preserved (Fig. S1). The dentary is relatively large, being 21 mm in length. As with Socognathus unicuspis (Gao and Fox, 1996), the jaw is relatively long, straight and shallow in lateral view; the dentary’s depth at midpoint is about 20% of its length. S. brachyodonalso resembles S. unicuspis in having a robust mandible; the lateral surface of the dentary is strongly convex and projects laterally well beyond the toothrow. Half a dozen small mental foramina are present on the lateral surface of the mandible.
Medially, the symphysis is well-developed. It is V-shaped with facets extending above and below the Meckelian canal, a derived feature of polyglyphanodontians. The ventral facet is enlarged, a derived feature shared with Socognathus unicuspis (Gao and Fox, 1996) and, to a lesser degree, Chamops segnis (4). The symphysis is angled relative to the long axis of the dentary, such that the two halves of the mandible would have diverged at approximately a 90º angle. The skull of Socognathus therefore would have been relatively short and broad. In ventral view, the dentary is strongly curved inward near the symphysis, but posteriorly the jaw is bowed inwards, as is typical of polyglyphanodontians.
The Meckelian canal is typical of polyglyphanodontians in being broadly open along the length of the jaw to accommodate a large splenial. The intramandibular septum terminates near the midpoint of the jaw, as in Chamops, and is concealed by the subdental ridge in medial view. The subdental ridge is typical of polyglyphanodontians. It is developed as a flat plate with a sharp dorsal edge that projects upwards to form a subdental shelf medial to the teeth. In medial view it is shallow posteriorly and becomes deeper anteriorly, where it projects ventrally to constrict the Meckelian canal at the tip of the jaw.
On the ventral surface of the subdental ridge there is a complex system of slots and ridges. A long slot between the subdental ridge and the intramandibular septum would have received the splenial. Medial to this slot there is a long ridge, which would have inserted into a corresponding slot atop the splenial. A similar arrangement is seen in other polyglyphanodontians, including Chamops, Leptochamops, Tchingisaurus, and as described above, Obamadon.
The dentary contains 19 teeth. As in other polyglyphanodontians, teeth implant high on the dentary, with the tooth bases extending about a third of the way down the medial surface of the dentary. In this feature, S. brachyodon is intermediate between the condition in primitive polygyphanodontians such as Leptochamops and Meniscognathus, where the teeth extend slightly less than halfway down the dentary (subpleurodont) and the more derived Chamops and Haptosphenus, in which the teeth lie along the dorsal margin of the jaw (subacrodont). In contrast to the typical squamate condition, in which multiple tooth positions are empty and replacement foramina are present, all tooth positions are occupied by a tooth, and replacement foramina are absent. This indicates that tooth replacement is greatly reduced or absent in adults, a derived character shared with most other polyglyphanodontians.
Tooth crowns are small and slender anteriorly, then become taller and more robust near the middle of the jaw, developing the typical ‘barrel-shaped’ crowns seen in other Chamopsiidae; they then become shorter again at the back of the jaw. The swelling of the crowns is more well-developed than in Socognathus unicuspis, in which the crowns are more columnar. Anterior teeth are unicuspid, with a central cusp and weak mesial and distal ridges. Posterior teeth have accessory cusps. The accessory cusps are more well-developed than in Socognathus unicuspis, but are reduced compared to polyglyphanodontians such as Chamops, Haptosphenus, and Leptochamops. Reduced accessory cusps appears to be a derived feature, and is shared with Stypodontosaurus and the Frenchman chamopsiid, suggesting that these animals may form a clade.
Systematics.Socognathus brachyodon shares numerous features with Polyglyphanodontia, including development of the dentary symphysis dorsal and ventral to the Meckelian canal, a subdental slot and ridge for the splenial, and subpleurodont implantation. The subacrodont tooth implantation and swollen tooth crowns indicate chamopsiid affinities. The robust symphysis suggests that Socognathus is most closely related to derived Chamopsiidae including Chamops and Haptosphenus, while the reduced accessory cusps suggests affinities with Stypodontosaurus and the Frenchman chamopsiid.
Fig. S3.Socognathus brachyodon n. sp, YPM-PU 16724 left dentary. A, medial view; B, ventromedial view; C, dorsal view, D, lateral view. Abbreviations: cor, coronoid facet; ims, intramandibular septum; sdr, subdental ridge; sym, dentary symphysis.
Material. UCM 42164, dentary.
Horizon and Locality. UCM 77062, upper Maastrichtian Laramie Formation, Weld County, Colorado.
Diagnosis. Chamopsiid with slender teeth in the anterior half of the jaw, and greatly enlarged teeth in the posterior half of the jaw; dentary tapers strongly anteriorly and dentaries weakly divergent in dorsal view.
Description. This species is known from a single, poorly preserved jaw (Fig. S4). The jaw appears to have been relatively long and slender, and the symphyseal region is strongly tapered in lateral view. The symphysis has the typical V-shaped configuration of polyglyphanodontians, however while the dorsal facet is robust, the ventral facet is more weakly developed, as in Leptochamops. The symphysis is weakly angled with respect to the dentary, such that the two dentaries would have diverged at an angle of roughly 45º. As in other chamopsiids, the subdental ridge is shallow posteriorly and deep anteriorly where it constricts the Meckelian canal. The subdental ridge supports a prominent subdental shelf.
There are 17 preserved teeth but the total number was probably 20 or more. Teeth are implanted in a strongly subacrodont fashion, similar to the condition in Chamops. All tooth positions are occupied and no teeth exhibit resorption pits, indicating cessation of replacement, as in other Chamopsiidae. Teeth are short and moderately robust anteriorly; towards the back of the jaw the teeth become broad and massive, and are 2-3 times the diameter of the anterior teeth. The crowns are heavily worn postmortem such that the enamel is missing, but the anterior teeth appear to have been unicuspid, and posterior teeth are strongly tricuspid.
Discussion. The Colorado chamopsiid can be assigned to Polyglyphanodontia on the basis of the V-shaped dentary symphysis and to Chamopsiidae on the basis of the subacrodont tooth implantation. Despite the poor preservation, the extreme disparity in size between anterior and posterior teeth makes this specimen unlike any of the other known chamopsiids; neither does the shape of the dentary match that of any of the other species. This specimen therefore appears to represent a distinct species, but better material is needed to characterize this taxon.