A new microvertebrate assemblage from the Mussentuchit Member, Cedar Mountain Formation: insights into the paleobiodiversity and paleobiogeography of early Late Cretaceous ecosystems in western North America

Introduction

The Cedar Mountain Formation, which spans both the uppermost Lower and lowermost Upper Cretaceous, records an important interval in the history of life coinciding with global climatic changes (Behrensmeyer et al., 1992) such as rising sea level (Haq, Hardenbol & Vail, 1987; Hallam, 1992; Fassell & Bralower, 1999; Frakes, 1999), global warming (Barclay, McElwain & Sageman, 2010), and the emergence of flowering plants (Crepet & Friis, 1987; Upchurch & Wolfe, 1987; Saward, 1992). Regionally, subduction of the Farallon Plate beneath North America’s west coast initiated the Sevier Orogeny—a north–south mountain range spanning from Canada to Mexico—that produced an eastward migrating forebulge (Currie, 1998; White, Furlong & Arthur, 2002; Eberth et al., 2006) followed by full inception of a foreland basin. These regional tectonic changes, coupled with global trends, resulted in the establishment of the North American Cretaceous Western Interior Seaway (WIS; Dickinson & Snyder, 1978; DeCelles, 1986; Neilson, 1986; Kauffman & Caldwell, 1993), a large, epieric body of water that flooded much of central North America by the early Late Cretaceous. Initially, rise of the Sevier Orogeny produced a period of increasing aridity that is recorded throughout most of the Cedar Mountain Formation (e.g., Yellow Cat through Ruby Ranch members) (Stokes, 1944; Young, 1960; Suarez et al., 2014; Hatzell, 2015). However, continued transgression of the WIS produced a dramatic climate shift in the capping Mussentuchit Member of the Cedar Mountain Formation, which records a rapidly changing paleoclimatic shift toward warmer, more humid conditions (Cifelli et al., 1997; Suarez et al., 2012, 2014).

Coupled with a shifting climate regime, animals inhabiting western North America during this interval also experienced a prolonged period of intercontinental faunal dispersal. Establishment of a land bridge across Beringia permitted faunal exchange between Asia and North America (Russell, 1993). This exchange has been linked to replacement of the endemic North American fauna by clades originating in Asia (Ostrom, 1970; Russell, 1993, 1995; Cifelli et al., 1997; Kirkland et al., 1997, 1999; Sereno, 1999; Nydam, 2002; Ji et al., 2003; Kobayashi & Azuma, 2003; Carpenter, 2006; Ji, Ji & Zhang, 2009; McDonald et al., 2010a; Zanno & Makovicky, 2011). Thus, paleofaunal assemblages of the Mussentuchit Member are key for recording the effects of climate change and faunal exchange across the Early to Late Cretaceous boundary in North America.

Previous microvertebrate studies of the Mussentuchit Member have documented the remains of more than 90 species (Gardner, 1994, 1996, 1999b; Cifelli et al., 1997, 1999; Fiorillo, 1999; Gardner & Cifelli, 1999; Kirkland et al., 1999, 2016; Nydam, 1999, 2000a, 2000b, 2002; Goldberg, 2000; Nydam & Cifelli, 2002; Cifelli, 2004; Garrison et al., 2007; McDonald et al., 2017), illuminating one of the most diverse Mesozoic assemblages in North America sampled to date. These taxa were discovered through extensive screen-washing and microvertebrate sampling during the latter part of the 20th century by the Oklahoma Museum of Natural History (Cifelli, 1996; Cifelli, Madsen & Larson, 1996; Cifelli et al., 1997, 1999) with a smaller effort aimed at studying the microvertebrates from a single dinosaur locality by the College of Eastern Utah (Garrison et al., 2007). Microvertebrate data collection continues through ongoing research by the North Carolina Museum of Natural Sciences and the Field Museum of Natural History.

Here, we report the taxonomic composition of a newly discovered site preserving an abundance of microvertebrate remains in the Mussentuchit Member—the Cliffs of Insanity (COI) microvertebrate locality. This site occurs in a restricted section of outcrop to the west of currently sampled localities, capturing microhabitat variation that adds critical information to the standing biodiversity during this transition period and new information on the timing and pattern of Laurasian exchange. We compare the abundance, biodiversity, and composition of the COI microvertebrate site to those from previous collection efforts. The methods used herein combine those from various fields of ecology in order to apply a more comprehensive quantitative approach to microvertebrate paleoecology.

Geologic setting

The Cedar Mountain Formation is stratigraphically bounded by the Upper Jurassic Brushy Basin Member of the Morrison Formation below and by the Upper Cretaceous Naturita Formation above (Kirkland et al., 1997, 1999; Carpenter, 2014). The formation is divided into six members spanning the Barremian/Aptian through the Cenomanian. Sedimentologically, the Mussentuchit Member consists predominantly of highly smectitic mudstone (formed from altered volcanic ash), containing localized, low grade coal beds (Kirkland et al., 1997, 1999) and has been interpreted as either a wet, lacustrine environment (Stokes, 1944, 1952; Craig, 1981; Garrison et al., 2007; Ludvigson et al., 2015) or a fluvial environment (Kirkland et al., 1997; Goldberg, 2000); more specifically, as a distal delta system (Sorensen, 2011; Holmes, 2017) at the western margin of the WIS in what is now central Utah (Fig. 1A). Radioistopic dates reported by Cifelli et al. (1997), Garrison et al. (2007), and Tucker, Makovicky & Zanno (in press) support a Cenomanian—early Turonian age (98.2 ± 0.6 to 93 Ma) for the Mussentuchit Member.

The microvertebrate fossils described here were recovered from a fine-grained sandstone sandwiched between siltstone-dominated units, approximately five meters below the contact with the overlying Naturita Formation. Five geologic samples above, within, and below the fossil layer were collected along a section ∼1 m high (Fig. 1E). These samples indicate the COI microfossil assemblage displays a coarsening upward sequence from siltstone below the fossil-bearing layer to a bentonitic, fine-grained sandstone within it and then a transition back to siltstone above the fossil-bearing layer, possibly representing an oxbow lake with an adjacent river migrating towards the lake. Precise locality information for this site is recorded at the NC Museum of Natural Sciences.

Microvertebrate sites published in Cifelli et al. (1999, fig. 2) indicate that the stratigraphically highest OMNH sites occur at about 13 and 17 meters below the contact with the Naturita Formation (named the Dakota Formation in the aforementioned reference). Site OMNH V824 may occur closer to the Naturita Fm.; however, Cifelli et al. (1999) state that the contact is covered in the field area of their localities so its exact distance below the contact is uncertain. The COI locality is therefore located considerably higher in section than OMNH sites previously reported, raising the likelihood of greater influence from the transgressing WIS.

Materials and Methods

Taxonomic referral and morphometrics

Taxonomic referrals are based on comparisons with previously collected specimens from the Mussentuchit Member of the Cedar Mountain Formation housed at the OMNH and a variety of comprehensive microfaunal studies. We take a conservative approach, using apomorphy-based identifications to assign specimens to the most inclusive taxonomic level possible. Fragmentary or poorly preserved material lacking autapomorphies was referred using morphological similarities and assigned to higher taxonomic levels. Dental terminology follows a number of authors depending on faunal group. These can be found in supporting descriptions and references therein.

Paleoecological studies of theropods are often difficult to conduct due to the rarity of their skeletal remains, which is in some part attributable to the small size and fragility of many theropod skeletons. By comparison, theropod teeth are more mechanically and chemically resistant, and often occur in abundance at microfossil localities. Therefore, paleobiological and faunal studies that utilize isolated teeth can provide more faunal information than those that rely strictly on skeletal elements (Rensberger & Krentz, 1988; Currie, Rigby & Sloan, 1990; Farlow et al., 1991; Fiorillo, 1999; Sankey, 2001, 2008a, 2008b; Heckert, 2004; Vullo, Neraudeau & Lenglet, 2007; Schoch et al., 2018). Furthermore, due to the wide morphological diversity of theropod teeth, they can be used to identify new taxa (Currie, Rigby & Sloan, 1990; Baszio, 1997a, 1997b; Sankey, 2001; Sankey et al., 2002; Larson, 2008; Gates & Scheetz, 2015). Some theropod clades that were present in the Mussentichit Member such as oviraptorosaurs (Makovicky, Zanno & Gates, 2015), or likely present such as ornithomimosaurs, were toothless and thus would not be represented in microvertebrate samples.

To better constrain the diversity of theropod species represented in our sample, we followed the approach of several recent studies (Smith, Vann & Dodson, 2005; Larson & Currie, 2013; Hendrickx, Mateus & Araújo, 2014; Williamson & Brusatte, 2014; Gates & Scheetz, 2015) by performing principal component (PCA) and discriminant (LDA) analyses in the program PAST version 3.19 (Hammer, Harper & Ryan, 2001) on two, independently derived, taxonomically comprehensive databases of theropod tooth measurements including the eight theropod teeth from the COI. Measurements follow Smith, Vann & Dodson (2005) and Larson & Currie (2013) (Fig. 2) and were taken in the program ImageJ (Rasband, ImageJ & National Institutes of Health, 2011). Both PCA and LDA identify the hypothetical variables (components) that account for the majority of variation in a multivariable dataset by transforming the new variables into linear combinations of the original variables (Davis, 1986; Harper, 1999). An LDA is a supervised analysis, which discriminates according to class, whereas a PCA is an unsupervised analysis that ignores class labels (Martínez & Kak, 2001).

We performed an LDA using a taxonomically comprehensive dataset (Hendrickx, Mateus & Araújo, 2014) that includes 11 principal measurements (CBL, CBW, CH, AL, MCL, MCW, MC, DC, CBR, CHR, and MCR) (Data S1). The Hendrickx, Mateus & Araújo (2014) dataset comprises 995 teeth assigned to one of 16 qualitative morphotype categories, each representing a clade, family, paraphyletic group, or a theropod species of uncertain affinity. All values were log-transformed to better reflect a normally distributed multivariate dataset. Three theropod teeth from the COI were recovered with missing apices. To account for this uncertainty, we estimated a maximum CH for each tooth (Data S1). Estimated maximum CHs were plotted in independent analyses in order to maintain statistical integrity, and resultant data points were then superimposed onto the output of prior analyses. Thus, the true point resides somewhere along the line, most likely close to the estimated CH point.

We performed a second series of PCA and LDA using a novel combination of numerical data derived from Larson & Currie (2013) and Williamson & Brusatte (2014) (sampled from Late Cretaceous North America theropods in contrast to the Hendrickx, Mateus & Araújo (2014) dataset, which sampled widely across the Mesozoic theropod diversity), then removed samples missing a measured FABL, CH, or BW (Fig. S1). These samples were removed because FABL, CH, and BW often represent the strongest lodgings in the multivariate analyses and there was uncertainty regarding how this missing data would be handled. The five principal measurements included in the combined database are FABL, CH, BW, PDM, and ADM. The modified dataset consists of 1,027 teeth belonging to one of six taxa (including two tooth morphotypes) identified to species by previous authors. These categories include Dromaeosauridae, Paronychodon, Richardoestesia, Saurornitholestinae, Troodontidae, and Tyrannosauroidea. Teeth were further separated into a total of 14 categories based on lithostratigraphic unit and measurement data that was log-transformed to better reflect a normally distributed multivariate dataset. This independent analysis was performed in order to check and contrast the results of the LDA using the Hendrickx, Mateus & Araújo (2014) database. However, the Hendrickx, Mateus & Araújo (2014) database is superior in terms of theropod diversity, tooth completeness, confidence of taxonomic identifications, and number of measured principal components. Therefore, we consider the LDA results using the Hendrickx, Mateus & Araújo (2014) database more heavily in our taxonomic identifications than those of our other analyses.

Results

First, we present our taxonomic assessment of the assemblage in a traditional “Systematic Paleontology” format, followed by subsequent analyses of biodiversity, paleoecology, and paleoenvironmental influences, based on comparisons of the COI assemblage with other microvertebrate assemblages from the Mussentuchit Member.

Systematic Paleontology

• OSTEICHTHYES Huxley, 1880

• ACTINOPTERYGII Klein, 1885

• NEOPTERYGII Regan, 1923

Approximately 112 teeth (Figs. 3A–3FF), 53 complete or partial scales (Figs. 3JJ–3LL), and additional skeletal and vertebral fragments were recovered representing Osteichthyes. Amiiform and lepisosteiform teeth constitute the majority of material, whereas only three teeth represent pycnodontids. Fish material unidentifiable to lower taxonomic levels are shown in Figs. 3GG–3LL.

• AMIIFORMES Hay, 1929 (sensu Grande & Bemis, 1998)

• AMIINAE Bonaparte, 1838

Amiid teeth from the COI possess a suite of characteristics typical of Amiidae as outlined by Estes & Sanchíz (1982), including a styliform morphology with a rounded crown situated atop a bony pedestal (Figs. 3A–3E). Well-developed carinae extend from the crown base to the apex and the teeth become increasingly transparent toward the apex similar to those described by Bryant (1988).

• LEPISOSTEIFORMES Hay, 1929

• LEPISOSTEIFORMES indent

Over 56 partial scales attributable to gars were discovered at the COI site, as is common for many nonmarine Cretaceous microvertebrate fossil concentrations. For taxonomic assignment we followed Grande (2010), Brinkman et al. (2013), and Brinkman, Newbrey & Neuman (2014) in considering isolated scales as generically indeterminate.

• LEPISOSTEIDAE Cuvier, 1825

There are three morphotypes of lepisosteiform teeth from the COI. Two of these were originally described by Bilelo (1969; but see also Estes & Sanchíz, 1982; Fiorillo, 1999) as dentary and palatal teeth of lepisosteids from the Lower Cretaceous Paluxy Formation of north-central Texas, and a third, lanceolate tooth type was described by Brinkman et al. (2013) from the Upper Cretaceous (Campanian) Kaiparowits Formation and the Upper Cretaceous John Henry Member of the Straight Cliffs Formation. McDonald et al. (2017) illustrated the first gar tooth from the Mussentuchit Member, which has striking similarities to specimens in our locality such as those in Figs. 3L–3P. Dentary (marginal?) teeth are short, broad, and conical with a smooth, translucent crown, and are occasionally falcate (Figs. 3F–3H). Teeth of the palatine are hemispherical with a smooth, convex crown (Figs. 3I–3K). The third tooth type accounts for the majority of recovered lepisosteiform material and is similar to the first type in that they are conical and translucent. However, the crowns possess elongated, narrow peduncles that constrict below the apex (Figs. 3L–3P). The apex is distinguished by short, well-developed, mesio-distal carinae.

• PYCNODONTIFORMES Berg, 1940

Pycnodontiform teeth from the COI are strongly falcate, with a smooth, translucent crown (Figs. 3Q–3Z). They are strongly labiolingually compressed and lack any striations or a carinae. They are identical to pycnodont teeth recovered at the Cifelli #2 Eolambia caroljonesa Quarry in Mussentuchit Member Wash of Utah (Garrison et al., 2007) as well as other localities across the Cretaceous of North America (Brinkman et al., 2013).

• SALMONIFORMES Greenwood, Rosen, Weitzman, and Myers, 1966

• cf. ENCHODONTIDAE Lydekker, 1889

NCSM 33308 may represent an Enchodus tooth fragment (Figs. 3AA–3FF), sharing a suite of features with those described in Fiorillo (1999) and Winkler, Murry & Jacobs (1990). The tooth fragment is slender and labiolingually compressed with a lenticular cross section in basal view. It is slightly recurved, and the apex is blunt and rounded. The enamel is smooth and thin, lacking striations, and there are weakly defined carinae on the mesial and distal margins. The fragmentary nature of NCSM 33308 makes an accurate referral to Enchodus tenuous, however, the presence of three Enchodus teeth that have been previously reported from the Mussentuchit Member (OMNH 026333, 026983, and 032537) may add support for this referral.

• LISSAMPHIBIA Haeckel, 1866

• ALLOCAUDATA Fox and Naylor, 1982

• ALBANERPETONTIDAE Fox and Naylor, 1982

• ALBANERPETON Estes and Hoffstetter, 1976

• ALBANERPETON

NCSM 33278 (Fig. 4) is a lissamphibian dentary bearing three complete teeth and one tooth fragment. Much of the jaw is missing and the lack of diagnostic elements makes assigning it to a lower taxonomic level difficult. However, Gardner (1999a) describes several dental characteristics of Albanerpeton arthridion that NCSM 33278 shares. These include teeth with strong labiolingual compression that are non-pedicellate, chisel shaped, and with strong pleurodont implantation. Additionally, teeth are straight and arranged closely together along the parallel orientations of their lengths. Oreska, Carrano & Dzikiewicz (2013) describe Albanerpeton material from the Lower Cretaceous Cloverly Formation resembling NCSM 33278.

• REPTILIA Linnaeus, 1758

• TESTUDINATA Klein, 1760

• HELOCHELYDRIDAE (SOLEMYDIDAE) Lapparent de Broin and Murelaga, 1996

The majority of turtle materials from the COI are referable to a new species of helochelydrid, previously referred to Naomichelys speciosa (Herzog, Zanno & Makovicky, 2015). Helochelydrid shell fragments, characterized by a dense arrangement of raised, cylindrical tubercles, are common throughout the Mussentuchit Member (Herzog, Zanno & Makovicky, 2015). Tubercles are typically 0.5–1 mm tall with planar apices and constricted bases (Figs. 5A and 5B). Microstructurally, COI helochelydrid shell samples are similar to Solemys vermiculata and Solemys sp. previously reported from Spain, and Naomichelys sp. previously reported from Canada and USA (Scheyer, Pérez-García & Murelaga, 2015). All samples possess a dipole structure with cortices of relatively equal thickness (Fig. 5C). The external cortex is characterized by an outer zone of ornamental tubercles composed of parallel-fibered bone (Fig. 5D). The lower zone of the external cortex is composed of coarse, interwoven structural bone fibers (Fig. 5E). The cancellous bone is comprised of long slender trabeculae with large intertrabecular regions (Fig. 5F). Secondary lamellar bone is often present along the walls of the trabeculae. The internal cortex is made of parallel fibered bone and occasionally grades into lamellar zonal bone (Fig. 5G).

• ADOCIDAE Cope, 1870

The COI contains shell fragments likely representing a new species of adocid (Figs. 5H–5O). Danilov, Sukhanov & Syromyatnikova (2011) describes adocid shell sculpturing as having a diagnostic arrangement of small and regular grooves and pits or dots. COI shell fragments ascribed to Adocidae are similar to those described by Danilov, Sukhanov & Syromyatnikova (2011) and Danilov et al. (2013) and are characterized by a smooth surface with small pits and rhomboidal sulci. A few shell fragments are sculptured, bearing small grooves and pits (Figs. 5H–5J). The identification of these specimens as adocid is supported by similarities of internal microstructure (Scheyer, Syromyatnikova & Danilov, 2017). All samples possess thick, densely packed external and internal cortical bone with well-developed interior cancellous bone (Fig. 5K). The external cortex is divided into upper and lower zones. The lower zone (bordering the internal cancellous bone) is made of interwoven structural fibers and is characterized by a reticular vascular pattern. The upper zone (bordering the external surface) is characterized by highly birefringent growth marks exhibiting a wavy pattern, and diagonally oriented Sharpey’s fibers (Fig. 5L). The cancellous bone is less spongy than found in Helochelydra, and is made of coarse, dense, short trabeculae with round to oval intertrabecular regions (Fig. 5N). Secondary lamellar bone is often present along the walls of the trabeculae. The internal cortex is made of parallel-fibered bone and occasionally grades into lamellar zonal bone (Fig. 5O).

• SQUAMATA Oppel, 1811

Squamata is represented by two isolated teeth (Figs. 6A–6J) and a jaw fragment bearing two teeth (Figs. 6K–6M). The poorly preserved and fragmentary condition of these specimens makes confident referrals to lower taxonomic levels difficult, however they share a suite of characteristics with squamate dental material described from the Mussentuchit Member (Nydam, 2002) and other Cretaceous-age sedimentary rocks (Nydam, Rowe & Cifelli, 2013).

• SCINCOMORPHA Camp, 1923

NCSM 33293 is a scincomorphan (paramacellodid–cordylid grade) (R. Nydam, 2018, personal communication) with subpleurodont implantation and deposits of cementum at the base (Figs. 6A–6E). The tooth is tall, narrow, conical, slightly recurved distally, with a weakly defined carina and several weakly developed striations on the lingual crown surface.

• SQUAMATA indet.

NCSM 33295 is a poorly preserved, isolated tooth possibly belonging to Squamata (Figs. 6F–6J). The tooth is tall, narrow, conical, lacks curvature, and is subpleurodont with heavy deposits of cementum at the base.

• SQUAMATA indet.

NCSM 33296 is a possible scincomorphan jaw fragment with two closely placed teeth (R. Nydam, 2018, personal communication) (Figs. 6K–6M). The complete tooth is tall, conical, lacks curvature, has heavy deposits of cementum at the base. The enamel is poorly preserved, improving slightly toward the tooth crown. The tooth crown is rounded and blunt, with a weakly defined carina.

• ARCHOSAURIA Cope, 1869

• CROCODYLIFORMES Hay, 1930, sensu Benton and Clark, 1988

• MESOEUCROCODYLIA Whetstone and Whybrow, 1983

By far the most abundant identifiable material from the COI is crocodyliform (Fig. 7), constituting approximately 3,657 teeth (∼94% of all teeth) as well as abundant osteoderm fragments. We note similarities of these teeth to four Late Cretaceous groups (bernissartids, atoposaurids, pholidosaurids (Pomes, 1988), and cf. Dakotosuchus). However, these assignments should be considered tentative as there is a great variability within and between tooth morphologies, with many teeth possessing a suite of characteristics along a gradational spectrum (Buscalioni et al., 2008). Following the example of McDonald et al. (2017), we simply consider the morphotypes under Mesoeucrocodylia with no further speculation to their taxonomic framework. Similarly, this approach was applied for all biodiversity analyses.

• MESOEUCROCODYLIA indet.

Teeth we identify as similar in morphology to bernissartids are apico-basally short, mesio-distally elongate, labiolingually compressed, and slightly reniform in occlusal view (Figs. 7A–7J). The crowns often possess an elliptical apical wear facet. The mesial and distal margins have poorly defined carinae and the labial and lingual faces are ornamented with widely spaced, well-defined, apicobasally oriented striations that converge toward the apex. Teeth matching this description have been recovered from the Lower Cretaceous Cloverly Formation of Wyoming and Montana and the Lower Cretaceous Rabekke, Jydegård, and Annero formations of Scandinavia (Schwarz-Wings, Rees & Lindgren, 2009; Oreska, Carrano & Dzikiewicz, 2013).

Teeth resembling atoposaurids from the COI are proportionately taller, labiolingually compressed, and reniform in occlusal view (Figs. 7K–7T). They are constricted toward the crown base and have well-developed, sometimes crenulated, mesial, and distal carinae. They are typically triangular in mesio-distal view, with a strongly convex labial face and a weakly convex lingual face. Tooth crowns range from strongly rounded to moderately acuminate and have varying degrees of recurvature. Striations run apicobasally, are sometimes anastomosing, and are typically more pronounced on the lingual face. Recent descriptions of similar teeth have come from the Lower Cretaceous Cloverly Formation of Wyoming and Montana and other sites in the Mussentuchit Member of the Cedar Mountain Formation (Garrison et al., 2007; Oreska, Carrano & Dzikiewicz, 2013; McDonald et al., 2017).

Teeth morphologically similar to those of pholidosaurids are considerably taller, strongly conical, and have nearly circular bases (Figs. 7Z–7NN). Smaller teeth are tall and slender, with slight lingual recurvature and sharp apices (Figs. 7Z–7II). Larger teeth tend to be more stout with rounded apices (Figs. 7JJ–7NN). Mesial and distal carinae extend from the base to the apex and are weakly defined. Striations are strongly defined and run apicobasally, sometimes anastomosing towards the apex. Larger, stouter teeth have numerous, thin, well defined striations that run apicobasally (Figs. 7JJ–7NN). These larger teeth have been previously reported from the Cedar Mountain Formation as a fifth group of teleosaurid teeth similar to Machimosaurus (Cifelli et al., 1997), which display a nearly complete sinusoidal curve outlined by the primary carinae in apical view (R. Nydam, 2018, personal communication).

NCSM 33362 and NCSM 33384 are mesoeucrocodylian osteoderms (Figs. 7OO and 7PP). Due to breakage there is no evidence of an anterior bar or posterior suture to aid identification and so we assign these osteoderms to Mesoeucrocodylia indet. NCSM 33384 possesses a random array of approximately equal-sized round pits, generally similar to the morphology on pholidosaurids and goniopholids. One feature on NCSM 33362 is a low median keel that barely rises above the level of the dermal body (Fig. 7OO).

• cf. DAKOTASUCHUS sp. Mehl (1941)

Teeth we identify as similar to Dakotasuchus are conical, sometimes slightly lingually recurved, and typically sub-circular in occlusal view (Figs. 7U–7Y) (Frederickson et al., 2017). They are somewhat labiolingually compressed, with the labial side slightly more convex than the lingual. Tooth bases are circular to somewhat elliptical and the crown apices are strongly pointed. Both the mesial and distal carinae extend from the base to the apex and are less pronounced than those of similar to atoposaurid-like teeth. Enamel ornamentation is characterized by multiple, weakly defined, apicobasally oriented, parallel striations. Teeth of Dakotasuchus are morphologically similar to goniopholid and pholidosaurid teeth (Garrison et al., 2007; Puértolas-Pascual, Rabal-Garcés & Canudo, 2015; Frederickson et al., 2017).

• DINOSAURIA Owen, 1842

• SAURISCHIA Seeley, 1887

• THEROPODA Marsh, 1881

Theropods from the COI are represented by eight complete to partially complete teeth, 20 tooth fragments, and 11 bone fragments. Tooth and bone fragments are too poorly preserved to allow for confident lower level taxonomic identifications. Therefore, only the eight well-preserved teeth are described here. These eight teeth are identified to varying degrees of taxonomic confidence based on qualitative features and the results of the morphological analyses (Fig. 8). Qualitative tooth descriptions are based on Hendrickx, Mateus & Araújo (2015).

• COELUROSAURIA Von Huene, 1914

• COELUROSAURIA indet.

NCSM 33268 (Figs. 9G–9N) is the largest theropod tooth recovered from the COI, with a preserved crown height of 17.5 mm, an estimated crown height of ∼20 mm, a FABL of 8.39 mm, and a basal width of 4.5 mm. The exact crown height is unknown because the apical portion of the tooth is missing. The tooth is robust, lacks recurvature in mesial view, and possesses a wide elliptical basal cross section. The enamel texture is veined and apicobasally oriented. Both the anterior and posterior denticles are subrectangular in shape and centrally positioned. The mesial carinae becomes unserrated towards the base as is typical with tyrannosaurids (Buckley et al., 2010). When considered as part of the Hendrickx, Mateus & Araújo (2014) database (Fig. 8), NCSM 33268 plots within the Coelophysoidea and Nuthetes morphospaces, the former being an improbable identification for a Late Cretaceous theropod and the latter, an enigmatic theropod of indeterminate affinity, but likely referable to dromaeosaurids or tyrannosauroids (Sweetman, 2004; Rauhut, Milner & Moore-Fay, 2010). However, in the PCA and LDA performed with the combined (Larson & Currie, 2013; Williamson & Brusatte, 2014) database NCSM 33268 consistently plots within or adjacent to Tyrannosauroidea and Dromaeosauridae (Fig. S1).

• TYRANNOSAUROIDEA Osborn, 1906

NCSM 33276 (Figs. 9A–9F) is a premaxillary tooth and the only specimen that can be confidently referred to as a tyrannosauroid based on a suite of diagnostic characteristics (Currie, Rigby & Sloan, 1990; Baszio, 1997b; Carr & Williamson, 2004; Zanno & Makovicky, 2011; Williamson & Brusatte, 2014). These include a D-shaped cross section, a lingually oriented carina that arches around the lingual side of the tooth, a greater arc length along the labial face than the lingual face, a lack of denticles and serrations, and a prominent ridge on the lingual surface. The tooth has a crown height of 6 mm and part of the lingual surface, near the apex is worn away.

• MANIRAPTORA Gauthier, 1986

• DROMAEOSAURIDAE Matthew and Brown, 1922

Two teeth are assigned to Dromaeosauridae from the COI. Diagnostic characteristics of dromaeosaurid teeth are outlined in Currie, Rigby & Sloan (1990) and Sankey et al. (2002). Maxillary and dentary teeth are laterally compressed and recurved in lateral view, lack basal constriction, and possess a flattened, oval cross section. Distal denticles are thin, sometimes point apically, and larger than mesial serrations, and the distal carina is often positioned toward the lingual margin. In both datasets, the dromaeosaurid convex hulls have wide regions that overlap with Tyrannosauridae and Richardoestesia. Thus, several teeth may in fact belong to other groups and should be considered tentatively assigned.

NCSM 33267 (Figs. 9O–9U) has a crown height of 10.15 mm, a FABL of 5.03 mm, and a basal width of 3.32 mm. The tooth lacks recurvature in mesial view and has an incrassate figure-eight-shaped cross section (sensu Hendrickx, Mateus & Araújo, 2015). Wear facets extend along the apical portion of the distal carina and across the smooth enamel of the labial surface. The mesial carina lacks serrations and twists lingually near the base. The distal denticles are subrectangular and the distal carina is positioned towards the lingual margin. When considered as part of the Hendrickx, Mateus & Araújo (2014) database, NCSM 33267 plots exclusively within Dromaeosauridae (Fig. 8).

NCSM 33275 (Figs. 9V–9CC) has a crown height of 5.83 mm, a FABL of 3.23 mm, and a basal width of 1.36 mm. The tooth is strongly compressed laterally, lacks recurvature in mesial view, and has a weakly figure-eight-shaped cross section. The enamel texture is smooth and a depression is present on the lingual face. The mesial carina is centrally positioned and the distal carina is positioned centrally at the apex, turning toward the lingual margin near the base. Both mesial serrations and distal denticles are subrectangular. When plotted with the Hendrickx, Mateus & Araújo (2014) database NCSM 33275 plots within Richardoestesia and close to Dromaeosauridae (Fig. 8). However, NCSM 33275 is assigned to Dromaeosauridae because it possesses strong recurvature in lateral view, a characteristic of dromaeosaurid teeth that is not accounted for in the dataset.

• RICHARDOESTESIA Currie, Rigby & Sloan, 1990

• RICHARDOESTESIA sp.

NCSM 33274 (Figs. 10A–10G) is complete and includes part of the root. NCSM 33274 is ascribed to Richardoestesia based on a suite of characteristics outlined in Currie, Rigby & Sloan (1990; see also Sankey et al., 2002) and from the results of the LDA (Fig. 8). NCSM 33274 has a crown height of 4.21 mm, a FABL of 1.65 mm, and a basal width of 0.98 mm. The tooth is tall and slender, lacks curvature in mesial view, is weakly recurved in lateral view, and has an oval to figure-eight-shaped cross section in basal view. The mesial carina lacks serrations and is centrally positioned, whereas the distal carina is positioned centrally at the apex, turning toward the lingual margin near the base. Distal denticles are small and sub rectangular. There is a depression near the base on both the lingual and labial surfaces and the enamel has a weakly braided texture (sensu Hendrickx, Mateus & Araújo, 2015). When considered as part of the Hendrickx, Mateus & Araújo (2014) database, NCSM 33274 plots slightly outside of the Richardoestesia convex hull.

NCSM 33288 (Figs. 10H–10N) has a crown height of 6.58 mm, an estimated crown height of ∼9.00 mm, a FABL of 4.05 mm, and a basal width of 1.9 mm. The exact crown height is unknown because the apical ∼third of the tooth crown is missing. The tooth is laterally compressed, weakly recurved in lateral view, lacks recurvature in mesial view, and has an elliptical cross section. The enamel texture is smooth and a subtle longitudinal depression is present on the lingual face. The mesial carina is centrally positioned, lacks serrations, and is worn away except near the base. The distal carina is positioned centrally at the apex, turning slightly towards the lingual margin near the base. Denticles along the distal carina are sub-rectangular to weakly apically oriented. When considered as part of the Hendrickx, Mateus & Araújo (2014) database, NCSM 33288 plots along a line representative of reconstructed crown height. Nearly all points along the line fall within the overlapping morphospace of Richardoestesia, Nuthetes, and Dromaeosauridae (Fig. 8).

• PARONYCHODON Cope, 1876

• PARONYCHODON sp.

Three teeth are referred to Paronychodon based on a suite of characters outlined by Currie, Rigby & Sloan (1990; see also Rauhut, 2002; Sankey et al., 2002). These teeth are recurved in lateral view, with a flattened lingual face, a convex labial face, numerous longitudinal ridges, oval cross section, and lacking denticles or serrations. Previous studies have tentatively referred Paronychodon to Dromaeosauridae (Antunes & Sigogneau-Russell, 1991), Troodontidae (Osmólska & Barsbold, 1990), and Aves (Rauhut, 2002).

NCSM 33277 (Figs. 10O–10T) has a crown height of 2.4 mm, an estimated crown height of ∼3.3 mm, a FABL of 1.57 mm, and a basal width of 1.00 mm. The exact height is unknown because part of the tooth crown is missing. NCSM 33277 is laterally compressed, recurved in lateral view, and has an elliptical cross section in basal view. The labial face is convex and the lingual face is flattened with a weakly defined longitudinal ridge. The distal carina is positioned lingually and lacks denticles. The mesial carina is worn away except at the base, positioned lingually, and lacks serrations. The enamel texture is smooth and ornamented by numerous longitudinal striations along the lingual and labial surface. When considered as part of the Hendrickx, Mateus & Araújo (2014) dataset, NCSM 33277 plots slightly outside of the troodontid morphospace (Fig. 8).

NCSM 33298 (Figs. 10U–10Z) has a crown height of 2.11 mm, a FABL of 1.16 mm, and a basal width of 0.77 mm. It is recurved in lateral view and has an asymmetrically salinon cross section (sensu Hendrickx, Mateus & Araújo, 2015) in basal view. The labial face is strongly convex and the lingual face is flattened with a well-defined longitudinal ridge. Both the distal and mesial carinae are positioned lingually and lack denticles or serrations and the distal carina is deflected inward. The enamel texture is smooth and there is a slight constriction at the base. When considered as part of the Hendrickx, Mateus & Araújo (2014) database, NCSM 33298 plots slightly outside of the troodontid morphospace (Fig. 8).

NCSM 33308 (Figs. 10AA–10EE) has a crown height of 2.75 mm, a FABL of 1.39 mm, and a basal width of 0.85 mm. It is laterally compressed, lacks recurvature, and has a lenticular cross-section in basal view. The labial and lingual faces are convex, and the lingual face has a weakly defined medial depression near the base. The distal and mesial carinae lack denticles or serrations and the mesial carina twists lingually at the base. The enamel texture is ornamented by numerous longitudinal striations with no real relief extending baso-apically along the lingual and labial surfaces. When considered as part of the Hendrickx, Mateus & Araújo (2014) database, NCSM 33308 plots slightly outside of the troodontid morphospace (Fig. 8). NCSM 33308 may belong to Aves based on a series of characteristics specific to this tooth morphotype that are not accounted for in the analysis such as recurvature, flattened lingual surface, absence of denticles, and an oval cross section. Therefore, NCSM 33308 may be a morphologically distinct bird tooth or possibly a mesially positioned theropod tooth.

• AVES Linnaeus, 1758

• Gen. et sp. indet.

Bird material from this locality is difficult to properly discern, as no complete skeletal material has been documented and teeth vary widely in morphology (Sankey et al., 2002; Mayr et al., 2007; Larson & Currie, 2013; Wilson, Chin & Cumbaa, 2016). Two morphologically distinct teeth from the COI are tentatively ascribed to Avialae.

NCSM 33299 (Figs. 11A–11E) and NCSM 33300 (Figs. 11F–11J) are similar in morphology and size to three teeth from the OMNH referred to Aves: OMNH 28735, 28736, and 28737. These teeth are very small, measuring ∼1 mm in height, are constricted at the base, and have circular cross sections. The crowns are asymmetrical, have weakly defined carinae on both anterior and posterior edges, and lack serrations or denticles. The enamel is smooth and moderately worn, and the crown apex is usually worn flat.

• DINOSAURIA Owen, 1842

• ORNITHISCHIA Seeley, 1887

• ORNITHISCHIA indet.

NCSM 33322 (Figs. 12A–12E) is a heavily worn tooth with a CH of 7.31 mm, a FABL of 9.59 mm, and a BW of 5.71 mm. There is a large wear facet that extends across the apical portion of the labial surface. Even after accounting for wear, the tooth is mesio-distally elongate and apico-basally short. The tooth is constricted below the crown and several worn ridges run apicobasally along the lingual surface. Due to the poor preservation, NCSM 33322 cannot be confidently referred to a lower taxonomic level. However, NCSM 33322 compares favorably with illustrations of worn ankylosaur teeth (e.g., Coombs, 1978, fig. 20.6d). Based on the megaherbivore clades known to have inhabited the Western Interior Basin of North America during the Late Cretaceous, a referral to Ankylosauridae is the most plausible.

NCSM 33318 (Figs. 12F–12G) is a 6.8 mm long tooth fragment, preserving five large denticles. NCSM 33318 compares most favorably with ankylosaurid teeth (Coombs, 1978).

NCSM 33316 (Figs. 12H–12L) is a small, triangular tooth with a CH of 1.4 mm, a FABL of 1.49 mm, and a BW of 0.79 mm, possibly pertaining to Neornithischia, Ankylosauria, or Pachycephalosauria. It has seven cusps, a typical characteristic of premaxillary teeth of basal euornithopods (Oreska, Carrano & Dzikiewicz, 2013), and lacks an elevated rim along with the primary and secondary ridges present in ornithopod teeth such as Zalmoxes (Virág & Ősi, 2017).

NCSM 33312 (Figs. 12M–12Q) is possibly the premaxillary tooth of a neornithischian (Fanti & Miyashita, 2009), with a CH of 0.95 mm, a FABL of 1.18 mm, and a BW of 0.73 mm. It is a small, triangular, scoop-shaped tooth with a constriction below the crown. The labial face is smooth and strongly convex. The lingual face is concave and possesses ∼15 well defined, apicobasally oriented ridges.

NCSM 33314 (Figs. 12R–12W) may represent a small ornithischian tooth, with a CH of 0.45 mm, a FABL of 0.55 mm, and a BW of 0.34 mm. It has a triangular crown that is weakly constricted between the base and root, and appears to have seven weakly defined denticles and apicobasally oriented striations.

• ORNITHOPODA Marsh, 1881

• HADROSAUROIDEA Cope, 1863

NCSM 33320, NCSM 33321, and NCSM 33323 (Figs. 13A–13O) are hadrosauroid teeth possibly referable to Eolambia caroljonesa, the only hadrosauroid yet described from the Mussentuchit Member (Cifelli et al., 1997; Kirkland, 1998; McDonald et al., 2017). The teeth show considerable wear and are marked by linear and branched tubule ridges along the transverse cross section. These tubules produce multiple shearing faces along the chewing surface (Erickson et al., 2012). Each tooth possesses a prominent main keel, and NCSM 33321 (Figs. 13F–13J) shows two additional smaller ridges running nearly parallel to the primary keel. Remnants of denticles on the apical edge of NCSM 33320 and NCSM 33321 demonstrate the presence of these features, but they are too worn to make comparisons.

• MAMMALIA Linnaeus, 1758

• METATHERIA Thomas Henry Huxley, 1880

• MARSUPIALIA Illiger, 1811

• SINBADELPHYS SCHMIDTI Cifelli, 2004

NCSM 33354 (Figs. 14A–14C) appears to be the mesial half of a right upper molar at the second locus, possibly belonging to Sinbadelphys schmidti but also similar to Adelodelphys muizoni (Cifelli, 2004). NCSM 33354 lacks a number of dental features except for the ectoflexus, post-metacrista, and metacone. This makes its precise taxonomic assignment on the basis of dental features difficult. Nonetheless, the dimensions of NCSM 33354 compare favorably to those of S. schmidti. The second molar of S. schmidti (OMNH 26451) has an anteroposterior length of 1.5 mm, and another possible second molar (OMNH 33088) has an anteroposterior length of 1.89 mm. NCSM 33354 has a predicted anteroposterior length ranging from 1.5 to 1.6 mm. Therefore, S. schmidti is the most appropriate known taxon to ascribe to NCSM 33354.

• MARSUPIALIA indet.

NCSM 33355 (Figs. 14D–14G) is the incomplete upper, second premolar of a marsupial, most likely belonging to A. muizoni. This assignment to a marsupial clade is based on comparisons with illustrations in Lillegraven (1969) and Clemens (1966). The dimensional proportions of NCSM 33355 compare favorably with Cretaceous marsupial teeth at the second locus, and the height of the principal cusp indicates it is a maxillary tooth. The length of the second premolar of Cretaceous marsupials is approximately 80% the length of the first lower molar (Lillegraven, 1969; Cifelli, 2004). If NCSM 33355 was complete it would be approximately 1.1 mm in length, 0.2 mm smaller than the estimated length of the M1 of A. muizoni (1.3 mm). Therefore, NCSM 33355 is mostly likely referable to A. muizoni although the specimen could also belong to the slightly larger S. schmidti, which has lower molars lengths between 1.4 and 1.7 mm and estimated premolar lengths between 1.2 and 1.4 mm.

• Ichnofossils

• ELONGATOOLITHIDAE Zhao, 1975

• MACROELONGATOOLITHUS Li, Yin, and Liu, 1995

• MACROELONGATOOLITHUS SP.

A total of 28 fragments of eggshell, belonging to the oogenus Macroelongatoolithus, were recovered from the COI. These fragments exhibit varying degrees of wear and range between 6–20 mm² and 1–3 mm thick (Figs. 15A–15C). Fragments usually possess a subtle curvature along a single axis, suggesting an elongated egg morphology (Zelenitsky, Carpenter & Currie, 2000). Shell material displays variation in ornamentation, similar to material previously collected in the Mussentuchit Member (Zelenitsky, Carpenter & Currie, 2000). These include dispersituberculate and linearituberculate ornamentation, with some fragments having well-defined, bulbous nodes and tubercles.

Biodiversity and paleoecology

We compared the faunal assemblage of the COI sight to 12 OMNH microvertebrate localities from the Mussentuchit Member sourced from Goldberg (2000) (see Data S2 for complete database). Results reveal that the total proportions of all specimens from all sites is skewed toward crocodylomorphs and osteichthyans, constituting 46% and 30%, respectively, whereas mammals are the third most abundant taxonomic group at 10%. Dinosaurian material as a whole makes up 10%, but when the two largest clades are separated their abundance proportion drops (Saurischia at 7% and Ornithischia at 3%). Squamata, Chondrichthyes, Chelonia, and Urodela have the lowest abundances, together making up the remaining 4% (Figs. 16 and 17).

Eight of the 13 Mussentuchit Member sites analyzed are dominated by mesoeucrocodylian remains. This pattern is also present at a site not included in this study (McDonald et al., 2017), raising that value to nine of 14 sites. The most common taxon of secondary abundance in mesoeucrocodylian-dominated localities are osteichthyan fish, although this proportion is nearly equal with sites containing a secondary abundance of mammalian remains (Goldberg, 2000; McDonald et al., 2017). A large percentage of sites are dominated by fish fossils, principally osteichthyans, with secondary abundances represented by mammals, mesoeucrocodylians, and saurischian dinosaurs.

Three sites (COI, OMNH V695, and OMNH V794) account for the majority of fossil material, each contributing nearly a quarter of the total specimens (Fig. 18). Whereas the COI contributes the most specimens to the dataset, it is the least sampled site, with only 183 kg of sediment processed. This gives the COI a fossil density of 22.4 fossils per kg, 53.33% higher than the next densest site (OMNH V240) with a fossil density of 0.42 fossils per kg (Table 1; Fig. 18).

Table 1:

Microfossil locality compilation.

	COI	OMNH V213	OMNH V234	OMNH V235	OMNH V236	OMNH V239	OMNH V240	OMNH V694	OMNH V695	OMNH V696	OMNH V794	OMNH V801	OMNH V868	Total
Total specimens	4,100	37	46	1,413	64	695	808	632	3,979	364	3,877	218	710	16,943
Matrix (kg)	183	775	1,825	4,690	910	1,960	1,915	3,645	14,580	955	11,390	1,820	2,780	47,428
Fossil density	22.4	0.05	0.03	0.3	0.07	0.35	0.42	0.17	0.27	0.38	0.34	0.12	0.26
Proportion	0.24	0.002	0.003	0.08	0.004	0.04	0.05	0.04	0.23	0.02	0.23	0.01	0.04
S–W index	0.45	1.61	1.26	1.35	1.36	1.37	0.91	1.05	1.29	1.38	1.40	0.58	1.61

DOI: 10.7717/peerj.5883/table-1

Note:

Specimen abundances, quantity of processed matrix, fossil density, and biodiversity indexes from all 13 localities.

Rarefaction of the sampled faunas across all localities (Fig. 19) shows that only four of the 13 localities (OMNH 239, 694, 695, 794) have plateaued. In other words, these sites have been sampled enough that it is unlikely a new vertebrate clade will be discovered with increased sampling. The COI site does not appear close to leveling off; therefore, a high probability exists to discover new clades not currently documented in the site fauna from further sampling. After estimating the rarefaction over 4,000 specimens, COI appears equal to OMNH V695 in clade representation. The direction of the rarefaction line for the COI locality is quite different from the OMNH localities, rising at a gradual pace to the end of the graph. This behavior is due to the high number of mesoeucrocodylian teeth and osteichthyan fossils in our sample along with the rarer individual numbers of additional clades. In short, the rarefaction of the COI database continuously provides excessive individuals of mesoeucrocodylians and fish along with occasional other clades, which when amalgamated across the entire resampling protocol produces the slowly increasing line. Compared to other OMNH localities there are key clades that are missing from the COI site, and recalculation of rarefaction curves at lower taxonomic levels might have different outcomes than presented here, yet more refined taxonomic classifications are not yet possible. Given the higher potential for COI to yield additional clades through further sampling, the COI locality could be one of the most taxonomically rich microsites yet described from the Mussentuchit Member.

In contrast, the biodiversity as calculated via the Shannon–Wiener index produces a low value, (0.41; Table 1) compared to other OMNH sites. The Shannon–Wiener index takes into consideration both the number of clades present and the number of individuals per clade in order to calculate biodiversity among sites. Sampling localities that have a larger number of clades and a relatively equal number of individuals per clade will receive a higher biodiversity score than sites that have either low number of clades or those sites that possess a large number of clades, yet a very few of those clades possess nearly all of the individual abundance. Therefore, the high number of crocodylomorph fossils from COI compared to all other clades in that site creates an uneven assemblage that effectively lowers this biodiversity measure. Locality OMNH V868 is the most diverse site by this measure, with a biodiversity index of 1.61. Figure 20 shows the results of the Shannon–Wiener COI subsampling. There is an obvious spread of biodiversity values for the COI locality under all subsampling simulations. However, the most distinguishing pattern is that in all cases the biodiversity of this site remains lower than every OMNH locality.

Interestingly, there is no correlation between the number of fossils collected and the Shannon–Wiener biodiversity index. The slope of the correlation was essentially zero (−5.77 × 10⁻⁵). The four sites that had plateaued rarefaction curves (OMNH V239, V694, V695, and V794) ranged in biodiversity from 1.05 to 1.40, which were all below the peak biodiversity of 1.61 (OMNH V868). This indicates that neither extensive sampling nor abundance of fossils can adequately predict the biodiversity of an individual site.

Similarity, as calculated by the Morisita–Horn index, of the subsampled sites has a much smaller distribution than the Shannon–Wiener values in every comparison, meaning that despite wide differences in biodiversity among subsamples, the similarity of faunas is not sensitive to subsampling techniques. In this index a value of 1 means that all of the clades are shared between localities and the proportions of those clades are also essentially equivalent. Sites OMNH V213 and OMNH V868 have a near identical array of clades compared to COI. The site that is least similar to COI is OMNH V696.

Paleoenvironmental influences

Goldberg (2000) characterized 11 of the 12 OMNH sites as either fluvial oxbow, floodplain, crevasse splay, or channel deposits, basing these inferences on a number of sedimentary features such as clast size, lag deposits, clay balls, slickensides, and plant debris. Among these localities, OMNH V236 and OMNH V239 were interpreted as channel deposits, OMNH V235 and OMNH V240 as splay environments, OMNH V694 and OMNH V695 as floodplain deposits, and OMNH V801 as an abandoned oxbow channel (Fig. 1D). Some uncertainty was noted for select sites including OMNH V794 and OMNH V868, which were interpreted as either channel deposits or splays, and OMNH V234, which was interpreted as either a floodplain or splay deposit.

Correspondence analysis of all 13 sites, using faunal data as a predictor of depositional environment (Fig. 21), shows that site faunal compositions can be associated with specific depositional environments; this ordination plots the COI close to OMNH V801, both of which are interpreted as representing oxbow lake environments. These sites trend to the bottom left of the ordination space due to their high concentration of Mesoeucrocodylia and Chelonia material. However, for the majority of sites taxonomic content is a poor predictor of depositional environment. These include channel or splay deposits and floodplain deposits.

Discussion

Conclusions

The recovery and analyses of over 4,000 new fossil specimens from the COI microvertebrate site has improved our understanding of the Cenomanian vertebrate fauna from Mussentuchit Member of the Cedar Mountain Formation. The site is diverse and includes the remains of Amiidae, lepisosteiforms, pycnodontiforms, salmoniforms, albanerpetontids, helochelydrids, adocids, mesoeucrocodylians, tyrannosauroids, dromaeosaurids, the theropods Richardoestesia and Paronychodon, avialans, a variety of ornithischians, metatherians, and trace fossils assigned to Macroelongatoolithus. The recovery of adocid turtle remains marks a new addition to the Mussentuchit Member assemblage and adds to a growing body of evidence refining the pattern and tempo of faunal interchange between Asia and North America prior to and post the Early/Late Cretaceous boundary. It also represents the earliest occurrence of Adocidae in North America, extending the record of this group by 5 million years. Documenting the fine-scale patterns of the Mussentuchit Member taxa can provide a better understanding of the effects of biogeographic complexity during the Cretaceous of North America.

Our morphometric tests of the eight theropod teeth from the COI using the Hendrickx, Mateus & Araújo (2014) database were informative, yet limited in their ability to accurately predict taxonomic associations. The inconsistencies between where a tooth may plot is likely due to the inability of existing databases to account for subtle shape variation and ambiguous taxonomic referrals due to their reliance solely on a limited series of linear measurements. Future attempts to refine this methodology will incorporate a larger sample set of theropod teeth from the Mussentuchit Member utilizing landmark data and three-dimensional shape analyses.

Finally, we find no consistent relationship between depositional environment and the abundance of vertebrate clades between microvertebrate localities in the Mussentuchit Member (with the possible exception of oxbow environments, which may preserve an overabundance of semiaquatic taxa). Yet also caution that detailed taphonomic studies are needed to parse out whether the lack of paleoenvironmental signal for most depositional environments is the result of non-isotaphonomy or is authentic.

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