Maastrichtian Hell Creek Formation Research Articles

New enantiornithine diversity in the Hell Creek Formation and the functional morphology of the avisaurid tarsometatarsus.

Enantiornithines were the most diverse group of birds during the Cretaceous, comprising over half of all known species from this period. The fossil record and subsequently our knowledge of this clade is heavily skewed by the wealth of material from Lower Cretaceous deposits in China. In contrast, specimens from Upper Cretaceous deposits are rare and typically fragmentary, yet critical for understanding the extinction of this clade across the K-Pg boundary. The most complete North American Late Cretaceous enantiornithine is Mirarce eatoni, a member of the diverse clade Avisauridae. Except for Mirarce, avisaurids are known only from isolated hindlimb elements from North and South America. Here we describe three new enantiornithines from the Maastrichtian Hell Creek Formation, two of which represent new avisaurid taxa. These materials represent a substantial increase in the known diversity of Enantiornithes in the latest Cretaceous. Re-examination of material referred to Avisauridae through phylogenetic analysis provides strong support for a more exclusive Avisauridae consisting of six taxa. Exploration of the functional morphology of the avisaurid tarsometatarsus indicates potential strong constriction and raptorial attributes. The lower aspect ratio of the tarsometatarsus facilitates a more biomechanically efficient lever system which in extant birds of prey equates to lifting proportionally heavier prey items. In addition, the proportional size and distal position of the m. tibialis cranialis tubercle of the tarsometatarsus is similar to the morphology seen in extant birds of prey. Together with the deeply-grooved metatarsal trochlea facilitating robust and likely powerful pedal digits, morphologies of the hindlimb suggest avisaurids as Late Cretaceous birds of prey.

A new oviraptorosaur (Dinosauria: Theropoda) from the end-Maastrichtian Hell Creek Formation of North America.

Caenagnathidae is a clade of derived, Late Cretaceous oviraptorosaurian theropods from Asia and North America. Because their remains are rare and often fragmentary, caenagnathid diversity is poorly understood. Anzu wyliei is the only caenagnathid species currently described from the late Maastrichtian Hell Creek Formation of the USA and is also among the largest and most completely preserved North American caenagnathids. Smaller, less complete caenagnathid material has long been known from the Hell Creek Formation, but it is unclear whether these are juvenile representatives of Anzu or if they represent distinct, unnamed taxa. Here, we describe a relatively small caenagnathid hindlimb from the Hell Creek Formation, and conduct osteohistological analysis to assess its maturity. Histological data and morphological differences from Anzu wyliei and other caenagnathids allow us to conclude that this specimen represents a new species of caenagnathid from the Hell Creek Formation, with a smaller adult body size than Anzu. This new taxon is also distinct from other small caenagnathid material previously described from the area, potentially indicating the coexistence of three distinct caenagnathid species in the Hell Creek Formation. These results show that caenagnathid diversity in the Hell Creek ecosystem has been underestimated.

Caenagnathids (Theropoda, Oviraptorosauria) from the uppermost Maastrichtian of the Scollard Formation of Alberta, Canada

Recent work has greatly improved our understanding of the diversity and paleobiogeography of caenagnathids in North America. However, the late Maastrichtian caenagnathid record remains poorly understood, with only two species established from this time interval. Here we describe three isolated elements of caenagnathids recovered from the base of the upper Maastrichtian portion of the Scollard Formation of Alberta, Canada. Two specimens, a metatarsal II and a pedal ungual, represent large-bodied species and are most similar to the same elements in Anzu wyliei (upper Maastrichtian Hell Creek Formation) and Caenagnathus collinsi (upper Campanian Dinosaur Park Formation). The third specimen, a pedal ungual from an immature individual, is most similar to Leptorhynchos gaddisi (upper Campanian Aguja Formation). Preservation of only isolated elements from the Scollard Formation and a scarcity of relevant comparative specimens from the upper Maastrichtian in general, hinders taxonomic assignment of these specimens to the genus level. Regardless, these specimens provide evidence for the widespread and northerly occurrence of caenagnathids, including large-bodied forms, in North America during the last one million years of the Cretaceous.

The promise of taphonomy as a nomothetic discipline: taphonomic bias in two dinosaur-bearing faunas in North America1

The fossil record of dinosaurs is a rich, if biased, one with nearly complete skeletons, partial skeletons, and isolated parts found in diverse, well-studied faunal assemblages around the world. Among the recognized biases are the preferential preservation of large dinosaurs and the systematic underrepresentation of small dinosaurs. Such biases have been quantitatively described in the Upper Cretaceous (Campanian) Dinosaur Park Formation of Alberta, where large, nearly complete dinosaurs were found and described early in collecting history and small, very incomplete dinosaurs were found and described later. This pattern, apparently replicated in the Maastrichtian Hell Creek Formation of Montana, is so striking that it begs the question of whether this is a nomothetic principle for the preservation of dinosaur faunas elsewhere. We tested this hypothesis by analyzing the very well-studied dinosaur fauna of the Upper Jurassic (Kimmeridgian) Morrison Formation of the western United States. The Morrison Formation fails to show any correlation between body size and completeness, order of discovery, or order of description. Both large and small dinosaurs of the Morrison include highly complete as well as highly incomplete taxa, and both large and small dinosaurs were discovered and described early in collecting history as well as more recently. The differences in preservation between the Dinosaur Park Formation and the Morrison Formation are so striking that we posit a Dinosaur Park model of dinosaur fossil preservation and a Morrison model. Future study will show whether either or both represent durable nomothetic models for dinosaur fossil preservation.

New manual unguals of Trierarchuncus prairiensis from the Hell Creek Formation, Montana, and the ontogenetic development of the functional alvarezsaurid hand claw

The Alvarezsauridae comprise a clade of small-bodied theropod dinosaurs with highly-specialized skeletal adaptations, primarily reduced but proportionately robust forelimbs which have been hypothesized as relating to an insectivorous ecology. Alvarezsaurids are known predominantly from Mongolia, China, and South America, with a comparatively sparse North American record, where alvarezsaurid remains are among the most rarely recovered vertebrate fossils from the Upper Cretaceous Western Interior. North American alvarezsaurids include Albertonykus borealis from the lower Maastrichtian Horseshoe Canyon Formation of Alberta, Canada, known from a partial skeleton and additional isolated remains, and the recently named Trierarchuncus prairiensis from the upper Maastrichtian Hell Creek Formation of eastern Montana, USA, known previously from isolated specimens, including three manual D-I unguals, a partial radius, and a partial metatarsal III. Here we describe two new manual D-I unguals referable to the alvarezsaurid Trierarchuncus from the Hell Creek Formation of eastern Montana, USA. These new unguals contribute additional detail to the currently known ontogenetic series of Trierarchuncus manual D-I unguals and indicate the sequence and degree of change that occurred through growth within these alvarezsaurid hand claws. These changes appear to be modifications and reinforcement related to stresses induced from the hypothesized ecology of alvarezsaurids as insectivores which would tear apart substrates in search of insect prey, and as such can be characterized as ecologically-driven ontogenetic changes. Additionally, we demonstrate morphological indicators for determining whether isolated alvarezsaurid manual D-I unguals pertain to the right or left manus.

Trierarchuncus prairiensis gen. et sp. nov., the last alvarezsaurid: Hell Creek Formation (uppermost Maastrichtian), Montana

The enigmatic Alvarezsauria (Dinosauria: Theropoda) are characterized by extremely short forelimbs with a single functional digit bearing a large, robust ungual. Alvarezsauria are first recorded from the Jurassic of China, but are otherwise mostly known from the Cretaceous of South America and Asia, including a number of relatively complete skeletons. North America has yielded only a fragmentary skeleton from the lower Maastrichtian Horseshoe Canyon Formation, Alberta, and a pubis, partial ischium, and metatarsal from the upper Maastrichtian Hell Creek Formation, Montana, and Lance Formation, Wyoming. Here we describe new alvarezsaurid material (Trierarchuncus prairiensis gen. et sp. nov.) from the Hell Creek Formation, Montana, comprising a metatarsal III, distal end of a radius, and three manual digit (MD)-I unguals, which form a progressive size series. One MD-I ungual is the most complete yet described, and is much more curved than typically depicted for Alvarezsauridae. Manual D-I unguals are of particular interest as they undergo a number of changes within the clade, including enclosure of the ventral blood vessel groove, development of a ventral sulcus, and increased robusticity and rugosity. Comparison among the new specimens suggests that these features also develop ontogenetically, which may have taxonomic implications. Stratigraphic data shows that alvarezsaurids occur through most of the ~85 m thick Hell Creek Formation, with the uppermost specimen occurring ~10 m below the upper contact with the Fort Union Formation. As such these are the youngest known alvarezsaurid remains and demonstrate that the clade survived at least until ~1–200 kyrs before the Cretaceous–Paleogene mass extinction.

Patterns of soft tissue and cellular preservation in relation to fossil bone tissue structure and overburden depth at the Standing Rock Hadrosaur Site, Maastrichtian Hell Creek Formation, South Dakota, USA

Recovery of soft tissues and cells from fossil bones is becoming increasingly common, with structures morphologically consistent with vertebrate osteocytes, blood vessels, fibrous/collagenous matrix, and potential intravascular contents now recognized from specimens dating back to the Permian. However, it largely remains unclear how bone tissue structure, early diagenetic regimes, and many other taphonomic variables influence or control the preservation potential of soft tissues in vertebrate fossils. To explore the influence of a few of these factors, we tested a suite of fossils from the Standing Rock Hadrosaur Site, a vast Edmontosaurus annectens bonebed in the Maastrichtian Hell Creek Formation of South Dakota, for preservation of cellular and tissue components. Demineralization of bone samples from each specimen yielded abundant microstructures morphologically consistent with vertebrate osteocytes, blood vessels, and collagenous matrix. This includes the first recovery of osteocytes and vessels from a fossil vertebral centrum and ossified tendons. Perhaps surprisingly, no correlation was found between soft tissue/cellular recovery and either bone tissue structure type (cortical vs. cancellous) or overburden depth at the time of discovery. A traditional taphonomic survey of the site, conducted in parallel and reported previously, affords a clear and detailed history of these remains, both pre- and postburial. Cumulative taphonomic evidence indicates the Edmontosaurus individuals died in a mass mortality event and their disarticulated remains were buried rapidly in a shallow floodplain pond during a crevasse splay event. Oxygenated flood waters and/or groundwater oxidized initially sideritic concretions to goethite during early diagenesis, facilitating rapid cementation of portions of the sediment that likely aided stabilization of soft tissues by shielding regions of the bones from prolonged exposure to pore fluids. Our findings support cancellous bone as a viable target for cellular analyses, corroborate previous propositions that iron-rich environments and rapid burial facilitate soft tissue preservation, and provide new details into early diagenetic environments conducive to such preservation.

TAPHONOMY OF THE STANDING ROCK HADROSAUR SITE, CORSON COUNTY, SOUTH DAKOTA

We present taphonomic analyses of the Standing Rock Hadrosaur Site (SRHS), a vast Edmontosaurus annectens bonebed in the Maastrichtian Hell Creek Formation of South Dakota, which yields important insights into hadrosaurid paleobiology and environmental settings recorded by basal Hell Creek strata. Though Edmontosaurus bonebeds have been described from other Late Cretaceous formations in the Western Interior, namely the Lance, Prince Creek, and Horseshoe Canyon formations, our study provides the first thorough description of an Edmontosaurus bonebed from the Hell Creek Formation. SRHS is also the first formally described bonebed of E. annectens. Taphonomically, representation of every skeletal element, horizontality of most bones, and rarity of weathering and abrasion suggest brief preburial exposure and transport with minimal sorting bias. Near-universal disarticulation and disassociation, localized orientation of bones, and infrequent preburial breakage indicate moderate flow energy during deposition. Additional fauna, though rare, are indicative of a fluvial-coastal setting, and palynofloral analyses signify deposition in a small, shallow floodplain lake surrounded by cypress forests. Cumulatively, these data indicate that a herd of primarily subadult and adult Edmontosaurus died in a nearby fluvial setting in a mass mortality event and, following brief decay and scavenging by theropods, their bones were buried in a shallow floodplain pond by a flooding event/crevasse splay. Our findings provide supporting evidence for the hypotheses of gregarious herding behavior in hadrosaurids and age structuring of Edmontosaurus herds.

The Hell Creek Formation and its contribution to the Cretaceous–Paleogene extinction: A short primer

Although it represents but one geographic data point, the uppermost Maastrichtian Hell Creek Formation (HCF), exposed in the upper Great Plains of the North American craton, remains the most studied source for understanding the final ∼1.5 Myr of the Mesozoic Era in the terrestrial realm. Because it lies conformably below the earliest Paleocene Fort Union Formation, and together these two units preserve a rich fauna and flora, much of what is understood about the terrestrial Cretaceous–Paleogene (K–Pg) boundary comes from this sequence.The HCF has been reconstructed as an expansive, fluvially drained, low coastal plain, built out, to the west, against the Laramide Orogen, and to the east, against the ultimate transgression (Cannonball) of the Western Interior Sea. Its meandering rivers and moist soils supported a multi-tiered angiosperm-dominated flora and rich insect and vertebrate faunas, including dinosaurs, crocodilians, squamates, turtles, and mammals. A dramatic facies change representing the initiation of catastrophic flooding is preserved, within available levels precision, at the K–Pg boundary.High-precision stratigraphy has proven difficult in this lenticular fluvial system. Where present, the boundary can be recognized by the bipartite boundary claystone; otherwise, palynostratigraphy has proven a powerful tool. Numerical dates have been successfully obtained from in tonsteins at the boundary and above, in the Fort Union; however, these have proven elusive below the boundary within the HCF. The K–Pg boundary in this region is dated at 66.043 Ma (Renne et al., 2013). Magnetostratigraphic studies have been carried out in the HCF; although all but one have lacked numerical dates, these have been used for correlations of widespread, disjunct exposures and for the estimation of sedimentation rates.The palynoflora is largely homogenous through the HCF; at the K–Pg boundary, it shows an abrupt ∼30% extinction. This makes it a powerful tool for identification of the K–Pg boundary, although because the boundary is identified on absence of Cretaceous taxa rather than presence of earliest Paleocene taxa, several competing methods have been applied to identifying the K–Pg boundary using pollen.The macroflora, consisting largely of leaves, consists of three successive floras, showing increasing diversity through the HCF. The ultimate of these three floras undergoes an abrupt 57% extinction; taken as a whole, however, the macroflora undergoes a 78% extinction at the K–Pg boundary.The best data available for dinosaurs – including archaic Aves – show an abrupt extinction. By contrast, salamanders and other lissamphibians, as well as chelonians, cross the boundary virtually without perturbation. Squamates appear to have suffered significant extinctions at the K–Pg boundary, as did euselachians (elasmobranchs) and insects. Mammals suffered a 75% extinction; however, some of this figure cannot be shown to have occurred in less than the last 500 kyr of the Cretaceous, and thus has been potentially attributable to causes other than a bolide impact. Taken together, the survivorship patterns are concordant with the catastrophic inception of ubiquitous flooding characterizing the K–Pg boundary.While the key K–Pg boundary question in the HCF was once the rate of the biotic extinction, it has moved to the distinction between single-cause scenarios, with the Chicxulub bolide as agent of extinction, and multi-cause scenarios, uniting habitat partitioning, Deccan flood-basalt volcanism, climate change, competition, and bolide impact. Not every potential environmental perturbation need be a mechanism for the extinction: parsimony and the data continue to be concordant with a bolide impact as the single agent of the terrestrial K–Pg mass extinction.

New material of the large-bodied caenagnathid Caenagnathus collinsi from the Dinosaur Park Formation of Alberta, Canada

A reexamination of large caenagnathid material from the Upper Cretaceous (Campanian) Dinosaur Park Formation of Alberta, Canada, reveals undescribed material referable to Caenagnathus collinsi. A femur, two astragalocalcanei, two metatarsals, two unguals, and a caudal vertebra provide anatomical information on Caenagnathus collinsi. Estimates of femoral length based on the proportions of other oviraptorids suggest that the non-femoral material represents a taxon intermediate in size between Chirostenotes pergracilis from the Dinosaur Park Formation and Anzu wyliei from the Maastrichtian Hell Creek Formation. The femur is within the range of predictions, and confirms the body size estimates based on the other material. The large size of the material and a number of morphological characters distinguish the material from Chirostenotes pergracilis and suggest that it is referable to Caenagnathus collinsi. The relative diversity of caenagnathids in the Dinosaur Park Formation is likely underestimated.

Basal ornithopod (Dinosauria: Ornithischia) teeth from the Prince Creek Formation (early Maastrichtian) of Alaska

A diverse and prolific record of polar dinosaurs comes from the Upper Cretaceous (Campanian–Maastrichtian) sediments of the Prince Creek Formation exposed on Alaska’s North Slope. Previous assignment of basal ornithopod material from this formation has been based solely on teeth, which have either been referred to “hypsilophodontid” indet. or Thescelosaurus sp. Here, we re-examine this material and describe several new specimens, including five isolated premaxillary teeth and three cheek teeth. The premaxillary teeth are most similar to those of Thescelosaurus , whereas the cheek teeth are more similar to its sister taxon Parksosaurus , for which premaxillary teeth are unknown. Referral of this new material to Thescelosaurus would represent the oldest occurrence of this taxon and considerably extend its stratigraphic range. A more likely possibility is that the premaxillary teeth are referable to Parksosaurus, an interpretation that is more parsimonious from a stratigraphic perspective. Intriguingly, one cheek tooth previously referred to as “hypsilophodontid” cannot be referred to either Thescelosaurus or Parksosaurus. Previously, faunal comparisons of the Prince Creek Formation have largely been made with non-contemporaneous formations, including the Campanian-aged Judith River and Aguja formations, or to the latest Maastrichtian Hell Creek Formation. On the basis of age and faunal similarities, a more appropriate comparison should be made with coeval rocks of the Horseshoe Canyon. This study expands our knowledge of Cretaceous ornithischian diversity at polar paleolatitudes and underscores the importance of small, rare, or easily misidentified fossils in paleoecological studies.

Comparative Taphonomy and Paleoecological Reconstruction of Two Microvertebrate Accumulations from the Late Cretaceous Hell Creek Formation (Maastrichtian), Eastern Montana

Although microvertebrate accumulations are commonly used for paleoecological reconstructions, taphonomic processes affecting the final taxonomic composition of an accumulation are often ignored. This research explores the effects of abiotic taphonomic processes on the taxonomic composition of terrestrial microvertebrate accumulations by comparing a floodplain and a channel lag deposit from the Maastrichtian Hell Creek Formation in eastern Montana. Distribution of skeletal elements with specific physical attributes and relative abundance of taxa correlate with the hydraulic indicators (i.e., grain size, sedimentary structures) of the depositional facies. Transport distances, hydraulic equivalencies of dominant skeletal elements, amount of hydraulic sorting and reworking, and degree of time averaging vary between deposits and significantly affect taxonomic distributions. Relative abundance data, in conjunction with chi-square test results and rank-order analysis, show that size, shape, abrasion, and taxonomic compositions vary significantly between assemblages. The fine-grained assemblage is dominated by tabular, low-density elements, such as cycloid scales and fish vertebrae. Dense, equidimensional elements, such as teeth and ganoid fish scales, dominate the sandstone assemblage. Rank-order analysis results demonstrate that relative abundance of hydraulically equivalent skeletal elements from morphologically similar organisms can be compared regardless of accumulation in nonisotaphonomic deposits. Statistical comparisons were made among osteichthyans using ganoid scales, caudates using vertebrae, ornithischians using teeth, and testudinates using shell fragments. Results show that portions of the assemblage analyzed using hydrodynamically equivalent elements are not significantly different, despite different depositional environments.

Microscale δ18O and δ13C isotopic analysis of an ontogenetic series of the hadrosaurid dinosaur Edmontosaurus: implications for physiology and ecology

Stable isotope analysis of vertebrate biominerals, primarily in mammals, has been used to address questions of paleodiet, paleoclimate, trophic level, migration, foraging zone, and thermophysiology with varying degrees of success. Isotopes have been used less commonly to study physiology and ecology through ontogeny in dinosaurs, generally due to (1) the lack of modern analogs as a basis for comparison of observed fossil isotope values, and (2) difficulty in sampling very thin tooth enamel (a non-remodeled biomineral). By utilizing a relatively new technique in microsampling, this study addresses the following questions: Do microscale analyses of oxygen and carbon isotopes from mineralized tissues of hadrosaurid dinosaurs record temporal variation? If so, is the cause of the variation physiological or ecological? Isotope values from the carbonate component of enamel ( δ 18O ec and δ 13C e) were obtained by microsampling multiple teeth in a temporal series from the dental batteries of a juvenile, sub-adult, and adult Edmontosaurus from the Late Cretaceous Maastrichtian Hell Creek Formation of South Dakota. To establish isotope variability in an extant archosaur, consecutive teeth in a temporal series from an extant Alligator mississippiensis specimen were microsampled for isotopic analysis and compared to those of Edmontosaurus. To test for diagenesis, bulk samples from the phosphate component ( δ 18O p) of modern and fossil tooth enamel, bone, and dentine from Edmontosaurus, A. mississippiensis, and extant ratites were analyzed and compared. Edmontosaurus bone and dentine indicate a greater degree of alteration than does enamel, and while absolute δ 18O ec values may be altered, the pattern of seasonal cycles appears to be preserved and can provide detailed information on hadrosaur physiology (tooth mineralization times, rates, and seasons) and ecology (dietary information). δ 18O ec seasonal patterns are preserved in Edmontosaurus specimens, and are interpreted to correlate with annual δ 18O variation of local meteoric waters rather than thermophysiology, changes in drinking water sources, or migration. All teeth were mineralized in <0.65 year with no consistent season of mineralization. Mean tooth mineralization times are shorter in the juvenile and sub-adult than the adult. Enamel mineralization rates are estimated to be ∼38 mm/year in Edmontosaurus and ∼36 mm/year in Alligator (consistent with mineralization rates for modern ungulates), although the length of time for tooth formation is shorter in the archosaurs compared with mammals. Heavier than predicted δ 13C e values are hypothesized to result from (1) enrichment of δ 13C in ingested plant material due to higher atmospheric δ 13C ( δ 13C atm) in the Late Cretaceous; (2) taxon-specific δ 13C effects of ingested plants (primarily gymnosperms); (3) isotopic enrichment of ingested plant material ( δ 13C p) due to osmotic stress from proximity to the Western Cretaceous Interior Seaway, (4) taxon-specific δ 13C diet– δ 13C e fractionation factors for Edmontosaurus that vary from those observed in modern mammals, and/or (5) diagenesis. Microsampling provides a detailed perspective on the physiological (tooth mineralization times, rates, and seasons) and ecological (dietary) mechanisms of oxygen and carbon isotope incorporation in dinosaur biominerals that is not obtainable through bulk sampling alone.

A new genus of derived pachycephalosaurian from western North America

ABSTRACT Pachycephalosaurian specimens from the Upper Cretaceous (upper Campanian) Kirtland Formation of northwestern New Mexico include a partial skull that preserves much of the basicranium. It represents a new genus and species, Sphaerotholus goodwini. A new species, S. buchholtzae, from the upper Maastrichtian Hell Creek Formation of Montana is referred to this genus. Additional New Mexico pachycephalosaurians include a partial dentary with associated skull material that is tentatively referred to S. goodwini and a partial frontoparietal that is referred to Pachycephalosauridae incertae sedis. On the basis of a hypothetical growth series of Stegoceras, we excluded characters based on dome development and suture closure from a cladistic analysis of pachycephalosaurian relationships. Ornatotholus browni, the only putative “flat-headed” pachycephalosaurian from North America, is considered a nomen dubium and may represent a juvenile Stegoceras. Gravitholus albertae is an adult Stegoceras sp.; Stegoceras edmontonense is a nomen dubium and is referred to cf. Sphaerotholus sp. Based on the results of a quantitative cladistic analysis, Stegoceras (including Stegoceras breve, S. lambei, S. stembergi, and UCMP 130051) is the sister taxon to all other domed pachycephalosaurians. Derived pachycephalosaurids consist of two principal clades: a lineage that includes Stygimoloch, Pachycephalosaurus, and Sphaerotholus and a lineage represented by the Asian taxa, Tylocephale and Prenocephale. With biogeographic occurrences mapped onto the phytogeny, a single dispersal event from Asia into North America, followed by dispersal of Prenocephale and Tylocephale into Asia prior to the late Campanian is indicated (ACCTRAN) or two independent dispersals into North America prior to the late Campanian is indicated (DELTRAN). Pachycephalosaurian phylogeny does not support Asian-North American contiguity throughout the Campanian and Maastrichtian.

Ugrosaurus olsoni, a new ceratopsian (Reptilia: Ornithischia) from the Hell Creek Formation of eastern Montana

Fragments of a ceratopsian dinosaur from the Maastrichtian Hell Creek Formation, Garfield County, Montana, are unlike any known ceratopsian. Diagnostic features include a low, rounded nasal boss, an especially deep rostral-premaxillary pit, and paired choanae. The snout is elongate. The new taxon, Ugrosaurus olsoni, is referred to the Ceratopsidae.