The teeth of the unenlagiine theropod Buitreraptor from the Cretaceous of Patagonia, Argentina, and the unusual dentition of the Gondwanan dromaeosaurids

The Unenlagiinae is a clade of Gondwanan dromaeosaurid theropods mainly known from incomplete skeletal material. The group includes two recently discovered theropods, Buitreraptor and Austroraptor, from which cranial remains are available with in situ maxillary and dentary teeth, thus allowing the study of tooth morphology. Among the derived traits that diagnose the dentition of unenlagiines are: (i) high tooth count, (ii) small size of individual teeth when compared with skull height, (iii) absence of denticles and carinae, and (iv) presence of longitudinal grooves on the tooth crown. This suite of dental characteristics, shared between Buitreraptor and Austroraptor, can be considered as diagnostic of the Unenlagiinae or, at least, a more exclusive clade within the group. The teeth of Buitreraptor exhibit a remarkable labiolingual compression, whereas Austroraptor possesses more conical teeth, probably respective autapomorphic features. On one hand, these dental morphologies differ from those observed in most Laurasian dromaeosaurids and, for instance, could be considered as further proof of the purported vicariant evolution of the lineage on the southern continents. On the other hand, the morphological similarities (e.g., absence of denticles) between the teeth of unenlagiines and other theropod lineages, including Mesozoic birds and ornithomimosaurs, can be considered as the result of parallel trends related to dental reduction

Theropod teeth from the upper Maastrichtian Hell Creek Formation “Sue” Quarry: New morphotypes and faunal comparisons

Isolated teeth from vertebrate microfossil localities often provide unique information on the biodiversity of ancient ecosystems that might otherwise remain unrecognized. Microfossil sampling is a particularly valuable tool for documenting taxa that are poorly represented in macrofossil surveys due to small body size, fragile skeletal structure, or relatively low ecosystem abundance. Because biodiversity patterns in the late Maastrichtian of North American are the primary data for a broad array of studies regarding non-avian dinosaur extinction in the terminal Cretaceous, intensive sampling on multiple scales is critical to understanding the nature of this event. We address theropod biodiversity in the Maastrichtian by examining teeth collected from the Hell Creek Formation locality that yielded FMNH PR 2081 (the Tyrannosaurus rex specimen “Sue”). Eight morphotypes (three previously undocumented) are identified in the sample, representing Tyrannosauridae, Dromaeosauridae, Troodontidae, and Avialae. Noticeably absent are teeth attributed to the morphotypes Richardoestesia and Paronychodon. Morphometric comparison to dromaeosaurid teeth from multiple Hell Creek and Lance formations microsites reveals two unique dromaeosaurid morphotypes bearing finer distal denticles than present on teeth of similar size, and also differences in crown shape in at least one of these. These findings suggest more dromaeosaurid taxa, and a higher Maastrichtian biodiversity, than previously appreciated

A basal ceratopsian with transitional features from the Late Jurassic of northwestern China

Although the Ceratopsia and Pachycephalosauria, two major ornithischian groups, are united as the Marginocephalia, few synapomorphies have been identified due to their highly specialized body-plans. Several studies have linked the Heterodontosauridae with either the Ceratopsia or Marginocephalia, but evidence for these relationships is weak, leading most recent studies to consider the Heterodontosauridae as the basal member of another major ornithischian radiation, the Ornithopoda. Here, we report on a new basal ceratopsian dinosaur, Yinlong downsi gen. et. sp. nov., from the Late Jurassic upper part of the Shishugou Formation of Xinjiang, China. This new ceratopsian displays a series of features transitional between more derived ceratopsians and other ornithischians, shares numerous derived similarities with both the heterodontosaurids and pachycephalosaurians and provides strong evidence supporting a monophyletic Marginocephalia and its close relationship to the Heterodontosauridae. Character distributions along the marginocephalian lineage reveal that, compared to the bipedal Pachycephalosauria, which retained a primitive post-cranial body-plan, the dominantly quadrupedal ceratopsians lost many marginocephalian features and evolved their own characters early in their evolution