Abstract
BackgroundTroodontids are a predominantly small-bodied group of feathered theropod dinosaurs notable for their close evolutionary relationship with Avialae. Despite a diverse Asian representation with remarkable growth in recent years, the North American record of the clade remains poor, with only one controversial species—Troodon formosus—presently known from substantial skeletal remains.Methodology/Principal FindingsHere we report a gracile new troodontid theropod—Talos sampsoni gen. et sp. nov.—from the Upper Cretaceous Kaiparowits Formation, Utah, USA, representing one of the most complete troodontid skeletons described from North America to date. Histological assessment of the holotype specimen indicates that the adult body size of Talos was notably smaller than that of the contemporary genus Troodon. Phylogenetic analysis recovers Talos as a member of a derived, latest Cretaceous subclade, minimally containing Troodon, Saurornithoides, and Zanabazar. MicroCT scans reveal extreme pathological remodeling on pedal phalanx II-1 of the holotype specimen likely resulting from physical trauma and subsequent infectious processes.Conclusion/Significance Talos sampsoni adds to the singularity of the Kaiparowits Formation dinosaur fauna, which is represented by at least 10 previously unrecognized species including the recently named ceratopsids Utahceratops and Kosmoceratops, the hadrosaurine Gryposaurus monumentensis, the tyrannosaurid Teratophoneus, and the oviraptorosaurian Hagryphus. The presence of a distinct troodontid taxon in the Kaiparowits Formation supports the hypothesis that late Campanian dinosaurs of the Western Interior Basin exhibited restricted geographic ranges and suggests that the taxonomic diversity of Late Cretaceous troodontids from North America is currently underestimated. An apparent traumatic injury to the foot of Talos with evidence of subsequent healing sheds new light on the paleobiology of deinonychosaurians by bolstering functional interpretations of prey grappling and/or intraspecific combat for the second pedal digit, and supporting trackway evidence indicating a minimal role in weight bearing.
Highlights
Troodontids are an intriguing clade of feathered theropods known to possess the smallest body sizes [1,2,3] and largest relative brain sizes [4] among non-avian dinosaurs, and widely hypothesized to be among the closest extinct relatives of birds [2,5,6,7]
Taxonomic Considerations Historically, Late Cretaceous North American troodontid materials were classified under the nomina Troodon formosus [23], Polyodontosaurus grandis [24], and Stenonychosaurus inequalis [22]
Soon thereafter Russell [26] synonymized Polyodontosaurus, and Ornithomimus altus [27] under Stenonychosaurus inequalis based on new materials of the latter, yet noted a lack of sufficient evidence to synonymize these materials with Troodon formosus
Summary
Troodontids are an intriguing clade of feathered theropods known to possess the smallest body sizes [1,2,3] and largest relative brain sizes [4] among non-avian dinosaurs, and widely hypothesized to be among the closest extinct relatives of birds [2,5,6,7]. The past decade has witnessed the discovery of several remarkable specimens that support a close relationship with Avialae and provide unprecedented glimpses into the biology of non-avian dinosaurs. The taxonomic resolution of North American troodontid materials is obfuscated by a paucity of well-preserved skulls and associated skeletons. This preservational bias has resulted in a series of reclassifications and synonymizations that span over 150 years of scientific inquiry and remain a continuous point of debate.
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