A microraptorine (Dinosauria–Dromaeosauridae) from the Late Cretaceous of North America

Abstract

The fossil assemblages of the Late Cretaceous of North America are dominated by large-bodied dinosaur species. Associated skeletons of small dinosaurs are exceedingly rare, and small (<10 kg) carnivorous theropods have not previously been reported from these beds. Here, we describe a small dromaeosaurid from the 75-millionyear-old Dinosaur Park Formation of Alberta, Canada. Hesperonychus elizabethae gen. et sp. nov. is represented by a pelvic girdle from an animal weighing Ϸ1,900 g. Despite its size, the pubes and ilia are coossified, indicating that the animal was somatically mature. This is the smallest carnivorous, nonavian dinosaur known from North America. Phylogenetic analysis of Hesperonychus reveals that it is not closely related to previously described North American dromaeosaurids. Instead, Hesperonychus is a member of the dromaeosaurid clade Microraptorinae, a group containing the 4-winged Microraptor and the feathered Sinornithosaurus, both from the Lower Cretaceous Jehol Group of China. Hesperonychus is the youngest known member of this lineage, extending the temporal range of the clade by 45 million years, and it is the first microraptorine known from North America, providing further evidence for an affinity between the dinosaur faunas of North America and Asia. Study of fossil collections from the Dinosaur Park and Oldman formations of Alberta has revealed numerous isolated bones of small, basal dromaeosaurids, which are tentatively referred to Hesperonychus. These fossils suggest that small dromaeosaurids were a significant component of the carnivore community in this Late Cretaceous biota.Campanian ͉ Dinosaur Park Formation ͉ microraptorinae ͉ theropoda

Mass extinction of lizards and snakes at the Cretaceous–Paleogene boundary

“…A number of small, nonavian dinosaurs are now known from the Late Cretaceous, including alvarezsaurids (37) and microraptorine dromaeosaurids (38), and taphonomic biases almost certainly obscure the true diversity of small dinosaurs (38,39). However, the fact remains that during the late Maastrichtian, small dinosaurs were vastly outnumbered by other small vertebrates, including a minimum of 30 squamates, 18 birds (15), and 50 mammal species (40).…”

Aerodynamic performance of the feathered dinosaur Microraptor and the evolution of feathered flight

“…Wind tunnel model tests included two different leg configurations, two main wing angles of incidence relative to the body and two different tail sizes, all consistent with described Microraptor fossils and known microraptorine (dromaeosaurid) anatomy 4,19 . Tests also encompassed all previously proposed (anatomically feasible) wing configurations for Microraptor (Methods) (Fig.…”

“…As our experiments show that achieving a high SCR was most important for Microraptor's flight and that differences in gliding-leg position only lead to very small differences in performance, sprawled or legs-down configurations are equally likely, especially for medium-length gliding flights. Of these, however, a legs-down gliding configuration is best supported by fossil data 19,23 and has been shown to be advantageous in stabilizing roll and yaw 28 .…”

“…The results, although showing much lower L/D ratio than theoretical predictions 15 , do indicate that gliding was feasible for Microraptor. Serious anatomical issues with the Microraptor model described by Alexander et al 16 have, however, been raised: based on the known 3D anatomy of closely related microraptorines 19 , it is unlikely that Microraptor could have attained the leg positions used for this model 23 .…”

“…Our model Microraptor was tested with all previously proposed leg configurations [15][16][17] , apart from the anatomically impossible, fully sprawled position 16 , as this would have been anatomically impossible 19,23 .…”