A new dromaeosaurid (Dinosauria: Theropoda) with Asian affinities from the latest Cretaceous of North America

Abstract

Dromaeosaurids from the Maastrichtian of North America have a poor fossil record and are known largely from isolated teeth, which have typically been referred to taxa based on more complete material from earlier Campanian strata. An almost complete maxilla with well-preserved dentition and an associated dentary from the Hell Creek Formation of Montana are used to establish a new dromaeosaurid taxon in the latest Maastrichtian, immediately prior to the end-Cretaceous extinction event. Acheroraptor temertyorum gen. et sp. nov. is differentiated from other dromaeosaurids on the basis of a hypertrophied postantral wall that projects posteriorly into the antorbital fenestra, a maxillary fenestra positioned low in the antorbital fossa and directly posterior to the promaxillary fenestra, and distinctive dentition with marked apicobasal ridges. The new material allows a dromaeosaurid from the Maastrichtian of North America to be placed within a phylogenetic framework for the first time. Phylogenetic analysis suggests Acheroraptor is a velociraptorine that is more closely related to Asian dromaeosaurids, including Tsaagan and Velociraptor, than it is to Dromaeosaurus, Saurornitholestes, or any other taxon from North America. As part of the Lancian Tyrannosaurus-Triceratops fauna, A. temertyorum is the latest occurring dromaeosaurid. Its relationships and occurrence suggest a complex historical biogeographic scenario that involved multiple, bi-directional faunal interchanges between Asia and North America during the Late Cretaceous.

Small Theropod Teeth from the Late Cretaceous of the San Juan Basin, Northwestern New Mexico and Their Implications for Understanding Latest Cretaceous Dinosaur Evolution

“…Several have a mesial carina that curves lingually, but the twist in the carina typically occurs at a point closer to the base of the tooth than the midpoint of the carina. Denticles are largest near the middle of the carina and decrease in size basally and apically as is typical for dromaeosaurids [24]. Denticles range from being rounded to asymmetrical with apically-hooked denticles.…”

“…The mesial denticles are smaller than the distal denticles, but the largest mesial denticle is more than half the size of the largest distal denticle. The denticles are rounded as in Acheroraptor [24] rather than strongly apically-hooked as in Saurornitholestes langstoni [21].…”

“…Most of the fossil evidence for small theropod dinosaurs in fossil faunas of the Western Interior are from isolated teeth [10], [17], [18], [19], [20], [21], [22], [23] which were typically recovered from microvertebrate fossil concentrations using screenwashing techniques. Amazingly, it was only very recently that the first diagnostic skeletal remains of a North American Maastrichtian dromaeosaurid were recovered, despite the recovery of thousands of teeth of these animals over the past several decades [24].…”

A Late Cretaceous diversification of Asian oviraptorid dinosaurs: evidence from a new species preserved in an unusual posture

“…This is not unprecedented: one of the very few well-sampled small theropod faunas, the Yixian Formation of northeastern China, exhibits a staggering diversity of small carnivorous and omnivorous dromaeosaurids53545556. It may be that a high diversity of small theropods was common in individual dinosaur faunas, but has gone unrecognized because of preservational bias against small dinosaur fossils5758.…”

First Ornithomimid (Theropoda, Ornithomimosauria) from the Upper Cretaceous Djadokhta Formation of Tögrögiin Shiree, Mongolia

“…A complex palaeobiogeographic aspect involving multiple dispersals between Asia and North America that has been supported for several Late Cretaceous dinosaur clades, including hadrosauroids, dromaeosaurids 10,57 Makovicky and his colleagues are proposed that the derived monophyletic clade, Struthiomimus and Ornithomimus , from North America accounted for at least one dispersal event from Asia to North America across Beringia 5 . Alternatively, one more dispersal event probably occurred into North America or a back to Asia if Ornithomimus and Struthiomimus are not immediate sister taxa 5 .…”