Halszkaraptor escuilliei and the evolution of the paravian bauplan

Abstract

The evolution of birds from dinosaurs is a subject that has received great attention among vertebrate paleontologists. Nevertheless, the early evolution of the paravians, the group that contains birds and their closest non-avian dinosaur relatives, remains very poorly known. Even the most basal members of one paravian lineage, the Dromaeosauridae, already show a body plan that differs substantially from their closest non-paravian relatives. Recently, the dromaeosaurid Halszkaraptor escuilliei was described from the Cretaceous of Mongolia. Halszkaraptor possesses numerous unserrated premaxillary teeth, a platyrostral rostrum with a developed neurovascular system, an elongate neck, bizarrely-proportioned forearms, and a foreword-shifted center of mass, differing markedly from other paravians. A reevaluation of the anatomy, taphonomy, environmental setting, and phylogenetic position of H. escuilliei based on additional comparisons with other maniraptorans suggests that, rather than indicating it was a semiaquatic piscivore, the body plan of this dinosaur bears features widely distributed among maniraptorans and in some cases intermediate between the conditions in dromaeosaurids and related clades. I find no evidence for a semiaquatic lifestyle in Halszkaraptor. A phylogenetic reevaluation of Halszkaraptorinae places it as the sister clade to Unenlagiinae, indicating the bizarre features of unenlagiines previously interpreted as evidence of piscivory may also represent a mosaic of plesiomorphic, derived, and intermediate features. The anatomy of Halszkaraptor reveals that dromaeosaurids still possessed many features found in more basal maniraptoran and coelurosaur clades, including some that may have been tied to herbivory. Rather than being a semiaquatic piscavore, Halszkaraptor was a basal dromaeosaurid showing transitional features.

The body plan ofHalszkaraptor escuilliei(Dinosauria, Theropoda) is not a transitional form along the evolution of dromaeosaurid hypercarnivory

“…Among these, the premaxillae of Erlikosaurus are the best preserved and are highly reminiscent of the premaxillae of Halszkaraptor in their clear lateral expansion in dorsal view […]". Contra Brownstein (2019), it is unlikely that the platyrostral morphology of Halszkaraptor is homologous to those he referred to other coelurosaurs. In Halszkaraptor, the platyrostral condition is acquired by the remarkable anteroposterior elongation and dorsoventral flattening of the prenarial region of the premaxilla, which also results in the posterior placement of the narial region relative to the snout anterior tip.…”

“…In the following references, the term "Ref./Refs." followed by one or more numbers refers to the reference list in Brownstein (2019).…”

“…Literature misreports and unsupported statements Brownstein (2019) compared the premaxilla of Halszkaraptor with those of ornithomimosaurs and therizinosaurians. He wrote: "moderately to strongly (=platyrostral) laterally expanded premaxillae are found in a variety of maniraptorans and maniraptoriforms […].…”

“…1). The "lateral expansion" of the premaxilla, claimed by Brownstein (2019), is thus produced by distinct elements in the two taxa (i.e., prenarial elongation and depression in Halszkaraptor, vs sub-and perinarial widening in Erlikosaurus), and could not be considered homologous (Fig. 1).…”

“…1). Brownstein (2019) wrote: "It is unclear how Cau et al [Ref. 32] observed retracted nares in Halszkaraptor, as the anterior nasals are not preserved in that taxon".…”