A new paravian dinosaur from the Late Jurassic of North America supports a late acquisition of avian flight.

Scott Hartman,William R Wahl,Jessica Lippincott,Dean R Lomax,David M Lovelace,Mickey Mortimer

doi:10.7717/peerj.7247

Abstract

The last two decades have seen a remarkable increase in the known diversity of basal avialans and their paravian relatives. The lack of resolution in the relationships of these groups combined with attributing the behavior of specialized taxa to the base of Paraves has clouded interpretations of the origin of avialan flight. Here, we describe Hesperornithoides miessleri gen. et sp. nov., a new paravian theropod from the Morrison Formation (Late Jurassic) of Wyoming, USA, represented by a single adult or subadult specimen comprising a partial, well-preserved skull and postcranial skeleton. Limb proportions firmly establish Hesperornithoides as occupying a terrestrial, non-volant lifestyle. Our phylogenetic analysis emphasizes extensive taxonomic sampling and robust character construction, recovering the new taxon most parsimoniously as a troodontid close to Daliansaurus, Xixiasaurus, and Sinusonasus. Multiple alternative paravian topologies have similar degrees of support, but proposals of basal paravian archaeopterygids, avialan microraptorians, and Rahonavis being closer to Pygostylia than archaeopterygids or unenlagiines are strongly rejected. All parsimonious results support the hypothesis that each early paravian clade was plesiomorphically flightless, raising the possibility that avian flight originated as late as the Late Jurassic or Early Cretaceous.

Highlights

Paravians are an important radiation of winged coelurosaurs more closely related to birds than to Oviraptor, that include dromaeosaurids, troodontids, unenlagiines, halszkaraptorines, and archaeopterygids in addition to derived avialans
A new paravian dinosaur from the Late Jurassic of North America supports a late acquisition of avian flight
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Summary

Introduction

Paravians are an important radiation of winged coelurosaurs more closely related to birds than to Oviraptor, that include dromaeosaurids, troodontids, unenlagiines, halszkaraptorines, and archaeopterygids in addition to derived avialans. This lack of phylogenetic resolution has been attributed to a combination of rapid rates of evolution at the base of Paraves (Brusatte et al, 2014) and a lack of sampling of taxa from outside the hugely prolific Late Jurassic and Early Cretaceous fossil beds of eastern China. Incomplete taxonomic sampling and unresolved relationships of basal avialans hinders tests of hypotheses for the origin of flight, and the order of acquisition of flight-associated characters in stem avians

Methods

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Discussion

Conclusion