Paravian Phylogeny and the Dinosaur-Bird Transition: An Overview

AGNOLIN, FEDERICO L.; MOTTA, MATIAS J.; BRISSÓN EGLI, FEDERICO; LO COCO, GASTÓN; NOVAS, FERNANDO E.

Frontiers in Earth Science

Springer

Año: 2019 vol. 6

Recent years witnessed the discovery of a great diversity of early birds as well as closelyrelated non-avian theropods, which modified previous conceptions about the origin ofbirds and their flight. We here present a review of the taxonomic composition and mainanatomical characteristics of those theropod families closely related with early birds,with the aim of analyzing and discussing the main competing hypotheses pertaining toavian origins. We reject the postulated troodontid affinities of anchiornithines, and thedromaeosaurid affinities of microraptorians and unenlagiids, and instead place thesegroups as successive sister taxa to Avialae. Aiming to evaluate previous phylogeneticanalyses, we recoded unenlagiids in the traditional TWiG data matrix, which resultedin a large polytomy at the base of Pennaraptora. This indicates that the TWiGphylogenetic scheme needs a deep revision. Regarding character evolution, we foundthat: (1) the presence of an ossified sternum goes hand in hand with that of ossifieduncinate processes; (2) the presence of foldable forelimbs in basal archosaurs indicateswidespread distribution of this trait among reptiles, contradicting previous proposalsthat forelimb folding driven by propatagial and associated tendons was exclusive to theavian lineage; (3) in basal paravians and avialans (e.g., Archaeopteryx, Anchiornis) thewings are relatively large and wide, with relatively short rectricial feathers, a roundedalar contour, and a convex leading margin. These taxa exhibit restricted forelimb foldingcapability with respect to more derived birds, their hands being preserved at anglesof flexion (with respect to the radius/ulna) of no less than 90. In more derived birds,however, the rectrices are notably elongate and the angle between the hand and forearmis much less than 90, indicating not only increased forelimb folding capability but also anincreased variety of wingbeat movements during flight. Because of the strong similaritiesin pectoral girdle configuration between ratites and basal avialans and paravians, it ispossible to infer that the main forelimb movements were similar in all these taxa, lackingthe complex dorsoventral wing excursion characteristic of living neognathans.