Paravian Phylogeny and the Dinosaur-Bird Transition: An Overview

Abstract

Recent years witnessed the discovery of a great diversity of early birds as well as closely related non-avian theropods, which modified previous conceptions about the origin of birds and their flight. We here present a review of the taxonomic composition and main anatomical characteristics of those theropod families closely related with early birds, with the aim of analyzing and discussing the main competing hypotheses pertaining to avian origins. We reject the postulated troodontid affinities of anchiornithines, and the dromaeosaurid affinities of microraptorians and unenlagiids, and instead place these groups as successive sister taxa to Avialae. Aiming to evaluate previous phylogenetic analyses, we recoded unenlagiids in the traditional TWiG data matrix, which resulted in a large polytomy at the base of Pennaraptora. This indicates that the TWiG phylogenetic scheme needs a deep revision. Regarding character evolution, we found that: (1) the presence of an ossified sternum goes hand in hand with that of ossified uncinate processes; (2) the presence of foldable forelimbs in basal archosaurs indicates widespread distribution of this trait among reptiles, contradicting previous proposals that forelimb folding driven by propatagial and associated tendons was exclusive to the avian lineage; (3) in basal paravians and avialans (e.g., Archaeopteryx, Anchiornis) the wings are relatively large and wide, with relatively short rectricial feathers, a rounded alar contour, and a convex leading margin. These taxa exhibit restricted forelimb folding capability with respect to more derived birds, their hands being preserved at angles of 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 forearm is much less than 90 • , indicating not only increased forelimb folding capability but also an increased variety of wingbeat movements during flight. Because of the strong similarities in pectoral girdle configuration between ratites and basal avialans and paravians, it is possible to infer that the main forelimb movements were similar in all these taxa, lacking the complex dorsoventral wing excursion characteristic of living neognathans.

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

“…Models of avian flight origins based on these taxa may be misinterpreting the sequence of character acquisition that resulted in crown avian flight. The embedding of putatively flighted Rahonavis, Archaeopteryx (though see Agnolin et al, 2019), and Microraptor within clades that lack evidence of aerial locomotion is consistent with prior studies that found the morphology of most non-avialan paravians as functionally more similar to terrestrial birds and mammals than arboreal ones (Dececchi & Larsson, 2011; Agnolin et al, 2019). This supports a non-volant terrestrial ecomorph as the basal condition for the major paravian clades, supporting numerous previous studies demonstrating that key flight preadaptations up to and including vaned feathers and well-developed wings evolved in terrestrial contexts millions of years prior to the origin of crown avian flight (Makovicky & Zanno, 2011; Brusatte et al, 2014; Dececchi, Larsson & Habib, 2016; Cau, 2018).…”

The distribution of dental features in non-avian theropod dinosaurs: Taxonomic potential, degree of homoplasy, and major evolutionary trends

“…We investigated dental features on teeth preserved within the upper and lower jaws as well as isolated teeth belonging to a total of 198 taxa bracketed phylogenetically between the basal saurischian Herrerasaurus ischigualastensis (Reig, 1963;Sereno and Novas, 1994) and the basal avialan Archaeopteryx lithographica (Godefroit et al, 2013a;Foth et al, 2014;Lefèvre et al, 2017; Appendix 1.1). The basal saurischians Daemonosaurus (Sues et al, 2011), Eodromaeus (Martínez et al, 2011) and Eoraptor (Sereno et al, 1993(Sereno et al, , 2013, as well as the Scansoriopterygidae (Czerkas and Yuan, 2002) and the Anchiornithinae (sensu Agnolin et al, 2019; 'Anchiornithidae' of 'Anchiorninae' of Hu et al, 2018), recently recovered as non-avialan theropods by some authors (Sues et al, 2011;Brusatte et al, 2014;Xu et al, 2015bXu et al, , 2017Cau et al, 2017;Müller et al, 2018;Hu et al, 2018), were also included in this study. Of these 200 taxa, we examined firsthand the dentition of 125 taxa deposited in 35 scientific collections from Argentina, France, Belgium, Germany, Italy, Spain, Portugal, Qatar, Switzerland, the United Kingdom, South Africa, China, Canada and the United States (Appendix 1.1).…”

Comments on the Morphology of Basal Paravian Shoulder Girdle: New Data Based on Unenlagiid Theropods and Paleognath Birds

“…We follow the phylogenetic framework proposed by Agnolín and Novas (2013); (see also Agnolín and Novas, 2011;Agnolín et al, 2019;Motta et al, 2020) in which Deinonychosauria is interpreted as a non-monophyletic group, and Dromaeosauridae is formed by Velociraptor, Dromaeosaurus, Deinonychus, and taxa more closely related to them than to Microraptor, Unenlagia, Rahonavis or Archaeopteryx. Microraptoria and Unenlagiidae are interpreted to form successively closer relatives of the clade Avialae (Figure 1).…”

“…Our interpretative scheme about the phylogenetic relationships among basal paravians departs from others (e.g., Norell and Clarke, 2001;Makovicky et al, 2005;Xu and Kim, 2011;Turner et al, 2012;Gianechini et al, 2017;Lefèvre et al, 2017;Pei et al, 2017;Hartman et al, 2019;Cau, 2020;Pei et al, 2020) in that we propose that Troodontidae, Dromaeosauridae, Microraptoria, Unenlagiidae and Anchiornithidae form successively closer relatives of Avialae. We choose to follow this scheme, because a detailed overview of other phylogenetic analyses (e.g., TWIG) resulted in poorly resolved phylogenetic relationships among derived paravians (Agnolín et al, 2019).…”