Deep reptilian evolutionary roots of a major avian respiratory adaptation.

Vertebral ribs of the anterior thorax in extant birds bear bony prongs called uncinate processes, which improve the mechanical advantage of mm. appendicocostales to move air through the immobile lung and pneumatic air sacs. Among non-avian archosaurs, broad, cartilaginous uncinate processes are present in extant crocodylians, and likely have a ventilatory function. Preserved ossified or calcified uncinate processes are known in several non-avian dinosaurs. However, whether other fossil archosaurs possessed cartilaginous uncinate processes has been unclear. Here, we establish osteological correlates for uncinate attachment to vertebral ribs in extant archosaurs, with which we inferred the presence of uncinate processes in at least 19 fossil archosaur taxa. An ancestral state reconstruction based on the infer distribution suggests that cartilaginous uncinate processes were plesiomorphically present in Dinosauria and arguably in Archosauria, indicating that uncinate processes, and presumably their ventilatory function, have a deep evolutionary history extending back well beyond the origin of birds.

Taxonomic revision of Eoalligator (Crocodylia, Brevirostres) and the paleogeographic origins of the Chinese alligatoroids.

Background. The primarily Neotropical distribution of living alligatoroids raises questions as to when and how the ancestors of Alligator sinensis migrated to China. As phylogeny provides a necessary framework for historical biogeographic issues, determining the phylogenetic positions of the Chinese alligatoroids is a crucial step towards understanding global alligatoroid paleobiogeography. Besides the unnamed alligatoroids from the Eocene of Guangdong Province, three Chinese fossil taxa have been referred to Alligatoroidea: Alligator luicus, Eoalligator chunyii and Eoalligator huiningensis. However, none of these fossil taxa has been included in a phylogenetic analysis. The genus Eoalligator was established to accommodate E. chunyii from Guangdong Province. E. huiningensis from Anhui Province was later erected as a second species, despite no distinctive similarities with E. chunyii. By contrast, the putative crocodyline Asiatosuchus nanlingensis was established based on material from Guangdong Province, close to the E. chunyii specimens geographically and stratigraphically. Furthermore, specimens of A. nanlingensis and E. chunyii share four distinctive characters, but display no evident differences. As a result, the taxonomic relationships of these three species require restudy. Methods. In this paper, all specimens of E. chunyii and E. huiningensis are reassessed in detail, and compared to specimens of A. nanlingensis. Detailed re-descriptions and revised diagnoses are provided, and a cladistic analysis is carried out to assess the phylogenetic positions of E. chunyii, E. huiningensis and A. nanlingensis. Results. The analysis recovers E. chunyii and A. nanlingensis as sister taxa among basal Crocodylidae, while P. huiningensis is posited as an alligatoroid. Two key characters support the monophyly E. chunyii + A. nanlingensis: sulcus within surangular, and anteroposteriorly oriented surangular-articular suture. The former character is unique to E. chunyii and A. nanlingensis among crocodyloids, although a smaller and more posteriorly positioned surangular fossa is known in Diplocynodon. Detailed comparisons show the two species to be synonymous, with E. chunyii as the junior synonym of A. nanlingensis based on page priority. Because E. chunyii was erected as the type species of Eoalligator, the genus is now invalid. We establish the new genus Protoalligator to accommodate "Eoalligator" huiningensis, an alligatoroid whose exact phylogenetic position is uncertain. In particular, P. huiningensis retains primitive characters such as a lacrimal that extends further anteriorly than the prefrontal, and a notch at the premaxilla-maxilla suture. However, P. huiningensis also appears to share one important derived character, a complete nasal bar, with alligators. Our taxonomic revisions imply that four alligatoroids are currently known from China, and these species must have dispersed from North America to Asia in more than one event.