Traditional studies (eg. Nesbitt 2011) nest all Archosauriformes in a ladder of taxa that includes a major split between the so-called “Pseudosuchia” (aetosaurs, poposaurs, rauisuchids, crocodylomorphs and several individual genera) and the so-called “Avemetatarsalia” (pterosaurs and dinosauriforms).
The present large heretical study, that includes many times more taxa, recovers a major split at the grade of Youngina and Youngoides. One branch includes the taller, narrower skull forms descending from sisters to Proterosuchus. The other branch includes the wider, lower skull forms that include choristoderes, phytosaurs and chanaresuchids along with related taxa.
Taxon Exclusion is the Problem
Traditional studies do not include dozens of key taxa due to a priori exclusion policies based on tradition and prejudice. The present heretical study minimizes those prejudices by including specimens from every corner of the reptile family tree.
It All Begins with Youngoides
Youngina and Youngoides (Fig. 2) have been recognized for decades as basal taxa to the Archosauriformes. That’s absolutely correct. What hasn’t been recognized is that the variety in Youngina is real. The skulls look crushed and distorted, but some really are taller than wide or wider than tall. Basically, that’s what sets euarchosauriforms (Proterosuchus and descendants) apart from pararchosauriforms (choristoderes and desendants). The nares drift dorsally. The rostrum becomes elongated. The orbits either are or are not elevated above the rostrum. After those basal traits, evolution produced bipeds, croc-like forms and pachypleurosaur-like forms, along with at least one plant-eater in both branches of the Archosauriformes.
Basal pararchosauriforms, like Doswellia and the Choristodera (Fig. 2), did not have an antorbital fenestra. These enlarged descendants of Youngoides elongated the snout and moved the naris dorsally. Lazarussuchus lost the lateral temporal fenestra and further elongated the pre-narial premaxilla.
Currently known from a single published skull and an unpublished post-crania, Proterochampsa (Figs. 1, 2) is the most basal pararchosaur to sport an antorbital fenestra and no fossa surrounded it. This flat-skulled form was probably aquatic and short-legged like its sisters.
The croc-like phytosaurs are a distinct clade sharing a long list of character traits. Even basal taxa have a longer rostrum than Proterochampsa.
Chanaresuchus and Tropidosuchus comprise the Chanaresuchidae, a clade of increasingly terrestrial forms culminating in the biped, Lagerpeton, a taxon commonly and mistakenly associated with dinosaurs by traditional workers.
Strangely, members of the Phytosauria and Chanaresuchidae have nested in traditional studies with pterosaurs, but this is patently ridiculous, a result of improper taxon inclusion and exclusion as demonstrated by the results of the large study.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
Broom R 1914. A new thecodont reptile. Proceedings of the Zoological Society of London, 1914:1072-1077.
Gardner NM, Holliday CM and O’Keefe FR 2010. The braincase of Youngina capensis(Reptilia, Diapsida): New insights from high-resolution CT scanning of the holotype. Paleonotologica Electronica 13(3):online PDF
Gow CE 1975. The morphology and relationships of Youngina capensis Broom and Prolacerta broomi Parrington. Palaeontologia Africana, 18:89-131.
Olsen EC 1936. Notes on the skull of Youngina capensis Broom. Journal of Geology, 44 (4): 523-533.