Earlier we learned that the antorbital fenestra, the hallmark of the Archosauriformes, actually had four other distinct and convergent origins in Chroniosuchians, Pararchosauriformes, Pamelaria + Jaxtasuchus and Fenestrasaurs.
Overlooked until now,
today we’ll look at the origin of the antorbital fenestra in the Archosauriformes.
Earlier we learned that the proximal outgroup taxon to the Archosauriformes was Youngoides romeri (FMNH UC 1528; Late Permian; Olson and Broom 1937; Figs.1, 2)
The antorbital fenestra in Archosauriformes
began as a small opening in the skull below the lacrimal and above the maxilla in Youngina and Youngoides specimens. As proterosuchid descendants grew larger (Fig. 3), so did the antorbital fenestra. Euparkeriid descendants were not much larger — at first. In this clade the antorbital fenestra enlargement came at the expense of the a lateral temporal fenestra reduction as the orbit shifted posteriorly. In proterosuchids, the lateral temporal fenestra became wither taller or longer, depending on the clade.
The clade of terrestrial younginiformes,
and all of the Youngina specimens need to be reexamined as they have not been studied thoroughly as a group since Gow 1975, which predates cladistic analysis using software. Once you have an outgroup taxon for the Archosauriformes (Fig. 6), then you have a good idea where to look for the origin of the antorbital fenestra, a subject missed by Witmer 1997. The AMNH 5661 holotype of Youngina may also have had an antorbital fenestra, but the skull has several damaged areas, that one among them.
The BPI 375 specimen of Youngina likewise seems to have had a small antorbital fenestra. If so, then the lack of an antorbital fenestra in most prolacertids (protorosaurs) represents a secondary loss of this trait. Gow (1975) drew the area with a dotted line (Fig 5), but the a DGS tracing of the specimen appears to show several possible fenestra between bones, the antorbital fenestra most prominent among them.
In at least three lineage of archosauriformes,
the Choristodera, the Crocodylia, and derived Ornithischia, the antorbital fenestra disappeared. We’ll look at the choristodere sequence in a future blog post.
Gow CE 1975. The morphology and relationships of Youngina capensis Broom and Prolacerta broomi Parrington. Palaeontologia Africana, 18:89-131.
Olson EC and Broom R 1937. New genera and species of tetrapods from the Karroo Beds of South Africa. Journal of Paleontology 11(7):613-619.
Witmer LM 1997. The Evolution of the Antorbital Cavity of Archosaurs: A Study in Soft-Tissue Reconstruction in the Fossil Record with an Analysis of the Function of Pneumaticity. JVP 17(1 supp):1–76.