Earlier we looked at the origin of the antorbital fenestra in archosauriforms and the early radiation of that clade in the Late Permian as demonstrated by a series of skulls. Several Late Permian Youngina and Youngoides specimens preceded several Early Triassic Proterosuchus specimens and Euparkeria at the base of the Archosauriformes in the large reptile tree.
A recent online paper
by Bernardi et al. (2015) presented a holistic approach to the study of early archosauriform evolution by integrating body and track records supporting a Late Permian – Early Triassic radiation. The skeletal record is largely Triassic for this clade. Only Archosaurus and a few other taxa are known from the Late Permian. The ichnite record provides evidence for an erect posture in the Late Permian. This is all very reasonable and well-documented,
There are other problems elsewhere on the Bernardi tree, as they nest crocs with rauisuchians and phytosaurs basal to avemetatarsalians (which includes the lepidosauromorph pterosaurs at its base). We’ve talked about the ‘by default’ nesting of phytosaurs close to pterosaurs earlier. A long list of workers need to look more closely at their cladograms for such ‘strange bedfellow’ problems, typically due to taxon exclusion. Evolution works gradually and all cladograms should provide a valid, tested, gradual accumulation of traits among all included taxa.
Here we’ve already looked at Early Permian scleroglossan lepidosaurs and Late Permian protosquamates. When you add taxa (now up to 555 in the large reptile tree) you can trace the ancestry of archosauriforms back to basal tetrapods. You find that mammals, ichthyosaurs and placodonts are more closely related to archosaurs than lizards are.
One note that did catch my interest
was the scaly pattern on the ichnite. Scalation is rarely preserved along with fossils or footprints, so this is a rare find. Cheirotherium tracks have been matched to Ticinosuchus (Fig. 3).
Ticinosuchus had an erect carriage (Fig. 3), as the Late Permian ichnites indicate, but body fossils are known only from the Middle Triassic. Obviously the ichnites were closer to the origin of the clade, while the body fossils represent a wider radiation or the dying days of the taxon.
By contrast, a traditional Middle Triassic Cheirotherium ichnite (Fig. 4) appears to presage dinosaurs (Fig. 5) with the reduction and rotation of the two lateral manual digits along with the reduction of digits 1 and 5 of the pes. Mechanically the lateral rotation of manual digits 4 and 5 remove them from contributing and/or reduce their contribution to forward propulsion.
And it’s also worthwhile,
while we’re at it, to compare the traditional Cheirotherium track (Fig. 4) to Gracilisuchus (Fig. 6), which nests between Ticinosuchus and pre-dinosaurs, but definitely closer to the croc lineage based on the absence of pedal digit 5 and other traits.
The Bernardi et al. study confirms our and other earlier studies that archosauriforms had their origin and early radiation in the Late Permian. Let’s hope that someday they get a larger taxon list to get their tree topology right.
Bernardi M, Klein H, Petti FM & Ezcurra MD 2015. The Origin and Early Radiation of Archosauriforms: Integrating the Skeletal and Footprint Record. PLoS ONE 10(6): e0128449. doi:10.1371/journal.pone.0128449