UNAPPRECIATED HIGH CRANIAL DISPARITY IN STEM ARCHOSAURS PREDATED THE DOMINANCE OF THE ARCHOSAUR CROWN

FOTH, C.; EZCURRA, M. D.; SOOKIAS, R. B.; BRUSATTE, S.L.; BUTLER, R. J.

Congreso; SVP anual meeting 2016; 2016

Archosaurs, the hugely successful group that includes modern crocodilians and birdsas well as dinosaurs and many extinct clades, began to proliferate in the Triassic after thePermo?Triassic mass extinction, but are part of a broader radiation that began with theorigin of the clade Archosauromorpha in the middle?late Permian. Previous research hasmainly focused on the origin and initial diversification of archosaurs and their majorsubgroups (particularly dinosaurs), whereas non-archosaurian archosauromorphs havebeen largely overlooked in macroevolutionary studies. Here, we analyze the temporalpattern of cranial disparity of late Permian to Early Jurassic archosauromorphs using twodimensionalgeometric morphometrics, allowing us to compare skull shape diversitybetween non-archosaurian archosauromorphs and archosaurs and between the crocodileand bird-lines of archosaurs (Pseudosuchia and Ornithodira). Shapes of hypotheticalancestors were estimated using a time-calibrated informal supertree and added to thedataset to compensate for poor sampling, which is a particular problem around thePermo?Triassic and Triassic?Jurassic boundaries. Non-archosaurian archosauromorphsshowed an increase in cranial disparity from the late Permian to the Early Triassic,followed by an unappreciated high peak in the Middle Triassic, and then abruptlydeclined during the Carnian (Late Triassic). By contrast, cranial disparity of archosaursrose from the Middle Triassic to the end of the Late Triassic, decreased around theTriassic?Jurassic boundary, but expanded again towards the end of the Early Jurassic.The Late Triassic archosaur peak is primarily based on high disparity values amongpseudosuchians, while the re-expansion in the Early Jurassic results from a continuousincrease of ornithodiran cranial disparity from the Carnian until the Sinemurian. Ourstudy indicates that non-archosaurian archosauromorphs were highly diverse componentsof terrestrial ecosystems in the Early and Middle Triassic, predating the major radiationof crown group archosaurs. There was a gradual faunal replacement of stem archosaursby the crown group (primarily Pseudosuchia) during the Ladinian and Carnian, includinga short interval of partial overlap in morphospace during the Ladinian. In addition,simultaneous decrease of pseudosuchian disparity and increase of ornithodiran disparityacross the Triassic?Jurassic boundary could be evidence for an opportunistic replacementscenario between the two major archosaur groups.