Endocranial anatomy and life habits of the Early Triassic archosauriformProterosuchus fergusi

Abstract

Proterosuchids are an important group of carnivorous basal archosauriforms characterized by a bizarre and enigmatic downturned premaxilla that overhangs the lower jaw. They are particularly significant because they radiated in the immediate aftermath of the Permian–Triassic mass extinction and represent one of the best known ‘disaster taxa’ following that event. While traditionally considered to be semi‐aquatic, recent histological studies and geological data have suggested that it is more likely that they inhabited terrestrial environments. By using computed tomographic (CT) data, we virtually reconstruct the brain endocast and endosseous labyrinths of two adult specimens of Proterosuchus fergusi from the earliest Triassic of South Africa, in an attempt to understand its life habits within the context of basal archosauriform evolution. Endocasts reveal that the brain cavity is tubular in shape and the endosseous labyrinths are highly pyramidal. The angle of the lateral semicircular canal suggests that P. fergusi naturally held its head upwards ~17°, while the length of the cochlear duct suggests its auditory abilities were specialized towards low‐frequency sounds. Furthermore, beam theory analysis suggests that the rostrum of P. fergusi is highly resistant to both bending and torsion when compared to modern crocodilians, although this resistance is neither enhanced nor reduced by the overhanging premaxilla. Comparative anatomical analyses suggest P. fergusi was probably a semi‐aquatic, generalist apex predator capable of surviving the harsh environmental perturbations of the Early Triassic.

The cranial morphology ofTanystropheus hydroides(Tanystropheidae, Archosauromorpha) as revealed by synchrotron microtomography

“…The flocculus is remarkably large and extends laterally between the anterior semicircular canal and the vestibule of the endosseous labyrinth. Large flocculi are also present in the early archosauromorphs Mesosuchus browni and Euparkeria capensis, whereas it is only poorly developed in Proterosuchus fergusi (Brown et al, 2019;Sobral & Müller, 2019;.…”

A test of the lateral semicircular canal correlation to head posture, diet and other biological traits in “ungulate” mammals

“…11 c). In Proterosuchus this upward tilting would be about 17° 27 , whereas in Lystrosaurus it would be between 19° and 23° 94 . However, an upward tilting of the neutral head posture is also habitually observed in many fully terrestrial species, such as Camelus dromedarius and was occasionally spotted in Capra ibex (Fig.…”

“…6 ), even though it would be expected of species that have to keep breathing above water level most of the time while immersed 27 , 30 . Recently, a semi-aquatic habit for the Triassic archosaur Proterosuchus and the therapsid Lystrosaurus has been hypothesized as these two species would have their head tilted upward anteriorly when the plane of the LSC is horizontal 27 , 94 (Fig. 11 c).…”

“…This is backed by the hypothesis that a horizontal orientation of the LSC would mechanically maximize the recording of rotational and linear head movements made in the horizontal plane by placing the sensory hair cells of the semicircular canal and its associated ampulla perpendicular to the horizontal plane [12][13][14][15][16] . The subsequent use of the orientation of the plane of the LSC as a proxy to infer head posture in fossil vertebrates has grown more popular among paleontologists, as it is being applied to dozens of extinct taxa such as archosaurs, including dinosaurs, and synapsids, including mammals 12,[17][18][19][20][21][22][23][24][25][26][27][28][29] . This has raised discussion on some crucial paleobiological questions, such as the evolution of bipedalism in ancient hominin 14,18 and paleodiets.…”

“…As browsers are expected to hold their head higher than grazers 30 , head posture has been invoked in reconstructing ancient diet in fossil herbivorous species 20,24 . Semi-aquatic species, on the other hand, would hold their head tilted upward 27 (but see Neenan and Scheyer 31 ). In addition, head posture is directly involved in discussions about the origin of endothermy, as blood pressure to perfuse the head, and particularly the brain, directly depends on head posture and thermophysiology (species with low metabolism have a lower blood pressure than species with a high metabolism, and therefore cannot perfuse their brain if their head is held far above their heart) 32 .…”