Post‐hatchling cranial ontogeny in the Early Triassic diapsid reptile Proterosuchus fergusi

Abstract

The phylogenetic position of Proterosuchus fergusi (Lower Triassic of South Africa) as one of the most basal archosauriforms means that it is critically important for understanding the successful evolutionary radiation of archosaurs during the Mesozoic. The excellent sample of the species provides a unique opportunity to understand early archosauriform ontogeny. Qualitative and quantitative analyses of cranial ontogenetic variation were conducted on an ontogenetic sequence, in which the smallest individual is 37% of the size of the largest one and osteohistological evidence suggests that four of 11 collected specimens had not reached sexual maturity. Through ontogeny the skull of Proterosuchus became proportionally taller, the infratemporal fenestra larger, and the teeth more isodont and numerous but with smaller crowns. The sequence of somatic maturity supports relatively high growth rates during early ontogeny. The skull of juvenile specimens of Proterosuchus closely resembles adults of the basal archosauromorph Prolacerta, whereas adult specimens resemble adults of more derived archosauriforms. As a result, a plausible hypothesis is that ontogenetic modification events (e.g. heterochrony) may have been key drivers of the evolution of the general shape of the skull at the base of Archosauriformes. These changes may have contributed to the occupation of a new morphospace by the clade around the Permo-Triassic boundary.

Redescription and Phylogenetic Relationships of the Proterochampsid Rhadinosuchus gracilis (Diapsida: Archosauriformes) from the Early Late Triassic of Southern Brazil

“…This interpretation is consistent with the presence of a completely unfused cervical centrum to the missing neural arch (Fig. 5: fna) (Brochu, 1996;Irmis, 2007) and the probable presence of a relatively large orbit (Ezcurra and Butler, 2015a). However, the evidence currently available to establish the ontogenetic stage of the specimen is weak and should be tested in the future with other lines of evidence (e.g., palaeohistology).…”

Endocranial anatomy and life habits of the Early Triassic archosauriformProterosuchus fergusi

“…). Similarly, in P. fergusi the more gracile skull and relatively longer teeth of juvenile specimens has been thought to suggest that different ontogenetic stages favoured different prey items, possibly to decrease intraspecific competition (Ezcurra & Butler ). Proterosuchus fergusi may have had a generalist diet, like that of modern crocodylians which change their diet through ontogeny from largely invertebrates to large terrestrial vertebrates.…”

“…To date no physiological function has been suggested and mechanical functions such as digging have been ruled out after macroscopic analysis of the premaxillary teeth (Ezcurra ). The ontogenetic changes in the snout morphology (Ezcurra & Butler ) and phenotypic costliness led Ezcurra () to propose social and sexual selection as a possible explanation.…”

“…). Both specimens are large adults from the Lystrosaurus AZ of the Karoo Basin of South Africa (Ezcurra & Butler ). The left side of SNSB‐BSPG 1934 VIII 514 is nearly complete, but the right side has been subject to severe deformation and loss of information (Fig.…”

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“…Gould, 1966a), diapsids (e.g. Ezcurra & Butler, 2015), dinosaurs (e.g. Horner & Goodwin, 2006, 2009, or mammals (e.g.…”