New data on the braincase of the aetosaurian archosaur (Reptilia: Diapsida) Stagonolepis robertsoni Agassiz

Abstract

New data on the braincase of the aetosaurian archosaur Stagonolepis robertsoni Agassiz are presented, based on new preparation, synthetic casting, and interpretation of fossil material from the Triassic Elgin Sandstones, Scotland. The metotic fissure is not divided by bone. The perilymphatic foramen is completely bound by bone, and faces away from the otic capsule in a posterolateral direction. A prominent subvertical ridge on the anterolateral edge of the exoccipital and upper part of the basioccipital cannot be directly associated with the subcapsular process of the chondrocranium of extant crocodilians. This ridge projects laterally beyond the ventral ramus of the opisthotic, and lies anterior to the external foramina for the hypoglossal nerve. The overall structure of the braincases (especially the otic region) of S. robertsoni and other aetosaurians, where known, is more similar (in terms of derived archosaurian characters) to those of crocodylomorphs than are the braincases of other major suchian groups. This provides evidence for the currently unorthodox hypothesis that, among major suchian clades, Aetosauria and Crocodylomorpha are each others’ closest relatives. Support for this hypothesis is found in features of the palatine and prefrontal that have not been considered in recent studies of suchian phylogeny. This alternative phylogenetic hypothesis demands further investigation but, combined with the new morphological data that it explains, it provides a framework for the understanding of the evolution of the derived and distinctive braincase structure of extant crocodilians. © 2002 The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136, 7−23.

The Early Evolution of Archosaurs: Relationships and the Origin of Major Clades

“…3). That hypothesis was supported by three studies of the braincase of pseudosuchians (Gower and Walker, 2002;Gower, 2002;Gower and Nesbitt, 2006).…”

“…Gower and Walker (2002) and Gower (2002) proposed the unique hypothesis that aetosaurs represent the sister taxon of Crocodylomorpha based on synapomorphies in the braincase shared by the aetosaurs (mainly Stagonolepis) and crocodylomorphs (mainly Sphenosuchus and Crocodylus). Gower and Walker (2002) also provided two potential cranial characters uniting the two taxa. Their data were limited to cranial characters; however, the hypothesis represented a clear alternative to a rauisuchian sister taxon.…”

The higher-level phylogeny of Archosauria (Tetrapoda: Diapsida)

“…Many studies advocate a position more derived than phytosaurs, but less derived than crocodylomorphs and 'rauisuchians' (Gauthier 1986;Benton 1999;Nesbitt 2003. However, other studies find Aetosauria in a basal polytomy with phytosaurs and other taxa (Benton 2004), as the sister group to various 'rauisuchians' Juul 1994), as the sister group to crocodylomorphs + some 'rauisuchians' (Parrish 1993), or as the sister group to Crocodylomorpha (Gower 2002;Gower & Walker 2002;). …”

“…Previous analyses have reached little consensus on the position of Aetosauria, but the majority recover this clade as a basal lineage nested between phytosaurs and crocodylomorphs + 'rauisuchians'. Gower (2002), however, presented evidence for a close relationship between aetosaurs and crocodylomorphs based on braincase characters (see also Gower & Walker 2002;). Our analysis is the first study taking into account data from the entire skeleton that recovers a close relationship between these clades.…”

“…Regardless, for our dataset an aetosaur and crocodylomorph clade is well supported relative to other suchian clades, as it takes an additional four steps to enforce a grouping of all 'rauisuchians' (including ornithosuchids) and crocodylomorphs to the exclusion of aetosaurs, as is advocated by alternative analyses. It is also worth noting that there are two additional characters discussed by Gower & Walker (2002) that may support an aetosaur and crocodylomorph clade: a restricted dorsal fossa on the palatine and a ventromedial process on the prefrontal that projects into the antorbital cavity; but they were excluded here since they are difficult to score in many taxa because of missing data and insufficient published figures. A close relationship between aetosaurs and crocodylomorphs deserves further testing, and authors should no longer use aetosaurs as an outgroup in analyses of 'rauisuchian' and crocodylomorph phylogeny, as this implicitly assumes that the latter two groups form a clade exclusive of aetosaurs (e.g.…”