Redescription of the phytosaursPaleorhinus(‘Francosuchus’)angustifronsandEbrachosuchus neukamifrom Germany, with implications for Late Triassic biochronology

Abstract

Phytosaurs are a diverse and morphologically distinctive clade of superficially crocodile-like archosauriforms that had a near global distribution during the Late Triassic. Because their remains are among the most abundant vertebrate remains recovered in many Upper Triassic terrestrial formations, phytosaurs are used extensively in long-range biochronological and biostratigraphic correlations. The biochronologically oldest and earliest branching known phytosaurs include an array of nominal species from the early Late Triassic of the United States, Germany, Poland, Morocco, and India that have been synonymized within the genus Paleorhinus, and subsequently used to define a global 'Paleorhinus biochron'. However, recent phylogenetic work suggested that the North American species previously referred to Paleorhinus are paraphyletic. Here, we reassess the systematics and anatomy of putative specimens of Paleorhinus from southern Germany. Two well-preserved basal phytosaur skulls from the Blasensandstein (Carnian) of Bavaria form the holotypes of Francosuchus angustifrons and Ebrachosuchus neukami, both of which were synonymized with Paleorhinus by previous workers. We demonstrate that Francosuchus angustifrons shares unique synapomorphies with specimens referred to Paleorhinus bransoni from the Late Triassic of Texas, and thus refer the species to Paleorhinus. By contrast, the longirostrine Ebrachosuchus is highly distinctive in morphology, and our new cladistic analysis of Phytosauria demonstrates that it represents a valid taxon that is more closely related to Phytosauridae than to Paleorhinus. We provide the first autapomorphybased support for a monophyletic but restricted Paleorhinus (supported by a nodal row on the jugal, and low paired ridges on the squamosal) and confirm that previous broader conceptions of Paleorhinus are likely to be paraphyletic.

Neural and endocranial anatomy of Triassic phytosaurian reptiles and convergence with fossil and modern crocodylians

“…The antorbital sinus is large and fills the antorbital fenestra, as well as the space between the palate and the palatal shelf of the maxilla. A small diverticulum also appears to enter the jugal via a foramen near the ectopterygoid-jugal contact (Butler et al, 2014) in both taxa. Rostrally, a large canal is present, which opens into the antorbital cavity.…”

“…The region rostral to the external nares comprises a large air-filled space for the entire length of the premaxilla medial to the neurovascular canal supplying the alveolar cavities. This region may have housed a premaxillary sinus (most likely as extension of the antorbital sinus) and/or neurovascular bundles (Butler et al, 2014) as in extant crocodiles (Leitch & Catania, 2012). The airway is simple and unbranched in Ebrachosuchus neukami and Parasuchus angustifrons and connects the external nares with the choanae and the olfactory region.…”

“…The studied specimens consist of the holotypes of Ebrachosuchus neukami (BSPG 1931 X 501; Bayerische Staatssammlung für Paläontologie und Geologie, Munich, Germany) and Parasuchus angustifrons (BSPG 1931 X 502) (Kuhn, 1936; Butler et al, 2014). The skull of Ebrachosuchus neukami is fully articulated and complete and preparation work has removed most of the sandstone matrix, with the exception of most internal cavities, which remain filled with matrix.…”

“…BSPG 1931 X 502 was scaled to the same dorsoventral dimensions as a less compressed skull of Parasuchus hislopi (ISI R42, Indian Statistical Institute, Kolkata, India), corresponding to a scaling factor of approximately 40%. The same scaling factor was assumed for BSPG 1931 X 501 based on the fact that both specimens were found in close proximity to one another on a single bedding plane and likely had a similar diagenetic history (Butler et al, 2014). …”

“…Here, we describe the endocranial anatomy (including the brain, inner ear, neurovascular structures and sinus systems) of the two non-mystriosuchine phytosaurs Parasuchus angustifrons (=“ Paleorhinus angustifrons ”) and Ebrachosuchus neukami (Butler et al, 2014) (see Kammerer et al, 2016, for recent taxonomic revisions) (Fig. 1) based on digital reconstructions.…”