Abstract
Phytosaurs are a clade of large, carnivorous pseudosuchian archosaurs from the Late Triassic with a near cosmopolitan distribution. Their superficial resemblance to longirostrine (long-snouted) crocodylians, such as gharials, has often been used in the past to infer ecological and behavioural convergence between the two groups. Although more than thirty species of phytosaur are currently recognised, little is known about the endocranial anatomy of this clade. 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 from the Late Triassic of Germany based on digital reconstructions. Results show that the endocasts of both taxa are very similar to each other in their rostrocaudally elongate morphology, with long olfactory tracts, weakly demarcated cerebral regions and dorsoventrally short endosseous labyrinths. In addition, several sinuses, including large antorbital sinuses and prominent dural venous sinuses, were reconstructed. Comparisons with the endocranial anatomy of derived phytosaurs indicate that Phytosauria is united by the presence of elongate olfactory tracts and longitudinally arranged brain architecture—characters which are also shared with Crocodyliformes. However, a substantial morphological variability is observed in the cephalic and pontine flexure and the presence of a pineal organ across the different phytosaur species. These results suggest that the endocranial anatomy in Phytosauria generally follows a plesiomorphic pattern, with moderate variation within the clade likely resulting from divergent sensory and behavioural adaptations.
Highlights
Phytosaurs are a group of fossil archosauriform reptiles commonly found in Upper Triassic (c. 235–202 Ma) sediments in North America and Europe, and less commonly in other regions such as India, Africa, East Asia, Madagascar and South America (Stocker & Butler, 2013)
The endocasts of Ebrachosuchus neukami and Parasuchus angustifrons are very similar in their morphology
Fossae for olfactory bulbs are preserved in both taxa, but only in Parasuchus angustifrons is a rostral separation into two olfactory bulbs visible (Fig. 2B)
Summary
Phytosaurs are a group of fossil archosauriform reptiles commonly found in Upper Triassic (c. 235–202 Ma) sediments in North America and Europe, and less commonly in other regions such as India, Africa, East Asia, Madagascar and South America (Stocker & Butler, 2013). Phytosaurs resemble extant crocodylians, longirostrine morphotypes such as gharials Members of both groups possess large elongate skulls equipped with conical teeth, rows of sculptured osteoderms covering the axial and appendicular skeleton, and are characterised by a quadrupedal, sprawling gate (Westphal, 1976). Phytosaurs and the earliest fossil crocodylians are significantly separated temporally (by about 100 million years) and phylogenetically, gross morphological similarities between the two groups have often been cited as evidence for ecological and behavioural convergence (Camp, 1930; Anderson, 1936; Hunt, 1989; Hungerbühler, 2002; Witzmann et al, 2014). Neuroanatomical adaptations to a specific ecology or behaviour in phylogenetically divergent groups as drivers for morphological similarities have been suggested in avemetatarsalian (‘‘bird-line’’) archosaurs (Witmer et al, 2003)
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