Osteology of a North American Goniopholidid (Eutretauranosuchus delfsi) and Palate Evolution in Neosuchia

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Abstract

The goniopholidid Eutretauranosuchus delfsi is currently known from two skulls from the Late Jurassic Morrison Formation of Colorado. Here we present a detailed osteology for a third Eutretauranosuchus delfsi specimen (AMNH FARB 570), consisting of a skull and fragmentary postcranium from Bone Cabin Quarry, Wyoming (Morrison Formation). Additional preparation, computed tomography (CT), and three-dimensional processing of CT images reveal matrix-covered structures on AMNH FARB 570, provide new information on goniopholidid cranial anatomy, and allow for the first description of an articulated North American goniopholidid braincase. These new data, along with restudy of the palatal anatomy in Eutretauranosuchus, provide valuable insight into the phylogenetic history of goniopholidid crocodyliforms. The skull of AMNH FARB 570 is extremely similar to other specimens of E. delfsi and Amphicotylus spp., especially in details of the architecture of the palate. In these Morrison Formation goniopholidids the secondary palate is incomplete such that the nasopharyngeal passage has no ventral floor. Additionally, there is a complex septum, constructed from the vomers and pterygoid, which extends ventrally near to the plane of the secondary palate. Characters diagnosing Eutretauranosuchus delfsi include: (1) an elongate, platyrostral skull; (2) a prominent depression on the posterolateral surface of the alveolar process of the maxilla; (3) minimal lateral undulation of the maxillary tooth row; (4) broad- ening of the nasals anterior to the prefrontals; (5) nasals that do not contact external nares; (6) a broad, flat interfenestral bar with raised rims along the supratemporal fenestrae; (7) a nasopharyngeal septum formed from diverging vomeral processes anteriorly and a midline pterygoid process posteriorly (also in Amphicotylus lucasii, AMNH FARB 5782); and (8) a vomeral septal complex that forms an X shape in cross section.Based on a revised phylogenetic analysis of 88 crocodylomorph taxa, a novel phylogenetic hypothesis is proposed in which a clade of Calsoyasuchus Sunosuchus is sister to a clade of Goniopholis baryglyphaeus (Goniopholis simus (Amphicotylus lucasii Eutretauranosuchus delfsi)). Examination of secondary palate characters in Goniopholididae suggests that this group “experimented” with changes to the bony secondary palate a number of times, resulting in morphologies not seen other mesoeucrocodylian clades.

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