The cranial osteology of Tyrannoneustes lythrodectikos (Crocodylomorpha: Metriorhynchidae) from the Middle Jurassic of Europe.

Davide Foffa,Mark T Young

doi:10.7717/peerj.608

Abstract

Tyrannoneustes lythrodectikos is one of numerous metriorhynchid crocodylomorph species known from the Oxford Clay Formation of England (Callovian-Oxfordian; Middle-Late Jurassic). This taxon is of evolutionary importance, as it is the oldest and most basal known macrophagous metriorhynchid. It has a mosaic of plesiomorphic and derived feeding related characteristics, including: teeth with microscopic, poorly formed and non-contiguous denticles; increased tooth apicobasal length; ventrally displaced dentary tooth row (increased gape); reduced dentary tooth count; and a proportionally long mandibular symphysis. However the type specimen, and current referred specimens, all lack a preserved cranium. As such, the craniofacial morphology of this taxon, and its potential feeding ecology, remains poorly understood. Here we describe two skulls and two lower jaws which we refer to T. lythrodectikos. Previously these specimens were referred to ‘Metriorhynchus’ brachyrhynchus. They share with the T. lythrodectikos holotype: the in-line reception pits on the dentary, dorsal margin of the surangular is strongly concave in lateral view, and the most of the angular ventral margin is strongly convex. Based on our description of these specimens, the skull of T. lythrodectikos has three autapomorphies: very long posterior processes of the premaxilla terminating in line with the 4th or 5th maxillary alveoli, deep lateral notches on the lateral surface of the maxillary with reception pits for dentary teeth, and the premaxilla forms the anterior margin of the first maxillary alveoli. Our description of the cranial anatomy of Tyrannoneustes lythrodectikos confirms that some macrophagous characteristics evolved during the Middle Jurassic, and were not exclusive to the clade Geosaurini. Moreover, the skulls further highlight the mosaic nature of Tyrannoneustes lythrodectikos and wide-gape macrophagous evolution in Geosaurinae.

Highlights

Metriorhynchid crocodylomorphs were a highly aberrant, but very successful clade
The former preserves an incomplete cranium in two separate pieces, an incomplete mandible, four cervical vertebrae and four dorsal vertebrae. The latter specimen is an almost complete skull dorsoventrally distorted in the postorbital area and only missing few elements from the braincase, the posterior palatal surface and left temporal area. Alongside these two crania we describe PETMG:R60, a complete but distorted lower jaw from King’s Dyke, which have been associated to PETMG:R176
The enamel ornamentation is composed of low-relief apicobasal ridges, which are visible in the holotype (Young et al, 2013a) and PETMG:R176 (Fig. 23), but in NHMUK PV R3939 they are not clearly visible, most likely due to the generous amount of consolidant applied during the restoration process

Summary

INTRODUCTION

Metriorhynchid crocodylomorphs were a highly aberrant, but very successful clade. They diversified and radiated extensively during the Middle and Late Jurassic, and continued to diversify throughout the Early Cretaceous (e.g., Eudes-Deslongchamps, 1867–1869; Fraas, 1902; Andrews, 1913; Hua et al, 2000; Gasparini & Iturralde-Vinent, 2001; Buchy et al, 2007; Buchy, 2008; Lepage et al, 2008; Pol & Gasparini, 2009; Young et al, 2010; Fernandez et al, 2011; Parrilla-Bel et al, 2013; Herrera, Gasparini & Fernandez, 2013; Young et al, 2014). These bones are only partially preserved and are severely damaged In both NHMUK PV R3939 and PETMG:R176 the dorsoventral compression of the preorbital areas, and the dorsal displacement of the jugal bars and posterior processes of the nasals, make it impossible to describe the contacts of the lacrimal with the surrounding elements. The exoccipital-opisthotic complex appears to be a single unit (the otoccipital), there could be a suture ventral to the paroccipital processes (Fig. 11) which suggests they may be separate elements (a similar suture is seen in Torvoneustes coryphaeus; Young et al, 2013b) This complex is the main ‘element’ seen on the occipital surface, and forms the lateral and dorsal margins of the foramen magnum. The enamel ornamentation is composed of low-relief apicobasal ridges, which are visible in the holotype (Young et al, 2013a) and PETMG:R176 (Fig. 23), but in NHMUK PV R3939 they are not clearly visible, most likely due to the generous amount of consolidant applied during the restoration process

Findings

DISCUSSION

CONCLUSIONS