A new metriorhynchid crocodylomorph from the Lower Kimmeridge Clay Formation (Late Jurassic) of England, with implications for the evolution of dermatocranium ornamentation in Geosaurini

Abstract

A new metriorhynchid crocodylomorph from the Lower Kimmeridge Clay Formation (Kimmeridgian, Upper Jurassic) of England is described. This specimen, a three-dimensionally preserved skull and left mandibular ramus, is referred to a new species: Torvoneustes coryphaeus sp. nov. Within the genus Torvoneustes, T. coryphaeus sp. nov. is unique as it has a long anteromedial process of the frontal, ornamented dermatocranium, and the supraorbital notch forms a strongly acute angle. Our phylogenetic analysis confirms the placement of this specimen in Torvoneustes. The dentition of T. coryphaeus sp. nov., like that of the type species, has a blunt apex, crown basal-mid regions with numerous tightly packed apicobasally aligned ridges, and apical region with an anastomosed pattern of ridges that interact with the carinae. Within Thalattosuchia these dental characteristics are only found in Torvoneustes and the teleosaurid Machimosaurus. The heavily ornamented dermatocranium of T. coryphaeus sp. nov. is in contrast to the unornamented (nasals and frontal)-lightly ornamented (maxillae and premaxillae) pattern seen in Torvoneustes carpenteri. Curiously, this pattern of reduction and loss of dermatocranium ornamentation is also observed in Metriorhynchus, Dakosaurus, and the subclade Rhacheosaurini. We hypothesize that the 'smooth' dermatocranium of Late Jurassic metriorhynchids evolved independently in each subclade (parallel evolution), and would have reduced drag, thereby making locomotion through water more energy efficient.

The evolution of the meatal chamber in crocodyliforms

“…In Metriorhynchidae, there is a clear separation between those two apertures in the occipital surface as in Suchodus brachyrhynchus (NHMUK R3700), Dakosaurus andiniensis (MOZ 6146P), Torvoneustes coryphaeus (Young et al. ) and Maledictosuchus riclaensis (Parrilla‐Bel et al. ).…”

Metriorhynchidae (Crocodylomorpha: Thalattosuchia) From Upper Jurassic–lower Cretaceous of Neuquén Basin (Argentina), With Comments on the Natural Casts of the Brain

“…The skull is incompletely preserved, thus an estimation of body length can not be made. several taxa have been found (e.g., Gasparini and IturraldeVinent, 2001;Buchy et al, 2007;Buchy, 2008;Young et al, 2012Young et al, , 2013. In Patagonia, Oxfordian-Early Kimmeridgian marine levels are restricted to marine platform limestones Some undescribed Patagonian specimens need to be described.…”

Occurrence of Torvoneustes (Crocodylomorpha, Metriorhynchidae) in marine Jurassic deposits of Oaxaca, Mexico

“…Three dental characters are different in these two species. In T. carpenteri the tooth crowns are slightly curved backward, slightly laterally compressed (about straight and circular in cross section), and the anastomosed pattern of enamel ornaments in the crown apex extends onto the carinae (Young et al, 2013b). Conversely, in T. coryphaeus the tooth crowns are notably curved, strongly laterally compressed, and its crown apex has a smooth area between the carinae and the anastomosed pattern of tooth enamel (Young et al, 2013b).…”

“…Among metriorhynchids, the characters 1 and 2 are synapomorphies of the tribe Geosaurini, while 3 and 4 are present only in the most derived geosaurinis. According to Herrera et al (2015), the tribe Geosaurini includes Purranisaurus potens Rusconi, 1948, from the upper Tithonian-lower Berriasian section of the Vaca Muerta Formation, Mendoza, Argentina; Plesiosuchus manselii (Hulke, 1870), from the upper Kimmeridgian to lower Tithonian deposits of England, and possibly also Spain (Ruíz-Omeñaca et al, 2010;Young et al, 2012); Dakosaurus (Quenstedt, 1843), represented by two nominal species and numerous ideterminate specimens from the upper Kimmeridgian to lower Berriasian marine deposits of Europe (England, France, Germany, Russia and Switzerland), South America (Argentina), and probably Mexico (see Vignaud & Gasparini 1996;Buchy, 2007Buchy, , 2008aYoung et al, 2012; among others); at least three nominal species of Geosaurus from the Tithonian to Valanginian deposits of Europe (Debelmas & Strannoloubsky, 1957;Young & Andrade, 2009;Young et al, 2013b; among others); and two nominal species of Torvoneustes from the Kimmeridgian deposits of England and some late Oxfordian specimen from England referred to this genus Young et al, 2013b). IGM 9026 cannot belong to genus Cricosaurus because even though the species of this genus have non-, uni, or even bicarinated teeth, none of them have denticles along the carinae (= non-ziphodont condition).…”

“…In T. carpenteri the tooth crowns are slightly curved backward, slightly laterally compressed (about straight and circular in cross section), and the anastomosed pattern of enamel ornaments in the crown apex extends onto the carinae (Young et al, 2013b). Conversely, in T. coryphaeus the tooth crowns are notably curved, strongly laterally compressed, and its crown apex has a smooth area between the carinae and the anastomosed pattern of tooth enamel (Young et al, 2013b). Although the cross sections of the teeth are unknown in IGM 9026, this specimen shows a distinctive combination of the other two characters; its teeth are strongly curved (as in T. coryphaeus) and the tooth enamel ornamentations form an anastomosed pattern reaching the carinae (as in T. carpenteri).…”