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New information on the skull roof of Protoichthyosaurus (Reptilia: Ichthyosauria) and intraspecific variation in some dermal skull elements

Published online by Cambridge University Press:  14 November 2019

Dean R Lomax Open the ORCID record for Dean R Lomax [Opens in a new window] ,
Judy A Massare  and
Mark Evans
Dean R Lomax*
Affiliation:
Department of Earth and Environmental Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK
Judy A Massare
Affiliation:
Department of Earth Sciences, State University of New York, College at Brockport, Brockport, NY14420, USA
Mark Evans
Affiliation:
Centre for Palaeobiology Research, School of Geography, Geology and the Environment, University of Leicester, Leicester LE1 7RH, UK British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK
*
Author for correspondence: Dean R Lomax, Email: dean.lomax@manchester.ac.uk
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Abstract

A previously unrecognized specimen of Protoichthyosaurus prostaxalis, LEICT G142.1991, from the Lower Jurassic of Barrow-upon-Soar, Leicestershire, UK, includes an almost complete three-dimensional skull that provides new information on the configuration of the skull roof. The position of the pineal foramen (between the frontals and the parietals) and an elongated internasal foramen in a depression along the midline of the nasals are clearly shown. The maxilla makes up a significant portion of the external naris ventral margin, an unusual character for the genus/species. This reflects intraspecific variation, not evidence of a new taxon. The specimen enables comparisons of skull roof morphology with Ichthyosaurus and Stenopterygius, two common Early Jurassic taxa. In particular, the position of the pineal foramen is similar to Stenopterygius, but distinguishes Protoichthyosaurus from Ichthyosaurus. The lack of a frontal–prefrontal contact and the posteriorly wide nasals distinguishes Protoichthyosaurus from Stenopterygius. We also present a revised reconstruction of the skull roof morphology of Ichthyosaurus. Three additional specimens of Protoichthyosaurus are referred to the genus: another partial skull, referred to P. prostaxalis, and two isolated forefins, identified by their unique morphology.

Keywords

intraspecific variationIchthyosaurusStenopterygiusBarrow-upon-SoarLower Jurassic
Type
Original Article
Information
Geological Magazine , Volume 157 , Issue 4 , April 2020 , pp. 640 - 650
Copyright
© Cambridge University Press 2019

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