Abstract
Numerous specimens of Ichthyosaurus are known, but only very few small examples (total length of <1 m) have been assigned beyond Ichthyosaurus sp. Here, we report on a very small specimen (preflexural length of 560 mm) that can be unequivocally assigned to Ichthyosaurus communis due to possessing a unique combination of diagnostic skull and postcranial characters that are found in larger examples of the species. Furthermore, the specimen is identified as a neonate because of the small size, large sclerotic ring relative to the orbital region, and poorly ossified (highly cancellous) bones of the skull and postcranium. It is not an embryo as it is not preserved within an adult specimen and stomach contents are clearly evident. This is therefore the first neonate Ichthyosaurus communis skeleton to be described. The specimen, in the Lapworth Museum of Geology, University of Birmingham, has no provenance data associated with it. A microfossil analysis of the matrix in which the ichthyosaur skeleton is preserved strongly suggests a stratigraphic range of uppermost Hettangian to lowermost Sinemurian age (Lower Jurassic), but does not provide any geographical information.
Highlights
Ichthyosaurus is the most common genus of Lower Jurassic ichthyosaur to be found in the UK and was the first to be recognised by science (De la Beche and Conybeare 1821)
It can be assigned to the genus Ichthyosaurus because of the following combination of characters: maxilla excluded from margin of the external naris by subnarial process of premaxilla and anterior process of lacrimal; forefin with at least five primary digits; hindfin with at least four primary digits with a bifurcation in the third row; coracoid with prominent anterior and posterior notches; and a tripartite pelvis with an unfused pubis and ischium
BU 5289 displays several features that are consistent with Ichthyosaurus communis, which led Massare and Lomax (2017a) to refer it to the species, a full description is provided here
Summary
Specimen BU 5289 is a practically complete skeleton lying on its left side, exposed in lateral view (Figure 1). The specimen can be further assigned to Ichthyosaurus communis because it shares the following unique combination of characters, as redefined by Massare and Lomax (2017a): a symmetric, triangular maxilla with an anterior process that extends beyond the external naris, and a posterior process that extends well under the orbit; the premaxilla supra- and subnarial processes are about equal in length, and extend about half way across the dorsal and ventral margins of the external naris, with the nasal contributing to about half of the dorsal margin; the anterior process of the jugal extends only slightly beyond the orbit, and the jugal dorsal ramus has a right angle dorsal bend; the humerus is much longer than wide, with a prominent dorsal process; and four elements are in the third row of the hindfin, owing to a bifurcation of tarsal 2. A fragment of the ilium is present, but an impression in the matrix suggests it was originally preserved and was probably shorter than the ischium Both femora are present, the left is isolated in the matrix and the right is articulated with a portion of hindfin (Figure 8). The preservation and texture is comparable to coprolitic material so this may represent a bromalitic mass
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