Ontogeny of the braincase inStenopterygius(Reptilia, Ichthyosauria) from the Lower Jurassic of Germany

Abstract

The fossil record of the ichthyosaur genus Stenopterygius comprises a large number of specimens of all ontogenetic stages. This makes the taxon a good model for ontogenetic studies in a Mesozoic marine reptile. Here, we describe the morphology of the braincase elements of Stenopterygius over ontogeny and compare it with that of other known ichthyosaur braincases. Substantial ontogenetic changes were observed in most elements. These included negative allometry of the exoccipitals relative to the basioccipital, changes in the morphology of the parabasisphenoid and proportional reduction of the ridge separating the right and left internal carotid arteries, variable relative length of the paroccipital process of the opisthotic in juvenile specimens, and relative ossification of the basioccipital peg. Furthermore, we observed variability in relative braincase ossification between embryos within a single gravid female.

Cranial anatomy ofBesanosaurus leptorhynchusDal Sasso & Pinna, 1996 (Reptilia: Ichthyosauria) from the Middle Triassic Besano Formation of Monte San Giorgio, Italy/Switzerland: taxonomic and palaeobiological implications

“…The paroccipital process is roughly as wide as it is tall when measured at its widest position. This morphology resembles the condition in more derived Euichthyosauria, in which the paroccipital process is usually stouter (e.g., Stenopterygius ; Miedema & Maxwell, 2019 ) and Temnodontosaurus trigonodon ( Maisch, 2002 ), than in the more basal Mixosaurus cornalianus or Phantomosaurus neubigi ( Maisch & Matzke, 2006 ). In these latter taxa the paroccipital process is longer than wide and tapers distally, which is not the case in the opisthotic associated with GPIT 1793/1 ( Maisch, Matzke & Brinkmann, 2006a ; Maisch & Matzke, 2006 ).…”

“…This was also noted by Maisch & Matzke (1997a) , who suspected this notch to be a neurovascular foramen similar to those described in Ophthalmosaurus icenicus and Ichthyosaurus ( Andrews, 1910 ; McGowan, 1973 ). This morphology is most similar to Stenopterygius , as that taxon also lacks distinct supraoccipital foramina in most observed specimens, but contrasts with the morphology of most other Euichthyosauria in which the supraoccipital is known, as many taxa have two distinct foramina on the lateral sides of the element ( Miedema & Maxwell, 2019 ).…”

“…On the dorsal surface of the bone, the two facets for the exoccipitals are well-defined; these are represented by two shallow drop-shaped depressions, tapered rostrally. The two exoccipital facets are also parallel and very close to each other, as seen in Stenopterygius ( Miedema & Maxwell, 2019 ), and are separated by a dorsal median ridge (i.e., the floor of the foramen magnum). Rostral to the medial ridge a prominent basioccipital peg emerges, laterally delimiting two subtriangular articular facets for the basisphenoid.…”

“…There may be a high degree of conservatism in the inner-ear shape of ichthyosaurs, although this needs further study. The shape of the indentation is different from the early adult morphology in Stenopterygius , although it is unsure whether the shape of the inner ear or the shape of the enclosing opisthotic is more affected by ontogeny ( Miedema & Maxwell, 2019 ). The indentation for the caudal vertical semicircular canal does not reach higher than the dorsal margin of the medial head—this leads to a morphology which differs from many post-Triassic taxa in which this indentation creates a second “process” as visible in caudal view (clearly present in Stenopterygius and Platypterygius australis and to some degree in Ichthyosaurus and Temnodontosaurus ( McGowan, 1973 ; Maisch, 2002 ; Kear, 2005 ; Miedema & Maxwell, 2019 )).…”

“…The area is slightly convex ventrally resembling Ichthyosaurus , but not as convex as in, e.g., Phantomosaurus ( Maisch & Matzke, 2006 ). A wide extracondylar area is characteristic of more basal ichthyosaurs, as the extracondylar area is severely reduced in Stenopterygius and virtually absent in Ophthalmosauridae ( Miedema & Maxwell, 2019 ). The convex occipital condyle is located above this area and occupies a medial portion of the basioccipital bone that is half the size of the extracondylar area.…”