Ontogenetic variation in the skull of Stenopterygius quadriscissus with an emphasis on prenatal development

Abstract

The availability of a large sample size from a range of ontogenetic stages makes Stenopterygius quadriscissus a good model to study ontogenetic variation in a fossil sauropsid. We qualitatively examined pre- and postnatal ontogenetic changes in the cranium of S. quadriscissus. The prenatal ossification sequence is similar to other diapsids, exhibiting delayed chondrocranial ossification compared to the dermatocranium. In the dermatocranium, the circumorbital area is more ossified earlier in development relative to other elements, especially those of the skull roof where ossification is comparatively weaker across prenatal stages. Perinatally all cranial elements are ossified, and many scarf and step joints are already closed. We propose four prenatal and three postnatal stages in S. quadriscissus on the basis of relative ossification, size and qualitative cranial characters pertaining to the jugal, parietal, frontal, pterygoid and surangular. These will provide a basis for determining ontogenetic stages in other ichthyosaurs. Moreover, our postnatal observations aid in refining ontogenetic characters for phylogenetic studies. Lastly, we observed that the antimeric sutures of the midline of the skull roof are open perinatally and that fusion of the midline only appears in the adult stage. We hypothesize that the loose connection of the midline functions as a fontanelle, limiting potential damage during birth.

Is the hyoid a constraint on innovation? A study in convergence driving feeding in fish-shaped marine tetrapods

“…The element does not resemble any braincase elements of Ichthyosaurus , and instead is similar to the hyoid corpora in Hauf fi opteryx and Stenopterygius . The hyoid corpus has a rounded triangular outline, as in these two taxa (Motani et al 2013; Maxwell and Cortés 2020; Miedema and Maxwell 2022). It differs from Hauf fi opteryx in not showing two distinct posterior indentations and from Stenopterygius in being slightly more triangular (Motani et al 2013; Maxwell and Cortés 2020; Miedema and Maxwell 2022).…”

“…A cartilaginous hyoid corpus situated at the midline of the skull has been suggested (Kiprijanoff 1881; Kear 2005; Motani et al 2013). However, the discovery of ossified hyoid corpora at different ontogenetic stages in Stenopterygius (Miedema and Maxwell 2022) and in Ichthyosaurus (Fig. 6) might indicate that ichthyosaurs did possess a hyoid corpus after all.…”

Heads or tails first? Evolution of fetal orientation in ichthyosaurs, with a scrutiny of the prevailing hypothesis

“…Cranial fetal material is present in PIMUZ T 1902 and PIMUZ T 4830. The high degree of ossification of the dermal skull bones (especially the skull roof elements) as well as an advanced degree of ossification in the chondrocranium suggests a developmental stage comparable to stage 4 of Stenopterygius [ 15 ], meaning the fetuses are at a stage where prenatal development is largely finished and birth is imminent. Thus, in conclusion, the fetuses in PIMUZ T 1902 and PIMUZ T 2262 display orientations in utero suggesting tail-first birth and those in PIMUZ T 4830 are in head-first presentation close to the cloaca.…”

“…In viviparous squamates (as in their oviparous counterparts), fetuses are tightly curled through lateral flexion, forming a compact rounded mass tightly enclosed within the fetal membranes [ 37 , 38 ]. In contrast, ichthyosaur fetuses are only curled up early in development (i.e., stage 1 Stenopterygius [ 15 ]); subsequently they are stretched out (elongated) in a cranial/caudal direction [ 2 , 4 , 9 , 14 ]. Given the differences in uterine morphology, fetal turning in ichthyosaurs can therefore not be ruled out.…”

Dietary niche partitioning in Early Jurassic ichthyosaurs from Strawberry Bank

“…A three‐dimensional cranial model is divided into a mesh of tiny elements for detailed observation of stress response throughout the structure, and parameterization of the model includes the input of material properties, most often bone or muscle, and the assignment of force magnitude and direction based on the length, size and position of the reconstructed cranial elements and musculature (Rayfield, 2007). We assigned only the material properties of bone to the models, discounting potentially cartilaginous elements and elements that might have been unfused in young specimens (Miedema & Maxwell, 2022). We cannot identify the ontogenetic stage of the specimens other than to say they are juveniles, but the cartilaginous elements in a juvenile skull would spread the forces rather than alter their general distribution, which is the focus of this study.…”