Morphology of the anterior vertebral region in elasmobranchs: special focus, Squatiniformes

The morphology of extant and extinct Squatiniformes is well conserved within this lineage, but differences are of consequence to character interpretations for phylogenetic analyses. Investigation of the extinct taxon dagger Pseudorhina alifera (Munster, 1842) from the Late Jurassic of Solnhofen with extant species of Squatina provide new evidence that members of Squatiniformes possess a basioccipital fovea. Presence of a basioccipital fovea is the ancestral condition in Neoselachians and hypothesized to be lost in members of Hypnosqualea. In addition, species of Squatina all posses a reduced occipital hemicentrum while the occipital hemicentrum in dagger Pseudorhina is unreduced, indicating the condition in Squatina is an example of postdisplacement heterochrony. Pristiophoriformes and Batoidea also are characterized by a lack of a basioccipital fovea and hemicentrum. However, extinct members of Batoidea, such as dagger Spathobatis, and modern taxa do possess a distinct notch in the posterior basicranium ventral to the foramen magnum. Except for the lack of an associated occipital hemicentrum, this notch is similar to the basioccipital fovea, but its homology is not yet addressed. Furthermore, within all species of Squatiniformes and Pristiophoriformes, as well as some members of Orectolobiformes and Carcharhiniformes, basiventral cartilages are laterally expanded, contributing to a broad articulation with the occipital condyle. The disparate taxa with modifications to the basiventral cartilages suggest a significant functional, rather than phylogenetic, signal for this feature.