Evolutionary relationships among bullhead sharks (Chondrichthyes: Heterodontiformes)

The evolution of modern sharks, skates and rays (Elasmobranchii) is largely
enigmatic due to their possession of a labile cartilaginous skeleton; consequently, taxonomic
assignment often depends on isolated teeth. Bullhead sharks (Heterodontiformes) are a group
of basal neoselachians, thus their remains and relationships are integral to understanding
elasmobranch evolution. Here we fully describe †Paracestracion danieli – a bullhead shark
from the Late Jurassic plattenkalks of Eichstätt, Germany (150–154 Ma) – for its inclusion in
cladistic analyses (employing parsimonious principles) using morphological characters from
complete †Paracestracion and Heterodontus fossil specimens as well as extant forms of the
latter. Results confirm the presence of two separate monophyletic clades within
Heterodontiformes based on predominantly non-dental characters, which show a strong divergence in body morphology between †Paracestracion and Heterodontus (the latter possessing a first dorsal fin and pectoral fins that are placed more anterior and pelvic fins that
are placed more posterior). This study emphasizes the importance of including non-dental
features in heterodontiform systematics (as compared to the use of dental characters alone)
and supports the erection of the family †Paracestracionidae. Further, phylogenetic analysis of
molecular data from five extant species suggests that crown heterodontiforms arose from a
diversification event 42.58 Ma off the west coast of the Americas.