SICB Division of Phylogenetics and Comparative Biology (DPCB)

DPCB Researchers Database Entry

Kerin Claeson

Chondrichthyes and Evolution of Vertebrate Morphology
Chondrichthyes (chimaeroids, sharks, and batoids) is a clade of fishes whose endoskeleton is composed entirely of cartilage. This contrasts with most vertebrates that have bony skeletons that began as an unmineralized cartilaginous scaffold, to be replaced later during endochondral ossification. In addition to its mineral composition, the chondrichthyan skeleton is highly modified compared to their distant bony relatives, most notably by the fusion of several skeletal regions. The continuous nature of the skeleton in cartilaginous fishes makes the objectivity of character delimitation even more difficult than it is for other groups of vertebrates. It is therefore of particular importance to supply explicit justification when proposing character states for phylogenetic studies of the Chondrichthyes and their inclusive clades, such as the Elasmobranchii (sharks and batoids). With such a deep ancestry, these fishes are particularly important for understanding the evolution of vertebrate morphology.

I study elasmobranchs, and other long-lineage fishes, searching for patterns in morphology and inferring processes of vertebrate evolution. To do this, I integrate comparative anatomy, developmental biology, paleontology, and phylogenetics. Different topological results are expected considering the fundamental differences between the types of data and search criteria used to infer phylogenies. However, it is when those hypotheses differ that new hypotheses about the criteria used to derive them are generated, and new lines of evidence ought to be examined. The inclusion of ontogenetic characters with paleontological characters may overturn the inferences of systematics based on almost entirely extant morphological data sets. With a robust phylogeny including fossils, we may be able to polarize molecular characters for higher-level taxonomic analyses, assess the time of origin of genes, and determine rates and patterns for lower level gene evolution.