Genetic Study Questions Traditional Shark Family Tree

A preprint posted to the bioRxiv server examined genomes from 48 cartilaginous fish species and produced conflicting results on the placement of Hexanchiformes, the order that includes frilled sharks and cow sharks. Protein-coding genes supported the conventional grouping in which all sharks form a single branch separate from rays and skates.

Ultraconserved elements instead placed Hexanchiformes as an early-diverging lineage that may sit outside that branch.

Background on the group Cartilaginous fishes have existed for at least 439 million years. Hexanchiformes retain six or seven gill slits and jaw structures that resemble conditions seen in older fossil vertebrates. Researchers have long debated whether these traits represent retained primitive features or later reversals.

The new genomic data favor the interpretation that they are retained primitive features.

Implications and remaining uncertainty An evolutionary biologist not involved in the study told Nature that sequence data can create blind spots when lineages diverged hundreds of millions of years ago. The authors of the preprint described the root of the shark tree as a hard phylogenetic problem.

More than a third of shark and ray species are listed as threatened, and several deep-sea Hexanchiformes remain poorly studied. Confirmation that some members sit near the base of the tree would affect how scientists interpret the evolution of jaws, body plans, and genome architecture in this ancient vertebrate group.