Genetic Study Challenges Shark Classification, Proposing New Lineage for Hexanchiformes

A genetic analysis posted on February 15 on bioRxiv suggests sharks may not form a natural group, with Hexanchiformes—including cow sharks and frilled sharks—potentially representing a sister lineage to all other sharks, rays, and skates. Researchers Thomas Near and Chase Brownstein of Yale University examined genomes from 48 chondrichthyan species, finding conflicting results: protein-coding sequences supported traditional shark grouping, while ultraconserved elements indicated a separate branch for Hexanchiformes. These sharks, with six or seven gill slits instead of five, retain ancestral jaw structures and include record-holding species like the sharpnose and broadnose sevengill sharks. The study, not yet peer-reviewed, could redefine Chondrichthyes, a group dating back 439 million years with over 1,200 living species. Brownstein notes that sharks hold more evolutionary history than any vertebrate lineage, emphasizing the need for an accurate family tree to inform conservation efforts amid modern threats. Some scientists, like Gavin Naylor of the Florida Museum of Natural History, caution that genetic data alone may be insufficient due to lost evidence over millennia, advocating for anatomical comparisons. The findings have implications for evolutionary biology and biomedical research on jaw development.