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Here’s what you’ll learn when you read this study:
For hundreds of millions of years, creatures we call sharks have roamed Earth’s waters.
A new study analyzing the genomes of nearly 40 sharks found that skates and rays likely evolved from the shark body plan, and that hexanchiform sharks are actually evolutionarily distinct.
This means that sharks are a paraphyletic group—they all share a common ancestor, but not all species are contained within the group.
Sharks are old—really old. Likely arising 450 million years ago, shark-like fish have existed on Earth longer than there have been trees. But precisely because they’ve been around for so long—and because these captivating apex predators are cultural superstars, thanks to vehicles like Shark Week and Hollywood’s first summer blockbuster, Jaws—it might be pretty surprising to learn that scientists don’t actually know what a shark is.
Okay, let’s back up a little bit. Marine biologists do have a pretty good idea of what sharks are in general terms: they belong to a class called Chondrichthyes (jawed, cartilaginous fishes) that also includes skates and rays. The Great White, for example? Definitely a shark. However, a new study published on the preprint server bioarXiv suggests that species within the order Hexanchiformes—such as cow sharks and frilled sharks—are actually part of an evolutionary lineage that’s distinct from what we call sharks entirely. This would mean that skates and rays (also known as batoids) are actually more closely related to sharks than hexanchiform sharks are.
“This result implies that rays and skates are just another type of shark, and that the shark body plan came first,” Chase Brownstein, evolutionary biologist at Yale University and co-author of the study, told Nature.
Central to this debate is whether sharks are monophyletic or paraphyletic. In taxonomic terms, a monophyletic group (or clade) includes an ancestor and all of its descendants, whereas a paraphyletic group includes an ancestor and some (but not all) of its descendants. By analyzing 38 published genomes of sharks, Brownstein and coauthor Thomas Near confirmed that the main lineages of sharks, skates, and rays do exist as currently defined, but as a whole, the clade we refer to as sharks is paraphyletic, due to the fact that hexanchiform sharks are evolutionarily distinct from monophyletic sharks.
While frilled and cow sharks definitely look like they belong in the shark family, they have some key physical differences, including more gills, primitive jaws, and eel-like bodies, according to Nature. This could mean that the divergence between modern sharks and hexanchiforms represents the deepest divergence of the shark superorder Squalomorphi. Further analysis by Brownstein and Near also confirms that the diversification of most sharks occurred during the Jurassic period, and further diversification of skates and rays took place later, in the Cretaceous-Paleogene.
“Genomes have the capacity to drastically modify hypotheses about the relationships of species,” the authors wrote. “By clarifying where phylogenetic uncertainty remains in the relationships of one of the two major divisions of crown-group jawed vertebrates, we illuminate the unexpectedly ancient evolutionary history of some chondrichthyans that are now under threat of extinction due to the current anthropogenic biodiversity crisis.”
Looks like Shark Week just got a bit more interesting.
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