Because they lack a bony skeleton, cartilaginous fishes like sharks are often thought of as primitive. And as such, when imagining the appearance of an ancestor of all jawed vertebrates—from reptiles and amphibians to birds, mammals, and people—we frequently imagine something resembling a shark. Now, researchers re-examining a 415-million-year-old fish head from Siberia reveal how the last common ancestor of jawed vertebrates was surprisingly not shark-like: Instead, it resembled bony fishes, like tuna or codfish. That means sharks had ancestors that were once bony too. The findings were published in Nature this week.
Currently housed in Estonia, the skull was discovered near Sida River in Siberia in 1972. Based on its external features, researchers previously thought it belonged to an early bony fish (or osteichthyan), a group that not only have skeletons made of bone, but also armor on their faces, like tuna, salmon, and even coelacanths. But now, Martin Brazeau of Imperial College London, together with Oxford’s Matt Friedman and Sam Giles, used X-ray CT scans to create a 3D virtual model of the fossil—revealing never-before-seen anatomical details inside the skull, such as the traces left by networks of blood vessels and nerves.
Underneath its bony skull roof was a braincase similar to that of cartilaginous fishes (or chondrichthyans), like today’s sharks and rays. The team placed it in a new position at the base of all jawed vertebrates, before bony fishes and sharks spilt.
Chondrichthyans are often treated as proxies for what the ancestral jawed vertebrate would have looked like because of their lack of a bony skeleton. “The results from our analysis help to turn this view on its head: The earliest jawed vertebrates would have looked somewhat more like bony fishes, at least externally, with large dermal plates covering their skulls,” Giles explains in a university statement. “In fact, they would have had a mix of what are now viewed as cartilaginous- and bony fish-like features, supporting the idea that both groups became independently specialized later in their separate evolutionary histories.”
That means that sharks lost their bony armor early in their evolution. "Losing your bony skeleton sounds like a pretty extreme adaptation," Friedman says, but new discoveries like these suggest that "the ancient ancestors of modern sharks and their kin started out just as 'bony' as our own ancestors.”
Because of its newfound dual-sided nature, the ancient fish was renamed Janusiscus schultzei after Janus, the double-faced Roman god.
"It's misleading to think of evolution as a ladder which we reconstruct with 'missing links'," Brazeau says in a news release. "It's more accurate to think of it as a family tree, and research into evolution is like solving a puzzle. Each new piece you find throws the surrounding pieces into context and helps you to understand them better. Janusiscus has helped us to look at sharks differently and will ensure they are no longer dismissed as being 'frozen' at a primitive stage of evolution."
Images: S. Giles et al., Nature 2015
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