The rise of jaws? Tiny fossil sheds light on fishy origins

A tiny Siberian fish fossil that’s a whopping 415 million years old may be part of a group of fish that was ancestor to all the jawed vertebrates living today, according to a team of European scientists.

Described online in the journal Nature, the fossil fish, Janusiscus schultzei, could help researchers flesh out - and perhaps redraw - portions of the ancient fish family tree.

The fossil, discovered near the Sida River in Siberia in 1972, is a strange, chimeric mishmash of features of very different lineages, which makes it valuable to scientists who have long struggled to sort out the ancient fish family tree between 443 million and 358 million years ago, during the Silurian and Devonian periods.

The vast majority of fish in the Earth’s waters today are bony fishes – descendants of a group known as osteichthyans. But back in fishes’ heyday, around 400 million years ago, osteichthyans weren’t the only group around. There were also cartilaginous fish called chondrichthyans (whose descendants include sharks and rays) as well as extinct armored fishes called placoderms.

These different groups of fish were all descended from gnathostomes, or jawed fish, and scientists want to know what those ancestral gnathostomes look like – in part because the answer could affect how we see our own origins.

Here’s a brief family history. Osteichthyans, the bony fishes, split into two main groups – the ray-finned fishes, which make up the vast majority of fish in the ocean today, and the lobe-finned fishes, whose descendants eventually crawled onto land. Those descendants, the “tetrapods,” evolved into all four-legged vertebrates on Earth today, from reptiles to mammals (and, yes, including humans).

So what did the ancient ancestors of all jawed vertebrate life on Earth today look like? It was long thought that gnathostome species would look more like the cartilaginous chondrichthyans – their living members, the sharks, are seen as “primitive,” living fossils that haven’t changed much over time. Bony structures, like the ones in our own bodies, appear much more complex, so it was assumed they evolved later. But recent studies have found that shark-like, cartilaginous bodies may have been a later development – and that the bony fishes might be the ones who inherited the more primitive traits.

For this study, the researchers subjected the fossil skull of this fish to an X-ray CT scan to examine tell-tale structures in the animal’s skull. Like other recently examined specimens, the Janusiscus schultzei fish fossil also has a strange mix of features that seem like they’re both from the bony osteichthyans and the cartilage-bodied chondrichthyans.

The structure of the skull roof bones look a lot like those of osteichthyans, but a number of features, including a flat-based braincase, don’t match osteichthyans at all. In fact, certain features of the braincase actually look a lot more like those in early chondrichthyan species.

“Janusiscus presents an unexpected suite of osteichthyan, chondrichthyan and generalized gnathostome traits,” the study authors wrote.

Because this fish’s head seems to tell at least two stories from two divergent groups of fishes, the species was named for the Roman deity Janus – god of doorways and transitions, often depicted with two faces.

The findings show that other fossils that were described as bony fishes based on “superficial” traits might need to be reexamined, the study authors pointed out.

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