Ratfish are weird. While they’re related to sharks and rays – also members of the cartilaginous fish club – they aren’t particularly like either, from their exceptionally-large eyes to the sex organs some male ratfish dangle from right between their eyes. And now, thanks to a fossil braincase found in South Africa, the origins of these ghost-like fish are starting to swim into focus.
The fossil, named in 1986, is nothing more than the brainpan of a 280 million year-old fish. Paleontologists know it as Dwykaselachus oosthuizeni, and, up until now, it was thought to represent a form of totally-extinct shark called a symmoriid. When paleontologist Michael Coates and colleagues CT scanned the braincase and had a closer look, however, they found that Dwykaselachus was something very different.
The details of the fish’s eyes, inner ear, and braincase indicate that Dwykaselachus was a chimaeroid. This is the group that contains all 47 of today’s ratfish. What researchers thought was a shark turned out to be a very ancient example of a totally different group.
Reshuffling Dwykaselachus has changed the ratfish family tree. Its new ratfish status has pulled all of the symmoriids – including forms with comb-like ornaments jutting from their backs – out of the shark group and into the greater ratfish family. Likewise, Coates and colleagues report, an evolutionary icon has flipped positions.
For decades the fossil fish Cladoselache was heralded as a prime example of early sharks, not to mention an emblem of an important stage in the evolution of jawed vertebrates. Now it turns out that Cladoselache was probably an archaic member of the ratfish group, separate from sharks. This isn’t the first time such an icon has been rebranded. In 2013, the buzzsaw-jawed Helicoprion was revealed not to be a shark, but a ratfish, too.
All this shifting alters the larger picture of ratfish history. For one thing, Dwykaselachus and its relatives already had large eyes. To Coates and coauthors, this suggests that these fish might have taken to dimly-lit, deepwater habitats very early in their history. On top of that, the new evolutionary arrangement of these flexible fishes pushes the split between the ancestors of ratfish and the forebears of sharks and rays to over 360 million years ago. It was about that time that ratfish evolution apparently took off, spinning off forms that started to fill the seas when oceans dominated by sharks were only a distant possibility.
Coates, M., Gess, R., Finarelli, J., Criswell, K., Tietjen, K. 2016. A symmoriiform chondrichthyan braincase and the origin of chimaeroid fishes. Nature. doi: 10.1038/nature20806