In evolutionary biology today, it’s the ugly guys who get famous. But that hasn’t always been the case. When paleontologists were assembling a library of prehistoric life in the 19th century, they wanted to find the fossils they could easily categorize. The freaks, the weirdos, and the oddities were less well received, square pegs that wouldn’t fit in the round holes on a tree of life. However, today, it’s those hard to categorize fossils that tell the richest stories to biologists seeking to map evolutionary history, with all of its strange tributaries.
When Ramsay Heatley Traquair first described a 332-million-year-old fossil of a holocephalan (a relative of sharks and rays), he gave it a name that marked it as an outcast: Chondrenchelys problematica. Both parts of the name revealed his frustration with finding the right category for the ancient fish – Chondrenchelys means “cartilage eel” (a kind of oxymoron), and problematica is obvious. But the same features that made it so hard to classify in 1935 – plus a frightful new feature never observed before – made the long extinct species especially interesting to John Finarelli and Michael Coates of the University of Chicago.
Despite their obscurity today, holocephalans are an important group to scientists looking at the origins of jawed vertebrates – a group that would eventually include us. The holocephalans, including still-existing oddballs such as ratfish, the rabbit fish, and the elephant shark, split off from the rest of the fish world 400 million years ago, and have evolved in their own direction ever since. Studying the biology of modern holocephalans can tell us about some of our earliest ancestors, and studying holocephalan fossils gives the field even more direct insight into the early days of vertebrates.
“It represents an awful lot of vertebrate evolutionary history, that’s why it’s important,” said Finarelli, a research associate professor in the Department of Organismal Biology and Anatomy (OBA). “You’re getting down to a fundamental split at a very early point in the vertebrate family tree with what’s around today.”
“You look at these specimens to see what kind of insight you can get into the very general properties or conditions of how to make a jawed vertebrate, because you’re getting this independent pipeline,” said Coates, professor of OBA and senior author of the study.
Yet Chondrenchelys problematica lingered in obscurity until two new specimens were unearthed in the Mumbie Quarry in Scotland, described for the first time last week in the Proceedings of the Royal Society B. More complete than previously described fossils of the species, Finarelli and Coates noticed something unusual and unexpected about the long, skinny fish’s dental structure that sets it apart from other creatures, alive or extinct.
“It’s got teeth where you shouldn’t have teeth. Imagine a full set of teeth in your lips – that’s what this thing has,” Finarelli said. “They’re just fundamentally different from everything else we’ve ever seen in the jaw.”
To confirm that they didn’t just stumble upon mutant representatives of the species, the researchers went back and looked at the older Chondrenchelys fossils. Their re-examination confirmed that the unusual teeth was present in those less well preserved specimens as well, meaning the extra set of choppers was a standard feature for the species – and for no other species seen before or since.
The researchers haven’t yet commissioned an artist to reconstruct the face – “It would be pug ugly,” Coates said – but its homeliness is useful to scientists. Chondrenchelys existed at a time when holocephalans were exploding in diversity and the species newly-discovered weirdness only adds to the possible forms the group can take.
“One of the things the fossil record does is it shows you the extent to which living forms represent a remnant, or a limited part, of a former wider repertoire of things that can be done within this group,” Coates said. “We’ve searched around and we haven’t found anything like it today.”
The type of tooth present outside the jaws might also shake up how evolutionary biologists classify ancient fish. Teeth are an important part of the fossil record, with their durability ensuring that they are plentiful and well-preserved when paleontologists find them. In the absence of more complete specimens, many decisions about where to classify ancient species are made according to dental features until more of the skeleton can be found. In the case of Chondrenchelys, one type of tooth newly discovered by the researchers was previously only seen in a family of sharks called Petalodonts, suggesting that kind of tooth may not be as good a classifier as once thought.
“We’re trying to understand how useful the tooth record is,” Coates said. “There’s lots of good work out there defining these tooth kinds and showing where they are distributed. Part of the interest in our project is that we said we’re going to try and attach bodies to teeth.”
So the real value of Chondrenchelys problematica is that it continues to be problematic – shaking up scientific assumptions of what is possible for vertebrate teeth and biological forms by putting its ugly face forward.
“It’s a weird set of animals,” Finarelli said. “That’s one of the wonderful things about looking at this fossil: scratching your head, and saying, ‘This is just a bizarre specimen.’ Now there’s a whole new batch of questions to ask; that which was settled is now completely open to questioning again. In many ways have to go back to square one.”
Photos by Shahzad Ahsan
Finarelli JA, & Coates MI (2011). First tooth-set outside the jaws in a vertebrate. Proceedings. Biological sciences / The Royal Society PMID: 21775333