Ancient toothless 'eel' is your earliest known ancestor

More than a century ago, scientists were stumped by the discovery of an unusual fossil unearthed from a Scottish quarry. The remains suggested a toothless eel-like creature with a potentially cartilaginous skeleton, and for 130 years after the mysterious creature — named Palaeospondylus gunni — was unearthed, it continued to defy classification. Now, with the use of high-resolution imaging, a research team has finally determined that this mysterious fish may very well be one of our earliest ancestors. 

"To place Palaeospondylus in the evolutionary tree, identification of each skeletal element is prerequisite," said Tatsuya Hirasawa, an associate professor of paleontology at the University of Tokyo in Japan, and lead author of a new study describing the fossil. The mysteries surrounding this little fish persisted for so long because of two factors: its diminutive size, with a body measuring just 2.4 inches (6 centimeters) long, and the unfortunate fact that fossilization dramatically compressed its skeleton, squeezing individual bones into a distorted mass that was a paleontological nightmare to unravel, Hirasawa told Live Science in an email. 

Prior to the new study, scientists knew that Palaeospondylus lived in the middle Devonian epoch, roughly 398 million to 385 million years ago. The fish had well-developed fins but lacked limbs. Curiously, it seemed to lack teeth, unlike most vertebrates of this time. 

Repeated attempts to place the fish on the evolutionary tree pinned it all over the map. In 2004, researchers confidently reported in the journal American Scientist that Palaeospondylus was a primitive lungfish. However, a 2016 study, published by Hirasawa in the journal Zoological Letters, suggested it was instead a hagfish relative. One year later, a team at Australian National University questioned the fish's hagfish status, proposing instead that it was a cartilaginous fish like modern sharks. 

Related: Fish sprouted fingers before they ventured onto land, fossil shows

Nor is this taxonomic tennis match a recent phenomenon. "This strange animal has baffled scientists since its discovery in 1890 as a puzzle that’s been impossible to solve," study co-author Yu Zhi (Daisy) Hu, a researcher in the Department of Materials Physics at the Australian National University in Canberra, said in a statement. 

Truly, it seems that the only thing that paleontologists could agree on was that nobody really knew the identity of this animal.

Recently, Hirasawa and Hu, armed with micro-computed tomography (CT) scanning technology, were able to produce the highest resolution digital images of Palaeospondylus to date. To gather the most accurate data, they had to select the best fossils. Since 1890, many Palaeospondylus specimens have been found but most were damaged in some way — either by fossilization or excavation — which may have contributed to previous errors in classification. To circumvent this issue, the authors of the new study chose specimens with heads that were completely encased in rock. "I looked for specimens exposing only tails, and finally found two specimens that exposed only the tail part on the surface," said Hirasawa.

Scans of these specimens revealed several key features. One was that the inner ear was composed of several semicircular canals, much like the ears of modern fish, birds and mammals. This is significant, the authors noted, because it places some evolutionary distance between Palaeospondylus and more primitive jawless fish like hagfishes, which lack this feature. The researchers were also able to identify cranial features that place Palaeospondylus in a group called the tetrapodomorphs, which contains all four-limbed creatures and their closest relatives. Most importantly, phylogenetic analysis of these outstanding features suggests that Palaeospondylus might not be just any garden-variety tetrapodomorph; it might be the ancestor of all tetrapods. 

"Our analyses provided an inference that Palaeospondylus was a close kin to vertebrates having limbs (with fingers) and those having limb-like fins," also known as "fishapods," said Hirasawa. According to the researchers' findings, Palaeospondylus was likely more closely related to limb-bearing tetrapods than to more ancient species like lungfishes and coelacanths, which would make Palaeospondylus a close aquatic predecessor of the first animals that crawled onto land.

Even if this phylogenetic mystery is now solved, a number of open questions still linger. Tetrapodomorphs usually possess teeth, but Palaeospondylus had none — or if it did, they failed to fossilize. It also lacked any obvious appendages, while its closest relatives typically had them. 

What might explain these anomalies? One possibility, Hirasawa suggested, is that teeth and limbs may have been evolutionarily lost in Palaeospondylus. Another possibility is that the known Palaeospondylus fossils might represent larval or juvenile forms of the animal. 

"Whether these features were evolutionarily lost or whether normal development froze half-way in fossils might never be known," Hirasawa said in a statement. 

While we now have a better idea of where Palaeospondylus sits on the evolutionary tree, there is still much work to be done. At the moment, just like at the time of its discovery, this fish closely guards many of its ancient secrets. 

This research was published May 25 in the journal Nature. 

Originally published on Live Science.