More than a century ago, scientists were baffled by the discovery of an unusual fossil unearthed in 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, through the use of high-resolution imagery, a research team has finally determined that this mysterious fish may very well be one of our earliest ancestors.
“To place Paleospondyl in the evolutionary tree, identifying each skeletal element is a prerequisite,” said Tatsuya Hirasawa, 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 have persisted for years. so long because of two factors: its small size, with a body measuring only 2.4 inches (6 centimeters) long, and the unfortunate fact that fossilization has greatly compressed its skeleton, squeezing the individual bones into a deformed mass that was a paleontological nightmare to unravel, Hirasawa told Live Science in an email.
Prior to the new study, scientists knew that Paleospondyl lived in the Middle Devonian period, about 398 to 385 million years ago. The fish had well-developed fins but lacked limbs. Curiously, it seemed to lack teeth, unlike most vertebrates of that 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 (opens in a new tab) this Paleospondyl was a primitive lungfish. However, a 2016 study, published by Hirasawa in the journal Zoological letters (opens in a new tab), suggested that it was instead a relative of hagfish. A year later, a team from the Australian National University questioned the status of fish hagfish, proposing instead that it was a cartilaginous fish like modern sharks.
Related: Fish sprouted fingers before venturing onto land, fossils show
This taxonomic tennis match is not a recent phenomenon either. “This strange animal has baffled scientists since its discovery in 1890 as an impossible puzzle,” said study co-author Yu Zhi (Daisy) Hu, a researcher in the Department of Materials Physics of the Australian National University of Canberra. said in a press release (opens in a new tab).
Really, it seems the only thing paleontologists could agree on was that no one really knew the identity of this animal.
Recently, Hirasawa and Hu, armed with computed tomography (CT) technology, were able to produce the highest resolution digital images of Paleospondyl nowadays. To collect the most accurate data, they had to select the best fossils. Since 1890, many Paleospondyl specimens have been found, but most have been damaged in some way – either by fossilization or excavation – which may have contributed to earlier misclassifications. To circumvent this problem, the authors of the new study chose specimens with heads completely encased in rock. “I looked for specimens that only exposed tails, and eventually found two specimens that only exposed the tail part on the surface,” Hirasawa said.
Analyzes of these specimens revealed several key characteristics. One was that the inner ear was made up 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 Paleospondyl and more primitive jawless fish like hagfish, which lack this feature. The researchers were also able to identify cranial features that place Paleospondyl in a group called the tetrapodomorphs, which contains all four-limbed creatures and their closest relatives. More importantly, phylogenetic analysis of these exceptional features suggests that Paleospondyl might not be just any garden variety tetrapodomorph; it could be the ancestor of all tetrapods.
“Our analyzes provided an inference that Paleospondyl was a close relative of vertebrates with limbs (with fingers) and those with limb-like fins,” also known as “fishapods,” Hirasawa said. According to the researchers’ findings, Paleospondyl was probably more closely related to limb-bearing tetrapods than to older species like lungfish and coelacanths, which would make Paleospondyl an aquatic predecessor close to the first animals that crawled on land.
Even though this phylogenetic mystery is now solved, a number of open questions still remain. Tetrapodomorphs usually have teeth, but Paleospondyl didn’t – or if they did, they failed to fossilize. He also lacked any obvious appendages, while his closest relatives usually had them.
What could explain these anomalies? One possibility, Hirasawa suggested, is that teeth and limbs could have been lost during evolution. Paleospondyl. Another possibility is that the known Paleospondyl the fossils could represent larval or juvenile forms of the animal.
“We may never know if these features were lost through evolution or if normal development froze halfway through the fossils,” Hirasawa said. said in a press release.
Although we now have a better idea of where Paleospondyl seat on the evolutionary tree, there is still a lot of work to be done. Now, just as when it was first discovered, this fish closely guards many of its ancient secrets.
This research was published May 25 in the journal Nature (opens in a new tab).
Originally posted on Live Science.