A baffling mystery fish may have finally been identified and assigned to the evolutionary tree of life.
Scientists were baffled by the finding of a strange fossil in a Scottish quarry more than a century ago. The remnants revealed a toothless eel-like creature with a potentially cartilaginous skeleton, and the unusual creature — dubbed Palaeospondylus gunni — defied classification for 130 years after it was discovered. A research team has finally discovered that this mysterious fish may very well be one of our oldest progenitors, thanks to the use of high-resolution imaging.
"Identifying each skeleton feature is required to position Palaeospondylus in the evolutionary tree," said Tatsuya Hirasawa, an associate professor of palaeontology at the University of Tokyo in Japan and lead author of a new research documenting the fossil. The mystery surrounding this small fish has persisted for so long due to two factors: its small size, with a body measuring just 2.4 inches (6 centimetres) long, and the unfortunate fact that fossilisation dramatically compressed its skeleton, squeezing individual bones into a distorted mass that was a paleontological nightmare to unravel, according to Hirasawa.
Palaeospondylus flourished in the middle Devonian epoch, around 398 million to 385 million years ago, according to previous research. The fish lacked arms but had well-developed fins. It had no teeth, which was unusual for a vertebrate at the time.
Attempts to position the fish on the evolutionary tree resulted in it being pinned all over the map. Palaeospondylus was a primitive lungfish, according to researchers who published their findings in the journal American Scientist(opens in new tab) in 2004. However, Hirasawa's 2016 study in the journal Zoological Letters(opens in new tab) suggested that it was actually a hagfish related. A team from Australian National University questioned the animal's hagfish identity a year later, claiming it was a cartilaginous fish similar to modern sharks.
This taxonomic tennis match is also not a new phenomena. "Since its discovery in 1890, this unique species has perplexed scientists as an unsolvable problem," 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.
Indeed, it appears that the only thing palaeontologists could agree on was that no one truly knew what this animal was.
Hirasawa and Hu have produced the best quality digital photographs of Palaeospondylus to date using micro-computed tomography (CT) scanning technology. They had to pick the best fossils to get the most accurate data. Many Palaeospondylus specimens have been discovered after 1890, but the majority have been damaged in some way — either by fossilisation or excavation — which may have led to past classification problems. The authors of the new study chose specimens with heads totally encased in rock to avoid this problem. "I sought for specimens that just showed the tails and eventually identified two specimens that only displayed the tail section on the surface," Hirasawa explained.
Several essential characteristics were discovered during scans of these specimens. One was that the inner ear, like the ears of modern fish, birds, and mammals, was made up of numerous semicircular canals. This is significant, according to the scientists, since it separates Palaeospondylus from more primitive jawless fish like hagfishes, which lack this characteristic. The researchers were also able to uncover cranial characteristics that place Palaeospondylus in the tetrapodomorphs, a group that includes all four-limbed organisms and their closest relatives. Most significantly, evolutionary examination of these distinguishing characteristics reveals that Palaeospondylus is not just any tetrapodomorph; it is the ancestor of all tetrapods.
"Our findings indicated that Palaeospondylus was a close relative of vertebrates with limbs (fingers) and those with limb-like fins," also known as "fishapods," according to Hirasawa. Palaeospondylus was more closely connected to limb-bearing tetrapods than to more ancient species like lungfishes and coelacanths, according to the researchers' results, making Palaeospondylus a near aquatic ancestor of the first animals to crawl onto land.
Even though this phylogenetic puzzle has been solved, there are still a few unanswered questions. Teeth are common among tetrapodomorphs, but Palaeospondylus didn't have any, or if it did, they didn't fossilise. It also lacked any visible appendages, despite the fact that its nearest cousins usually did.
What could be the cause of these strange occurrences? Teeth and limbs may have been lost in Palaeospondylus as a result of evolution, according to Hirasawa. Another option is that the fossilised Palaeospondylus represents the animal's larval or juvenile forms.
In a statement, Hirasawa added, "Whether these qualities were evolutionarily lost or whether normal development stalled half-way in fossils may never be understood."
While we now have a better understanding of Palaeospondylus' evolutionary position, there is still much more work to be done. This fish is currently guarding many of its old secrets, just as it was when it was discovered.
Written by:
HASHIR BIN SHAHID
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