Your earliest known ancestor is a toothless "eel" from the distant past.

A puzzling mystery fish has perhaps been put on the evolutionary tree of life for the first time.

 

Over a century ago, researchers were perplexed by the finding of a unique fossil that had been excavated from a quarry in Scotland. 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. The creature's name comes from the fact that it was discovered in India. Thanks to the application of high-resolution imaging, a group of researchers have finally concluded that this enigmatic fish could very well be one of our oldest progenitors.

 

According to Tatsuya Hirasawa, an associate professor of palaeontology at the University of Tokyo in Japan and the lead author of a new study describing the fossil, "To place Palaeospondylus in the evolutionary tree, it is prerequisite that each skeletal element be identified." The statement was made about the fossil. The mysteries that surrounded this small fish persisted for such a long time due to two factors: its diminutive size, with a body measuring just 2.4 inches (6 centimetres) long, and the unfortunate fact that fossilization dramatically compressed its skeleton, squeezing individual bones into a shapeless mass that was a paleontological nightmare to unravel, Hirasawa told Live Science in an email. The fossilization process dramatically compressed its skeleton, creating a distorted mass that was

 

Before this recent discovery, researchers had previously determined that the Palaeospondylus species existed throughout the middle Devonian epoch, which spanned from 398 million to 385 million years ago. The fish's fins were highly developed, but they did not have any limbs. In contrast to the majority of vertebrates that existed during this period, they appeared to have no teeth.

 

The position of the fish in the evolutionary tree has been reassessed numerous times, but the results have been inconsistent. In 2004, researchers published their findings with complete conviction in the journal American Scientist, stating that Palaeospondylus was a primitive species of lungfish. On the other hand, a study conducted in 2016 by Hirasawa and published in the journal Zoological Letters revealed that it was hagfish related. A year later, a group of researchers from the Australian National University cast doubt on the hagfish classification of the fish and proposed that it was a cartilaginous species similar to modern sharks.

 

This taxonomic tennis match is also not a recent occurrence in the world. Since its discovery in 1890, "this odd animal has perplexed scientists as a problem that's been impossible to solve," Yu Zhi (Daisy) Hu, a researcher in the Department of Materials Physics at the Australian National University in Canberra, said in a statement. He is a co-author of the study.

 

It would appear that the only point on which palaeontologists could reach a consensus was that no one truly understood the nature of this animal's identity.

 

Recent research conducted by Hirasawa and Hu, using micro-computed tomography (CT) scanning technology, has produced digital photographs of Palaeospondylus with the best resolution to date. They had to pick the greatest fossils to study to get the most accurate information. Since 1890, many specimens of Palaeospondylus have been discovered; however, the vast majority of these specimens were harmed in some way, either by the process of fossilization or excavation, which may have contributed to earlier errors in categorization. The new study's authors chose specimens with heads that were entirely encased in rock to get around this problem. Hirasawa explained, "I looked for specimens that exposed just the tail section on the surface and ultimately found two specimens that did so." "I looked for specimens that displayed only the tail part on the surface."

 

The scans of these specimens revealed a few important characteristics. One of these was that the inner ear was made up of several semicircular canals, quite similar to how the ears of fish, birds, and mammals are structured. The scientists highlighted that this is noteworthy since it establishes a degree of evolutionary distance between Palaeospondylus and jawless fish that are more primitive, such as hagfish, which do not possess this characteristic. The researchers also successfully recognized cranial characteristics that place Palaeospondylus in a group known as the tetrapodomorphs. This group includes all animals with four limbs and their closest relatives. Most crucially, an evolutionary examination of these remarkable characteristics reveals that Palaeospondylus may not be a typical tetrapodomorph; rather, it may be the ancestor of all tetrapods.

 

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

 

Even if this evolutionary puzzle has been unravelled at this point, there are still a lot of unanswered questions. Tetrapodomorphs typically have teeth, but Palaeospondylus did not have any, or if it did have teeth, they were not preserved in the fossil record. In addition, it did not have any apparent appendages, unlike its closest cousins, who normally possessed appendages.

 

What may account for these irregularities? Hirasawa proposed that teeth and limbs may have been lost throughout Palaeospondylus' evolutionary history. There is also the possibility that the Palaeospondylus fossils that have been discovered so far reflect the animal in a larval or juvenile stage.

 

According to a statement released by Hirasawa, "Whether these qualities were lost due to the natural process of evolution or whether normal development froze half-way in fossils might never be known."

 

There is still much more work to be done, even though we have a better notion of where Palaeospondylus falls on the evolutionary tree now than we did before. This fish keeps many long-forgotten secrets close to its chest, just as it did when they first discovered it.

Article source : https://www.livescience.com/ancient-fishlike-weirdo-tetropod-ancestor

Image source   : https://pixabay.com/id/photos/lanzarote-el-golfo-semprot-201346/