A 390-million-year-old creature that looks like a four-limbed fish is probably not what you would expect to find in your family tree.
But a new study has shown that the creature, called Palaeospondylus gunni, could be one of our first ancestors.
Eel-like creature fossils have been abundant in Caithness, Scotland, since they were first discovered there in 1890.
Since then, specialists have found it difficult to place it on the evolutionary tree, as the Paleopond was only about two inches (5 cm) long, making it difficult to reconstruct the skull.
Now, Shigeru Kuratani at the RIKEN Cluster for Pioneering Research in Japan has revealed evidence that the creature had a jaw and four limbs.
The findings place the animal at the bottom of the family tree for vertebrates – including humans.
Palaeospondylus gunni is an ancient vertebrate that scientists believe could have been one of the first ancestors of four-limbed creatures – including humans. In the photo: reconstruction of Paleospondylos with X-ray computed tomography
Scientists from the University of Tokyo and the RIKEN Cluster for Pioneering Research have discovered cranial features that place the Invertebrate in the category of quadrupeds
Prior to these new discoveries, the creature was thought to share characteristics with both jawed and jawless fish.
Meet the family! Palaeospondylus gunni
Palaeospondylus gunni is an ancient vertebrate with a flat head, eel-like body and lived in the bed of a deep freshwater lake.
They fed on leaves, animal remains and other organic debris that fell to the bottom from the surrounding land communities.
They date back to 390 million years ago, when the first vertebrates began to emerge from the water.
For these pioneering fish, the fitting of the fins at the ends facilitated the transition – creating later mammals, birds and reptiles.
Until now, creatures were thought to share characteristics with both jaw and jawless fish.
No fossils have been found to suggest that the Paleopondylon – which lived in the Devonian period about 390 million years ago – had teeth or skin bones.
The creature had a flat head, an eel-like body and lived in the bed of a freshwater lake in the northeastern Highlands.
He had a strange device like a basket on his muzzle and a well-developed cartilaginous spine – but without visible fins.
The researchers found that Palaeospondylus was most likely a member of Sarcopterygii, a group of fish with lobed fins, due to its cartilaginous skeleton and lack of paired components.
The marine organism fed on leaves, animal remains and other organic remains that fell to the bottom of the lake from the surrounding land.
At that time, the Scottish mainland was south of the equator, where central Africa is today, so the environment was hot and semi-arid.
Palaeopondylus dates back to a critical point in history, when the first vertebrates began to emerge from the water.
Adapting their fins to the edges facilitated the transition – later creating mammals, birds and reptiles.
Researchers at RIKEN used X-rays from the SPring-8 synchronizer to create high-resolution micro-CT scans of the fish.
Kuratani and his team carefully selected fossils in which the heads remained completely embedded in the rock to obtain the most accurate image of the skull.
Lead author Tatsuya Hirasawa, of the University of Tokyo, said: “The selection of the best samples for micro-CT scans and the careful cutting of the rock surrounding the fossilized skull allowed us to improve the scan analysis.
“Although not quite cutting-edge technology, these preparations were definitely the key to our achievement.”
Images created from Paleospine fossils show that it had a flat head, an eel-like body, a device like a basket at its muzzle and a cartilaginous spine.
Scientists found three semicircular canals that confirmed that the creature most likely had a jaw.
“As a quadruped, the vertebrate had an extremely small lower jaw relative to the skull and the mouth opening was retracted,” Hirasawa added.
This is seen in a group of limbless amphibians living today called caecilians.
The “retracted” jaw, along with an unusually flat head shape, probably represented an adaptation to a bottom-dwelling habitat, as it allowed suction feeding.
The researchers also discovered cranial features that placed the Paleospinus in the category of quadrupeds or quadrupeds.
Cranial skeleton of Palaeospondylus gunni reconstructed with synchrotron X-ray from A: dorsal side, B: abdominal side and C: left side view
A: Location of the Palaeospondylus cranial skeleton embedded in a rock, B: Spinal aspect of the cranial skeleton, C: Abdominal view of the cranial skeleton, D: Separate skeletal sections
Teeth, skin bones and paired parts have never been associated with Paleopathy.
Professor Hirasawa said: “If these traits were evolutionarily lost or if normal growth froze halfway through the fossils, it might never be known.
“Nevertheless, this development may have facilitated the development of new features such as edges.”
Professor Hirasawa added: “The strange morphology of the Palaeospondylus, which is comparable to that of four-limbed larvae, is very interesting from a developmental genetic point of view.
“With that in mind, we will continue to study the developmental genetics that brought about this and other morphological changes that occurred during the transition from water to land in the history of vertebrates.”
HOW DID WE DISCOVER THE PALEOPONDYLOS?
Paleospine fossils were first discovered in the Achanarra fish bed in Caithness, Scotland around 1890.
They were found by amateur paleontologists Marcus and John Gunn – cousins who lived near the Ahanara shale quarry.
Other specimens have since been excavated at the same site, while some were found at two nearby sites.
The species is not known anywhere else in the world and is a unique example of the early life of fish on Earth.
The research was conducted with Dr Daisy (Yuzhi) Hu at the National University of Australia.
The graduate doctor said: “This strange animal has puzzled scientists since its discovery in 1890 as a puzzle that was impossible to solve.
“Morphological comparisons of this animal have always been extremely challenging for scientists.
“However, recent improvements in high-resolution 3D segmentation and visualization have made this hitherto impossible task possible.
“Finding a sample so well preserved with the ones we used is like winning the lottery, or even better!”
The new findings mean that scientists could unlock a number of unknown morphological features and evolutionary history of four-limbed animals.
“Despite the research, it is still difficult to determine who the animal was with 100 percent accuracy,” Dr. Hu added.
“Even with this new information, long-term research is needed with the joint effort of scientists from around the world to give us the perfect answer to what Palaeospondylus gunni really is.”