A 390 million year old fish-like creature with four limbs is probably not what you would expect to find in your family tree.
But a new study has shown the creature, called Palaeospondylus gunni, may be one of our earliest ancestors.
Fossils of the eel-like creature are plentiful in Caithness, Scotland, where they were first discovered in 1890.
Experts have since struggled to place it on the evolutionary tree, as Palaeospondylus was only about two inches (5cm) long, making cranial reconstructions difficult.
Now, Shigeru Kuratani of the RIKEN Cluster for Pioneering Research in Japan has discovered evidence that the creature had one jaw and four limbs.
The findings place the animal at the very bottom of the vertebrate family tree – including humans.
Palaeospondylus gunni is an ancient vertebrate that scientists believe may be an early predecessor to four-limbed creatures, including humans. Pictured: A reconstruction of the Palaeospondylus by computed tomography X-ray
Scientists from the University of Tokyo and the RIKEN Cluster for Pioneering Research discovered cranial features that placed Palaeospondylus in the tetrapodomorph category
Prior to these new findings, the creature was thought to share characteristics with jawed and jawless fish.
Until now, the creature was thought to share characteristics with jawed and jawless fish.
No fossils have been found to suggest that Palaeospondylus – which lived in the Devonian around 390 million years ago – had teeth or skin bones.
The creature had a flat head, an eel-like body, and lived on the bed of a freshwater lake in the far northeast of the Highlands.
It had a strange basket-like device on its snout and a well-developed cartilaginous spine – but no visible fins.
The researchers found that Palaeospondylus was most likely a member of Sarcopterygii, a group of lobe-finned fishes, due to its cartilaginous skeleton and lack of paired appendages.
The marine organism fed on leaves, animal remains and other organic debris that fell to the bottom of the lake from surrounding land.
At the time, the landmass of Scotland was south of the equator, where central Africa is today, so the environment was hot and semi-arid.
Palaeopondylus dates back to a crucial moment in history, when the first vertebrates began to emerge from the water.
The adaptation of their fins into limbs facilitated the transition – later giving rise to mammals, birds and reptiles.
RIKEN researchers used X-rays from the SPring-8 synchrotron to generate high-resolution micro-CT scans of the fish.
Kuratani and his team carefully selected fossils in which the heads remained completely embedded in rock to obtain the most accurate cranial image.
Lead author Tatsuya Hirasawa, from the University of Tokyo, said: “Choosing the best specimens for the micro-CT scans and carefully carving the rock surrounding the fossilized skull allowed us to improve the resolution of the scans.
“Although it is not state-of-the-art technology, these preparations have certainly been key to our success.”
Images created from Palaeospondylus fossils show it had a flat head, an eel-like body, a basket-like apparatus on its snout, and a cartilaginous spine
Scientists found three semicircular canals which confirmed that the creature probably had a jaw.
“As a tetrapod, Palaeospondylus possessed an excessively small lower jaw compared 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, together with an unusually flat head shape, probably represented an adaptation for bottom habitat as it allowed for suck-feeding.
The researchers also discovered cranial features that placed Palaeospondylus in the tetrapodomorph, or four-limbed, category.
Cranial skeleton of Palaeospondylus gunni reconstructed by synchrotron radiation computed tomography from A: dorsal view, B: ventral view and C: left oblique lateral view
A: position of the cranial skeleton of Palaeospondylus embedded in the rock, B: dorsal view of the cranial skeleton, C: ventral view of the cranial skeleton, D: separated skeletal parts
Paired teeth, dermal bones, and appendages have never been associated with Palaeospondylus.
Professor Hirasawa said: “Whether these features were lost during evolution or if normal development froze halfway through the fossils may never be known.
“Nevertheless, this evolution could have facilitated the development of new features like members.”
Professor Hirasawa added: ‘The strange morphology of Palaeospondylus, which is comparable to that of tetrapod larvae, is very interesting from a developmental genetic point of view.
“With this in mind, we will continue to investigate the developmental genetics that caused this and other morphological changes that occurred during the water-land transition in vertebrate history.”
The research was conducted with Dr Daisy (Yuzhi) Hu at the Australian National University.
The PhD holder said: ‘This strange animal has baffled scientists since its discovery in 1890 as an impossible puzzle.
“Morphological comparisons of this animal have always been extremely difficult for scientists.
“However, recent improvements in segmentation and high-resolution 3D visualization have made this previously impossible task possible.
“Finding a specimen as well preserved as the ones we used is like winning the lottery, or even better!”
The new findings mean scientists could unlock a range of unknown morphological characteristics and the evolutionary history of four-limbed animals.
“Despite the investigation, it is still difficult to determine what the animal was with 100% accuracy,” Dr Hu added.
“Even with this new information, long-term investigations with the joint effort of scientists around the world are needed to give us the perfect answer on what Palaeospondylus gunni really is.”