A 390 million-year-old four-limbed fish-like creature is probably not the one 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.
Fossils of the eel-like creature are abundant in Caithness, Scotland, after being first discovered there in 1890.
Since then, experts have found it difficult to place it on the evolutionary tree, as the Palaeospondylus was only about two inches (5 cm) long, making it difficult for cranial reconstructions.
Shigeru Kuratani of the RIKEN Cluster for pioneering research in Japan has now discovered evidence that the creature had a jaw and four limbs.
The findings place the animal at the bottom of the family tree of vertebrates, including humans.
Palaeospondylus gunni is an ancient vertebrate that scientists believe could be one of the first predecessors of four-limbed creatures, including humans. In the photo: a reconstruction of the Palaeospondylus using computed tomography X-ray
Scientists at the University of Tokyo and RIKEN Cluster for Pioneering Research discovered cranial features that placed the Palaeospondylus in the tetrapodomorph category.
Prior to these new discoveries, the creature was thought to share characteristics with jawed and jawless fish.
Meet the family! Palaeospondylus gunni
Palaeospondylus gunni is an ancient vertebrate with a flat head, an eel-like body that lived in the bed of a deep freshwater lake.
It fed on leaves, animal remains and other organic remains that fell to the bottom of the surrounding terrestrial communities.
They date back 390 million years, when the first vertebrates began to emerge from the water.
For these pioneer fish, the adaptation of the fins to the limbs facilitated the transition, later giving rise to mammals, birds, and reptiles.
Until now, the creature was thought to share features with jawed and jawless fish.
No fossils have been found to suggest that Palaeospondylus – which lived in the Devonian period about 390 million years ago – had dermal teeth or bones.
The creature had a flat head, an eel-like body, and lived in the bed of a freshwater lake at the northeastern tip of the Highlands.
He had a strange apparatus resembling a basket on his snout and a well-developed cartilaginous spine, but no apparent fins.
The researchers found that Palaeospondylus was probably a member of Sarcopterygii, a group of fish with lobed fins, due to its cartilaginous skeleton and the absence of paired appendages.
The marine organism fed on leaves, animal remains and other organic debris that fell to the bottom of the lake from the surrounding land.
At that time, the land mass of Scotland was south of the equator, where Central Africa is today, so the atmosphere was warm and semi-arid.
Palaeopondylus dates back to a crucial time in history, when the first vertebrates began to emerge from the water.
The adaptation of its fins in limbs facilitated the transition, giving rise later to mammals, birds and reptiles.
RIKEN researchers used X-rays from the SPring-8 synchrotron to generate high-resolution micro-CT scans of fish.
Kuratani and his team carefully selected fossils in which the heads remained completely embedded in the rock to obtain the most accurate cranial image.
Leading author Tatsuya Hirasawa of the University of Tokyo said: “Choosing the best specimens for micro-CT scans and carefully cutting out the rock surrounding the fossilized skull allowed us to improve the resolution of the scans.
“While technology is not at all cutting edge, these preparations were certainly key to our achievement.”
Images created from Palaeospondylus fossils show that 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 that confirmed that the creature probably had a jaw.
“As a tetrapod, Palaeospondylus had an excessively small lower jaw relative to the skull and the opening of the mouth was retracted,” Hirasawa added.
This is seen in a group of limbless amphibians living today called Cecilians.
The “retracted” jaw, along with an unusually flat head shape, probably represented an adaptation for a background habitat, as it allowed for suction feeding.
Researchers also discovered cranial features that placed the Palaeospondylus in the category of tetrapodomorph, or four-limbed.
Palaeospondylus gunni cranial skeleton reconstructed by synchrotron radiation computed tomography from A: Dorsal view, B: Ventral view, and C: Left oblique side 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: Separate skeletal portions
Teeth, dermal bones, and paired appendages have never been associated with Palaeospondylus.
Professor Hirasawa said: “It is possible that it will never be known if these features were lost evolutionarily or if normal development froze halfway in the fossils.”
“However, this evolution could have facilitated the development of new features such as limbs.”
Professor Hirasawa added: “The strange morphology of Palaeospondylus, which is comparable to that of tetrapod larvae, is very interesting from a genetic point of view of development.
“With that in mind, we will continue to study the developmental genetics that caused this and other morphological changes that occurred in the transition from water to land in the history of vertebrates.”
HOW DID WE DISCOVER THE PALAEOSPONDIL?
Palaeospondylus fossils were first discovered in the Achanarras fish bed in Caithness, Scotland around 1890.
They were found by amateur paleontologists Marcus and John Gunn, cousins who lived near the Achanarras slate quarry.
Other specimens have since been excavated at the same site and a few more have been found at two nearby sites.
The species is not known anywhere else in the world and is a unique example of the first lives of fish on Earth.
The research was conducted with Dr. Daisy (Yuzhi) Hu of the National University of Australia.
The doctoral student said: “This strange animal has baffled scientists since its discovery in 1890 as a puzzle that has been impossible to solve.
“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 well-preserved copy like the ones we used is like winning the lottery, or even better!’
The new findings mean that scientists could unblock a number of unknown morphological features and the evolutionary history of four-limbed animals.
“Despite the research, it is still difficult to determine which animal was 100 percent accurate,” Dr. Hu added.
“Even with this new information, long-term research is needed with the joint effort of scientists around the world to give us the perfect answer to what Palaeospondylus gunni really is.”