According to researchers, a 390 million-year-old ancient fish discovered in a Caithness graveyard is one of the early relatives of four-limbed mammals, including humans.
The prehistoric creature was discovered in a prehistoric graveyard in Caithness in 1890. A study identified it as a “missing link” in vertebrate evolution and named it Palaeospondylus gunni.
Only two inches long, it had a flat head, an eel like body and lived on the bed of a deep freshwater loch, feeding on leaves and other organic debris.
At the time, Scotland’s landmass lay south of the equator – where central Africa is today – and was arid and “semi-hot” according to scientists.
Palaeopondylus dates back the first vertebrates making their way out of water. For these pioneering fish, the adaptation of fins into limbs gave rise to mammals, birds and reptiles.
Lead author Professor Tatsuya Hirasawa, of Tokyo University in Japan, said: “Palaeospondylus gunni, from the Middle Devonian period, is one of the most enigmatic fossil vertebrates.
“Its phylogenetic position has remained unclear since its discovery in Scotland more than 130 years ago.”
The tiny fossil has been the cause of controversy since its discovery by amateur palaeontologists Marcus and John Gunn – cousins living near Achanarras slate quarry.
Other specimens have since been dug up at the same site with a few more found at two nearby locations. The species is not known anywhere else in the world.
It’s a unique example of the early lives of fish on Earth – with features unlike any others known to science.
Palaeopondylus had a strange basket-like apparatus on its snout and well-developed cartilaginous vertebral column but no fins.
High resolution images revealed three semi-circular canals in the skull indicating the inner-ear shape of jawed vertebrates.
Experts have found Palaeospondylus was a primitive type of tetrapod, more closely related to them than cousins that still retained fins.
Prof Hirasawa said: “As a tetrapod, Palaeospondylus possessed an excessively small lower jaw relative to the skull and the mouth opening was retracted.”
This is seen in the caecilians, a group of limbless amphibians that live today. The’retracted’ jaw, as well as the unusually flat head shape, were most likely adaptations for a bottom-dwelling habitat.The toothless and well-developed jaw suggested it was a suction-feeder – like most aquatic animals today.
Unlike previous studies that have used excavated heads, Prof Hirasawa and colleagues used carefully selected fossils in which the heads remained completely embedded in the rock.
He said: “Choosing the best specimens for the micro-CT scans and carefully trimming away the rock surrounding the fossilised skull allowed us to improve the resolution of the scans.
“Although not quite cutting-edge technology, these preparations were certainly keys to our achievement.”
Prof 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.
“Taking this into consideration, we will continue to study the developmental genetics that brought about this and other morphological changes that occurred at the water-to-land transition in vertebrate history.”
Nature published a description of Palaeospondylus gunni. Its fossils indicate that the first backboned animals on land may have lived in Scotland, possibly in the far north east Highlands.