A strange fish-like fossil has been discovered.

One of the most perplexing fossil vertebrates has been recognised for the first time since its discovery 130 years ago, helping us to gain a better understanding of the Earth's first crawlers.

"Since its discovery in 1890, this unique species has perplexed scientists as an unsolvable enigma," said Dr Daisy (Yuzhi) Hu, a researcher and PhD graduate from The Australian National University (ANU).

Palaeospondylus gunni is a curious fish-like fossil animal with a peculiar morphological profile, including the absence of teeth and dermal bones in the fossil record.

Around 390 million years ago, a little mammal with an eel-like body thrived. Despite its antiquity, its evolutionary position has remained a mystery until now.

The species was likely one of the oldest progenitors of four-limbed creatures, according to research headed by the University of Tokyo in collaboration with ANU and the RIKEN Evolutionary Morphology Laboratory.

"For biologists, morphological comparisons of this species have always been incredibly difficult," Dr Hu remarked.

"However, recent advances in high-resolution 3D segmentation and visualisation have enabled this hitherto difficult feat."

"It's like winning the jackpot, or even better, to find a specimen as beautifully preserved as the ones we utilised!"

Placing this fossil on the evolutionary tree has always been difficult due to previously damaged or poorly preserved specimens of the animal.

Palaeospondylus was most likely a member of the Sarcopterygii family of lobe-finned fishes, according to the researchers, because of its cartilaginous skeleton and lack of paired appendages.

"Despite the examination, it's still difficult to pinpoint exactly what the animal was," Dr Hu stated.

Scientists may be able to uncover a variety of previously unknown morphological traits and evolutionary history of four-limbed animals as a result of the new results.

"Even with this new knowledge, long-term research including the collaboration of experts from all around the world are required to provide us with a definitive explanation as to what exactly is Palaeospondylus gunni."