Researchers at the RIKEN Laboratory of Evolutionary Morphology have found that the ancient fish-like vertebrate Palaeospondylus was probably one of the earliest ancestors of four-legged animals, including humans.

Palaeospondylus is a small fish-like vertebrate about 5 cm long, eel-like, and lived in the Devonian period, about 390 million years ago. Scientists have had many of its fossils, but due to its small size and low-quality data on skull reconstructions, it has been difficult to place it on an evolutionary tree.

Palaeospondylus was thought to resemble both jawed and jawless fish. And among the unusual features, it was noted that vertebrates lack teeth and integumentary bones.

To comprehensively study Palaeospondylus, the researchers used the extremely powerful RIKEN SPring-8 synchrotron to perform high-quality microcomputed tomography using X-rays.

The high-resolution scan revealed a few key features. First, the researchers found three semicircular canals that clearly show the morphology of the inner ear, which is also found in maxillary vertebrates. This solved one of the problems, as previous studies suggested that Paleospondylus was evolutionarily closer to primitive jawless vertebrates. Later, they discovered the main features of the skull of Palaeospondylus, which should be placed in the category of tetrapodomorph – it consists of animals with four limbs and their closest ancient relatives.

Several analyzes have shown that Palaeospondylus is more closely related to the quadruped than to most other known tetrapodomorphs that still have fins.

The lack of teeth and bones surprised scientists. They suggested that they were dealing with an embryo they had not yet developed. But it’s also possible that Palaeospondylus lost them due to the “whims of evolution” and then began to develop in another way – perhaps by developing front and hind limbs.