London, Jan 13 (IANS): An analysis of a 415 million-year-old fish skull has suggested that the last common ancestor of all jawed vertebrates, including humans, was not very shark-like.

The findings add further weight to the growing idea that sharks are not 'primitive' as earlier thought.

"This 415 million year-old fossil gives us an intriguing glimpse of the 'Age of Fishes', when modern groups of vertebrates were really beginning to take off in an evolutionary sense," said one of the study authors Matt Friedman from Oxford University.

"It tells us that the ancestral jawed vertebrate probably does not fit into our existing categories," Friedman added.

The fossil skull was originally found near the Sida river in Siberia in 1972 and is currently held in the Institute of Geology at the Tallinn University of Technology, Estonia.

The fish fossil's 'two faces' led to it being named Janusiscus after the double-faced Roman god Janus.

The fossil is a mix of two distinct evolutionary branches of early fish: cartilaginous fishes (chondrichthyans) such as sharks and rays and the bony fishes (osteichthyans), the findings showed.

Bony fish include familiar fish such as cod and tuna as well as all land-dwelling creatures with backbones, such as humans.

The researchers used X-ray CT (computed tomography) scanning to look inside the skull.

The findings were published in the journal Nature.