Fossils of a four-legged fish have filled in our understanding of the evolution of land-based vertebrates.

Initially described in 1994, early specimens of Ventastega curonica were fragmented, and hard to interpret. New examples from Latvia have now allowed researchers to reconstruct the head, shoulders and part of the pelvis of the ugly looking beast (press release, research paper in Nature).

The editor’s summary in Nature notes that the new work shows Ventastega has the skull shape of an early tetrapod but the proportions of a fish. It provides new insights in the evolution of early land-dwelling vertebrates (called tetrapods) some 370 million years ago in the Late Devonian period.

“From a distance, it would have looked like an alligator,” says study author Per Ahlberg, of Uppsala University in Sweden (BBC). “But closer up, you would have noticed a real tail fin at the back end, a gill flap at the side of the head; also lines of pores snaking across head and body. In terms of construction, it had already undergone most of the changes from fish towards land animal, but in terms of lifestyle you are still looking at an animal that is habitually aquatic.”

Ahlberg speculates that it was crawling around on sandy banks and eating stranded fish in tidal creeks (AP).

Ted Daeschler, paleontologist at the Academy of Natural Sciences in Philadelphia, explains to National Geographic that although we have a general outline of the transition between fish and tetrapods there’s a lot we don’t know. It’s like building a house, he says: “We’ve got the frame built. We know what the rooms are shaped like. But we haven’t put in the electricity, installed the lamps, or put Sheetrock on the walls.”

Picture upper: Philip Renne and Per Ahlberg

Picture lower: Ventastega in side view / Per Ahlberg