Wednesday, April 05, 2006

Tiktaalik roseae, New Fish-Like Tetrapod

A Devonian tetrapod-like fish and the evolution of the tetrapod body plan. 2006. Edward B. Daeschler, et al. Nature 440: 757-763

Illustration by Shawn Gould, © National Geographic Society

Abstract: The relationship of limbed vertebrates (tetrapods) to lobe-finned fish (sarcopterygians) is well established, but the origin of major tetrapod features has remained obscure for lack of fossils that document the sequence of evolutionary changes. Here we report the discovery of a well-preserved species of fossil sarcopterygian fish from the Late Devonian of Arctic Canada that represents an intermediate between fish with fins and tetrapods with limbs, and provides unique insights into how and in what order important tetrapod characters arose. Although the body scales, fin rays, lower jaw and palate are comparable to those in more primitive sarcopterygians, the new species also has a shortened skull roof, a modified ear region, a mobile neck, a functional wrist joint, and other features that presage tetrapod conditions. The morphological features and geological setting of this new animal are suggestive of life in shallow-water, marginal and subaerial habitats.

© credit Kalliopi Monoyios.

From National Geographic
A new fish-like tetrapod has been found in the 375 million year old Devon rocks of the Canadian Arctic.
The 3 metre-long new species, Tiktaalik roseae, had a flattened, crocodile-like head and strong, bony leg-like fins. Ellesmere Island is more than 970 kilometers north of the Arctic Circle in Canada's Nunavut territory.

The creature is being hailed as a crucial missing link between fish and land animals—including the prehistoric ancestors of humans. The fish shows other features characteristic of land animals, including ribs, a neck, and nostrils on its snout for breathing air.

Researchers say the fish shows how fins on freshwater species first began transforming into limbs some 380 million years ago. The change was a huge evolutionary step that opened the way for vertebrates—animals with backbones—to emerge from the water.

The discovery marked the culmination of a five-year, 650 kilometer fossil hunt across the Arctic's frozen tundra.

Shubin says the fish's wide head and sharp teeth suggest it hunted much like a crocodile and that it also breathed air. "Look at the side of the snout. It has a nice big pair of external nostrils," he said. The creature's long snout seems to be adapted for snapping at prey and hunting with its head above water like a crocodile, Clack said.

The paleontologists say the new fish form goes a long way toward filling the evolutionary gap between fish and the earliest amphibians.

"Our remote ancestors were large, flattish, predatory fishes," they write. "Strong limblike pectoral fins enabled them to haul themselves out of the water."