Missing link crawls out of muck

Paleontologists have discovered fossils of a species that provides the missing evolutionary link between fish and the first animals that walked out of water onto land about 375 million years ago. The newly found species, Tiktaalik roseae, has a skull, a neck, ribs, and parts of the limbs that are similar to four-legged animals known as tetrapods, as well as fishlike features such as a primitive jaw, fins, and scales.

These fossils, found on Ellesmere Island in Arctic Canada, are the most compelling examples yet of an animal that was at the cusp of the fish-tetrapod transition. The new find is described by scientists at Harvard University, the University of Chicago, and the Academy of Natural Sciences in Philadelphia in two related research articles highlighted on the cover of the April 6 issue of Nature.

“This previously unknown, extinct animal represents the beginning of the emergence of fish onto land, and the evolutionary transformation of fins into limbs,” says Farish A. Jenkins Jr., Alexander Agassiz Professor of Zoology at Harvard and curator of mammalogy and vertebrate paleontology at Harvard’s Museum of Comparative Zoology. “The skeletal material is three-dimensional and exquisitely preserved; most material this old tends to be flattened or otherwise distorted. The geometry of the limb joints clearly indicates that segments of the fin could move independently. The ‘shoulder’ and ‘elbow’ could flex, and the ‘wrist’ could extend, converting the fin into postures appropriate to support the body from below and propel the animal on land.”

“Tiktaalik blurs the boundary between fish and land-living animal both in terms of its anatomy and its way of life,” says Neil Shubin, professor and chairman of organismal biology at the University of Chicago.

Tiktaalik was a predator with sharp teeth, a crocodilelike head, and a flattened body. The well-preserved skeletal material from several specimens, ranging from 4 to 9 feet long, enabled the researchers to study the mosaic pattern of evolutionary change in different parts of the skeleton as fish evolved into land animals.