The Fish That Was Not a Fish

By Hannah Hoag

Nature, 7/30/02

http://discover.com/science_news/newsflash/gthere.html?article=news_fish.html

Call it a case of mistaken identity.

For a quarter of a century, a 350-million-year-old fossilized skeleton lay in the basement of the Hunterian Museum in Glasgow, Scotland. Discovered by Peder Aspen in Dunbarton, Scotland, the unnamed fossil was labeled "Rhizodont Fish" but was never fully pried from its limestone casing, making an accurate classification impossible. In 1996 a graduate student brought the rock-covered fossil into the lab of Jenny Clack, a vertebrate paleontologist at the University of Cambridge's Museum of Zoology. She knew within minutes that what she had before her was not a fish but a key missing link, one that is only now giving up its secrets.

A 350-million-year-old fossil was labeled a fish, until close examination revealed a pelvis and femurs. Most of the pelvis is on the underside of the fossil, but part of it is visible (arrow, far right), a wide arc of bone above the darker-colored rock and below the vertebrae.
Photo by S. M. Finney, University Museum of Zoology, Cambridge.

One of life's greatest transitions--the move onto dry land--is poorly preserved in the fossil record. Various remains indicate that vertebrates and tetrapods (four-legged animals) became land creatures in the Lower Carboniferous period, which started about 363 million years ago. Right around that time, however, there is a perplexing 20-million-year-long gap in which few fossils have been found, leaving scant information about what happened in between. "At one end there are paddlelike feet for swimming, and at the other end there are fully terrestrial creatures running about," says Clack, who has been studying the fish relatives of four-legged animals for more than 20 years. Clack and her colleague Michael Coates call this blank period Romer's Gap, for the late Harvard paleontologist Alfred Sherwood Romer, who searched for fossils from the Lower Carboniferous during the 1940s. Not only are tetrapod fossils scarce during Romer's Gap, but there is also a paucity of invertebrate and plant fossils.

In the mislabeled fish fossil gathering dust in the Museum of Zoology, Clack saw what others had missed: a creature that could help plug Romer's Gap. When she examined the back end of the fossil, she observed plates of bone--a pelvis--and large broken bones that resembled femurs. Fish do not have hips, so clearly this creature was not a fish. "It began to dawn on us that we had something from the very early Carboniferous Period," Clack says. "It was extremely exciting."

It took four years to prepare the specimen before Clack could study it in its entirety. Once the limestone was chipped away, she realized that she had the best-preserved, most complete fossil dating from the time of Romer's Gap--only the tail was missing. And this creature sports a unique piece of tetrapod anatomy: "It is the first truly walking foot," says Clack. She discovered a more primitive tetrapod, Acanthostega, in 1987, but its eight-toed feet were placed at an angle that promoted swimming. This new, salamanderlike creature, which Clack has named Pederpes finneyae, has feet pointing forward, suggesting that it walked. Also, its five-digit feet resemble those of the terrestrial tetrapods found later in the Carboniferous Period, in the Viséan Epoch, between 333 million and 352 million years ago.

The hind limb of Pederpes shows (left) two complete digits, plus pieces of the first and fourth digits. The fifth digit is at the middle bottom of the photo, slightly behind the fourth digit, pointing backwards. The bone above the first digit is part of a femur. The shape and positioning of the foot bones suits walking more than swimming.
Photo by S. M. Finney, University Museum of Zoology, Cambridge.

Clack has returned to the Dunbarton site where Pederpes was found to search for additional tetrapods, but she has found only a few fish scales. "Pederpes was a very lucky find," she says. Although paleontologists have made slow progress in pursuing Romer's passion, "there is hope," says Robert Carroll, a vertebrate paleontologist at the Redpath Museum at McGill University in Montreal. "The evidence comes sporadically, but we've learned a lot recently."

RELATED WEB SITES:

Clack, Jennifer A. "An Early Tetrapod From 'Romer's Gap,'" Nature. July 4, 2002. Carroll, Robert. "Early Land Vertebrates," Nature. July 4, 2002. Both at: www.nature.com/nature

For more about Clack and Ancanthostega, see "Coming Onto the Land" by Carl Zimmer. Discover, June 1995. http://208.245.156.153/archive/output.cfm?ID=516