Tetrapods' old age revealed

Newly discovered tetrapod footprints suggest that the evolution of limbed vertebrates may have occurred nearly 20 million years earlier than scientists previously believed, according to a study published this week in Nature.

Pencil drawing of Acanthostega
gunnari, an early tetrapod
Image: Wikimedia commons,
linkurl:Nobu Tamura;http://www.palaeocritti.com

"This is a very important discovery," said paleontologist Philippe Janvier of the linkurl:Muséum National d'Histoire Naturelle;http://www.mnhn.fr/museum/foffice/transverse/transverse/accueil.xsp in Paris, France, who did not participate in the research. "Up to now, we thought the divergence between tetrapods and lobe-finned fish was in the period called the Givetian that was about 390 million years ago at the latest. But this was really a big step back in time -- it implies that the evolution of limbs with digits appeared much earlier." Working in the Holy Cross Mountains of southeastern Poland, paleontologist Per Ahlberg of linkurl:Uppsala University;http://www.uu.se/ and his colleagues identified numerous footprints and trackways of what appeared to be limbed animals, many of which contained digit marks -- a sign of a well evolved tetrapod. The prints dated to about 397 million years ago, approximately 18 million years older than the earliest tetrapod body fossils. "It was really a surprise," said Janvier, who wrote an accompanying review in Nature. "It changes the timing of the [evolutionary] tree by at least 18 million years, if not more." The age of the footprints also puts them about 10 millions years earlier than the oldest elpistostegids -- a group of lobe-finned fish believed to immediately precede tetrapods. This raises the possibility that the two organisms may have coexisted for a significant period of their evolutionary history. "We saw those fishes as a very short lived transitional phase, [but] we can now see that that is not the case," Ahlberg said. "These sort of intermediate forms must have evolved alongside the early tetrapods." As a result of this shift in the evolutionary timeline, "I suspect that this discovery will raise a great interest for any lobe-finned fish coming from the Eifelian period, [which were previously] regarded as too old to be relevant to the origin of tetrapods," Janvier added. In addition to the age of the fossilized tracks, their size and location also suggested some twists on the classical story of fish to tetrapod evolution. The prints varied in size; some appeared to come from animals more than 2 meters in length, indicating that "some of these early tetrapods were very large indeed, which is surprising," linkurl:Jenny Clack,;http://www.theclacks.org.uk/jac/ a professor and curator of vertebrate paleontology at the University Museum of Zoology, Cambridge, wrote in an email to The Scientist. "You'd expect a smaller animal would more easily become terrestrial." Furthermore, the prints were found in a marine environment, as opposed to the freshwater marshes or coastal lakes and rivers, where the earliest tetrapods were thought to have evolved. While previous studies had begun to suggest a marine setting for this evolutionary transition, "this new work seems to nail it," Clack wrote. There is one major concern regarding these new findings: They are just footprints. "Trace fossils such as these presumed vertebrate tracks and trackways are a notoriously difficult class of evidence to interpret with full confidence," linkurl:Ted Daeschler,;http://clade.ansp.org/vert_zoology/people/daeschler/ associate curator and chair of vertebrate zoology at the Academy of Natural Sciences in Philadelphia, wrote in an email. "With all respect to the scientists involved in this study, there may be other explanations for these suggestive tracks and traces in the shallow water limestones in Poland." "Footprints and trackways are always a bit frustrating because you really want to know more about the animal," Janvier agreed. "These footprints are so crisp that one can easily understand the anatomy of the limbs of this animal, but just a bone would have been wonderful to get from such an old age." On the other hand, "the beauty of actual trackways [is that] we can be sure that this is the environment where the animals were active," Ahlberg said, as opposed to where the bodies washed up after the animals had died. Still, until more evidence accumulates, such as body fossils of these ancient tetrapods, "I am not ready to discard the established paradigm for the fish-tetrapod transition, which has proven to have considerable explanatory and predictive power," Daeschler wrote. "No doubt that I will keep an open mind and keen eye on future developments."
**__Related stories:__***linkurl:Ancient fingers and toes;http://www.the-scientist.com/blog/display/55022/
[21st September 2008]*linkurl:Walking from water;http://www.the-scientist.com/article/display/20505/
[4th July 2002]