Ancient fingers and toes

Were animals with four limbs the first to evolve fingers and toes-- or did such digits evolve long before?

By | September 21, 2008

Were animals with four limbs the first to evolve fingers and toes-- or did such digits evolve long before? A linkurl:study published today; (September 21) in Nature claims to resolve this long-standing question. For many years, most paleontologists debated whether digits arose 380 million years ago as a novel evolutionary trait in tetrapods, or four-footed creatures. The new study, led by Catherine Boisvert, at Uppsala University in Sweden, identifies distinct digit-like structures, called distal radials, in the fin of a 385 million-year-old fish, Panderichthys. This fish is one of eight ancient fish species that paleontologists have specimens of, whose fossils date from between 385 and 365 million years ago, and are thought to be early predecessors of tetrapods. The finding suggests that the underpinnings of digits may have evolved in such fish, which did not walk on land, but lived in coastal shallow regions. "Here we get the first one of these archaic fish that has a tetrapod pattern" of bone structure, linkurl:Robert Carroll,; vertebrate paleontologist at McGill University, who was not involved in the study, told The Scientist. The findings are particularly convincing, noted Carroll, because the fish fossil that the researchers analyzed is more intact than previous specimens. Using a sophisticated CT scanning system, the researchers produced a 3D model of the ancient fish, which allowed them to identify radial distals in its fin, and concluded that "the lack of finger equivalents [in past specimens] was an artifact of the preservation of the fossils," Boisvert wrote to The Scientist in an Email. Previous data from another ancient fish called Tiktaalik showed distal radials as well -- although the quality of that specimen was poor. And the orientation of the radials did not seem to match the way modern fingers and toes radiate from a joint, parallel to each other. "The disposition of distal radials in Panderichthys are much more tetrapod-like than in Tiktaalik," Boisvert wrote. "Combined with fossil evidence from Tiktaalik and genetic evidence from sharks, paddlefish and the Australian lungfish, it is now completely proven that fingers have evolved from distal radials already present in fish that gave rise to the tetrapod." The next step in this work, said Carroll, will be to support the poor fossil record from this period of evolutionary history with genetic work to demonstrate the structural similarity between modern creatures and their supposed water-dwelling ancestors. Below is a movie of the 3-D imaging from the CT scans of Panderichthys. The orange pieces represent the distal radials.
Fish fingers point to origin of digits (Video credit: Catherine A. Boisvert, Elga Mark-Kurik, and Per E. Ahlberg)


Avatar of: Neville Woolf

Neville Woolf

Posts: 1

September 22, 2008

The five digits that appear in early tetrapods are\nnot just in the line of vertebraetes. Starfish also seem to prefer the grasping power of five digits.\n\nWhereas 4 feet allows a balance on three while the fourth one moves, the digits seem to have a different role, and one wonders about the rationale - is five fingers the least number that can get a good grasp of a slippery object?\n\nWhen two digits are used, any slight misalignment results in a torque that makes an object rotate out of the grasp. Three digits will grasp a linearly extended object moderately well, though it may slip along the line defined by the two digits on one side. Four digits creates\ntwo orthogonal directions of holding, but an irregularly shaped object will tend to have non parallel edges) and these will not be well held.\n\nA fifth digit allows the contacts to exert pressure at angles of ~72 degrees which seems to\nallow a much greater control Experiments to explore this issue would be very interesting.

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