Fossils suggest earlier land-water transition of tetrapod

Apr 17, 2009

New evidence gleaned from CT scans of fossils locked inside rocks may flip the order in which two kinds of four-limbed animals with backbones were known to have moved from fish to landlubber.

Both , known as Ichthyostega and Acanthostega, lived an estimated 360-370 million years ago in what is now Greenland. Acanthostega was thought to have been the most primitive tetrapod, that is, the first vertebrate animal to possess limbs with digits rather than fish fins.

But the latest evidence from a Duke graduate student's research indicates that Ichthyostega may have been closer to the first tetrapod. In fact, Acanthostega may have had a terrestrial ancestor and then returned full time to the water, said Viviane Callier, who is the first author of a report on the findings to be published in today's issue of the journal Science.

"If there is one take-home message, it is that the evolutionary relationship between these early tetrapods is not well resolved," Callier said.

Co-author Jennifer Clack of the University Museum of Zoology in Cambridge, England -- where she supervised Callier's work for a master's degree -- found the fossils embedded in rocks collected from East Greenland.

Rather than trying to remove them -- an action that would have destroyed much of the evidence -- the researchers studied the fossils inside the stone with computed tomography (CT) scanning. Callier "reconstructed" the animals using imaging software (Amira and Mimics) to analyze the CT scans, focusing on the shapes of the two species' upper arm bones, or humeri.

The CT slices revealed that Clack had found the first juvenile forms of Ichthyostega. Previously known fossils of Ichthyostega had come from adults.

Anatomies can morph as animals move towards adulthood, Callier said. And such shifts can help scientists deduce when in development the animal acquired the terrestrial habit. The fossils suggest that Ichthyostega juveniles were aquatically adapted, and that the terrestrial habit was acquired relatively late in development. The fossils bore evidence that the muscle arrangement in adults was better suited to weight-bearing, terrestrial locomotion than the juvenile morphology. It is possible that Ichthyostega came out of the water only as a fully mature adult.

In contrast, in Acanthostega "there is less change from the juvenile to the adult. Although Acanthostega appears to be aquatically adapted throughout the recorded developmental span, its humerus exhibits subtle traits that make it more similar to the later, fully terrestrial tetrapods," Callier said

Because the shapes of its adult limbs seemed the most fin-like, scientists had previously concluded that Acanthostega was "more primitive," Callier said. "But now, if we look at the details of the humeri, Ichthyostega's are actually more similar to earlier fishes."

Ironically, the shape of Acanthostegas limb's, in both adult and the newly-discovered juvenile forms, is more "paddle-like" than Ichthyostega's, Callier said. "They would have been really good swimmers. So, although Acanthostega had limbs with digits, we don't think it was really terrestrial. We think even the adults were aquatic."

"If Ichthyostega is actually more primitive than Acanthostega, then maybe animals evolved towards a terrestrial existence a lot earlier than originally believed," she said. "Maybe Acanthostega was actually derived from a terrestrial ancestor, and then, went back to an aquatic lifestyle."

Per Ahlberg, a Swedish paleontologist who was previously Clack's graduate student, also joined Clack in a comparative analysis of other more fish-like species living at about the same time as Ichthyostega and Acanthostega.

Those include Tiktaalik, another animal that has made the news because of scientists' deductions that it was in transition from water to land.

"It seems like there were different species evolving the same or similar traits independently -- evidence of parallel evolution," Callier said. "The terrestrial environment posed new challenges like feeding and moving on land and breathing air, to which the first tetrapods had to evolve solutions. Sometimes different lineages stumbled upon similar solutions."

Ahlberg, now professor at the University of Uppsala in Sweden, is corresponding author of the new Science report. The research was funded by the Winston Churchill Foundation and the Swedish Research Council.

Source: Duke University (news : web)

Explore further: Researchers create methylation maps of Neanderthals and Denisovans, compare them to modern humans

add to favorites email to friend print save as pdf

Related Stories

Four-legged fish an evolutionary mistake

Sep 03, 2005

The "four-legged fish" Ichthyostega is not the "missing link" between marine and land animals, but rather one of several short-lived "experiments". This is what scientists from Uppsala and Cambridge universities maintain ...

Did our ancestors breathe through their ears?

Jan 19, 2006

A fossil fish skull from Latvia that researchers from Uppsala University, Sweden, describe in this weeks issue of Nature shows that the earliest land animals probably breathed through their ears.

Primordial fish had rudimentary fingers

Sep 22, 2008

Tetrapods, the first four-legged land animals, are regarded as the first organisms that had fingers and toes. Now researchers at Uppsala University can show that this is wrong. Using medical x-rays, they found rudiments ...

Coelacanth fossil sheds light on fin-to-limb evolution

Aug 01, 2007

A 400 million-year-old fossil of a coelacanth fin, the first finding of its kind, fills a shrinking evolutionary gap between fins and limbs. University of Chicago scientists describe the finding in a paper ...

Recommended for you

Crowd-sourcing Britain's Bronze Age

Apr 17, 2014

A new joint project by the British Museum and the UCL Institute of Archaeology is seeking online contributions from members of the public to enhance a major British Bronze Age archive and artefact collection.

Roman dig 'transforms understanding' of ancient port

Apr 17, 2014

(Phys.org) —Researchers from the universities of Cambridge and Southampton have discovered a new section of the boundary wall of the ancient Roman port of Ostia, proving the city was much larger than previously ...

User comments : 0

More news stories

Clippers and coiners in 16th-century England

In 2017 a new £1 coin will appear in our pockets with a design extremely difficult to forge. In the mid-16th century, Elizabeth I's government came up with a series of measures to deter "divers evil persons" ...

Airbnb rental site raises $450 mn

Online lodging listings website Airbnb inked a $450 million funding deal with investors led by TPG, a source close to the matter said Friday.

Health care site flagged in Heartbleed review

People with accounts on the enrollment website for President Barack Obama's signature health care law are being told to change their passwords following an administration-wide review of the government's vulnerability to the ...