Navigation Links
Fossils suggest earlier land-water transition of tetrapod

DURHAM, N.C. -- 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 extinct species, 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.


Contact: Monte Basgall
Duke University

Related biology news :

1. Cretaceous octopus with ink and suckers -- the worlds least likely fossils?
2. Dinosaur fossils fit perfectly into the evolutionary tree of life
3. Hobbit fossils represent a new species, concludes University of Minnesota anthropologist
4. Rare Antarctic fossils reveal extinction of tundra before full polar climate arrived
5. Antarctic fossils paint a picture of a much warmer continent
6. Fossils and layers: Solving the terrane puzzle of the North American cordillera
7. Living fossils have long- and short-term memory
8. Oldest Australian crayfish fossils provide missing evolutionary link
9. Fossils excavated from Bahamian blue hole may give clues of early life
10. Rodent fossils allows to determine climate of the Iberian Peninsula 6 million years ago
11. Living fossils have hot sex
Post Your Comments:
(Date:11/10/2015)... LONDON , Nov. 10, 2015 /PRNewswire/ ... segmented on the basis of product, type, ... segments included in this report are consumables, ... this report are safety biomarkers, efficacy biomarkers, ... in this report are diagnostics development, drug ...
(Date:11/2/2015)...  SRI International has been awarded a contract of ... to the National Cancer Institute (NCI) PREVENT Cancer Program ... modern testing and support facilities, and analytical instrumentation to ... studies to evaluate potential cancer prevention drugs. ... Drug Development Program is an NCI-supported pipeline to bring ...
(Date:10/29/2015)... health pioneer, Joseph C. Kvedar , MD, describes ... wellness, and the business opportunities that arise from it ... Healthy Things . Long before health and wellness ... vice president, Connected Health, Partners HealthCare, was creating a ... the hospital or doctor,s office into the day-to-day lives ...
Breaking Biology News(10 mins):
(Date:12/1/2015)... Today the Allen Institute announced the opening ... South Lake Union neighborhood, the city,s biotechnology hub. ... Westlake Avenue North, the 270,000 square foot life sciences ... Science and the Allen Institute for Cell Science. ... the Allen Institute. "We started by building a map ...
(Date:12/1/2015)... MOUNTAIN VIEW, Calif. , Dec. 1, 2015 ... introduce the Transformational Health program. This program addresses ... this period of change. ... --> Logo - ... Understanding every aspect of healthcare, as well as ...
(Date:12/1/2015)... 2015 ... "2016 Europe Cell Surface Markers: Country ... Strategies, Opportunities for Suppliers--France, Germany, Italy, Spain, ... --> ) has announced the ... Surface Markers: Country Volume and Sales Forecasts, ...
(Date:12/1/2015)... MD (PRWEB) , ... December 01, 2015 , ... ... solutions provider, announces that its best selling system laboratory animal colony management software ... Cloud today, without investing in on-site IT resources., , ...
Breaking Biology Technology: