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Forget this, I'm going back in the water!

  • 19-04-2009 1:24am
    Registered Users Posts: 30,746 ✭✭✭✭

    Things just got confusing...
    So apparently certain tetrapods evolved 'backwards' to be more suited to life in the water. Significantly, this could hint that the first tetrapods to walk on land came about much earlier than first taught.
    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 published in the recent 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 analyse 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 palaeontologist 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.

    Image by Maurílio Oliveira


  • Registered Users Posts: 30,746 ✭✭✭✭Galvasean

    In a related story (from 2007) researchers demonstrated that early amphibians bit their prey (like a crocodile) rather than sucked it in (like a fish). Originally the research suggested that this trait originated in the water for a hunting strategy similar to that of modern day crocodiles and alligators. However the newer research (in the OP) would suggest it was adopted while living on land and taken back into the water later.
    (one of the researchers has the same middle initial and last name as me :))
    Ancient aquatic amphibians developed the ability to feed on land before completing the transition to terrestrial life, researchers from Harvard University report this week in the Proceedings of the National Academy of Sciences.

    Their work is based on analysis of the skulls of the first amphibians, which arose 375 million years ago, and their fish ancestors. The shapes of the junctions between adjacent skull bones - termed 'sutures' - in the tops of these fish and amphibian skulls reveal how these extinct animals captured prey, say authors Molly J. Markey and Charles R. Marshall.

    'Based on experimental data obtained from living fish, we found that the shapes of sutures in the skull roof indicate whether a fish captures its prey by sucking it into the mouth - like a goldfish - or by biting on it directly, like a crocodile,' says Markey, a postdoctoral researcher and lecturer in Harvard's Department of Earth and Planetary Sciences. 'A biting or chewing motion would result in a faint pushing together of the frontal bones in the skull, while a sucking motion would pull those bones ever so slightly apart. By comparing the skull roofs of living fish to those of early amphibians and their fishy ancestors, we were able to determine whether the fossil species fed by suction or by biting.'

    Using this approach, Markey and Marshall found that in one key transitional species, the aquatic amphibian Acanthostega, the shapes of the junctions between adjacent skull bones are consistent with biting prey. This finding, the scientists say, suggests that the water-dwelling Acanthostega may have bitten on prey at or near the water's edge.

    'Going from the aquatic realm to land involved a series of adaptations to facilitate changes in locomotion, respiration, reproduction, sensation, and feeding,' Markey says. 'In water, suction is an efficient method of feeding, but it does not work in the much less dense air environment. Early terrestrial inhabitants would thus have had to develop the means for chomping prey.'

    Markey and Marshall first measured the skull roof sutures, those areas where the bones of the skull roof meet, in the living fish Polypterus as it fed. They then analysed the same cranial junctions in several fossils - the early amphibian Acanthostega, its fishy ancestor Eusthenopteron, and the extinct terrestrial amphibian Phonerpeton - to determine how these bones may have moved relative to each other during feeding. By analysing the tiny forces that the sutures experienced during feeding, such as tension or compression, the researchers could determine how the skull roof likely deformed as the animals ate.

    Living fish exhibit an incredible array of tooth and jaw shapes, suggesting that, ironically, direct analysis of fossil jaws would be a less precise means of determining the feeding methods of extinct species, Markey says.

    'Analysis of the sutures of the early amphibian species Acanthostega revealed that, while it had many adaptations to an aquatic lifestyle, it was more likely a biter than a sucker,' Markey says. 'The analysis suggests that amphibians evolved a bite before emerging onto land as fully terrestrial animals.'

    Charles R. Marshall is professor of biology and geology in Harvard's Faculty of Arts and Sciences. His work with Molly J. Markey was supported by funding from Harvard