Fossil discoveries fill crucial gap in land animal evolution
7 March 2012, by Tamera Jones
A run of recent fossil discoveries in Scotland has helped plug a major 15-million-year gap in the fossil record and provide crucial information about the early evolution of land animals.
An early tetrapod, Pederpes finneyae.
UK palaeontologists led by Professor Jennifer Clack from the University of Cambridge discovered the fossils over a 23-year period. After searching in Devonian-age rocks in Scotland for over 20 years, Clack's colleague Stanley Wood and Tim Smithson finally chanced upon the fossils.
They include animals both with and without backbones – vertebrates and invertebrates – which lived either in water or on the land between 360 and 345 million years ago. This 15-million-year stretch runs from the end of the Devonian to the early Carboniferous periods.
The findings help bridge a puzzling gap in the fossil record between Devonian tetrapods, which were primitive, four-legged, aquatic animals, and the more complex, mostly land-dwelling post-Devonian tetrapods. These more-developed terrestrial animals are essentially our distant ancestors.
'The break has been frustrating, because you wouldn't expect evolution to jump from simple aquatic creatures to complex, terrestrial animals without something in between,' explains Clack.
The gap, named Romer's Gap after the palaeontologist who discovered it, was an even longer 30 million years when Romer first identified it. Although the gap is slowly closing, the break that covers the key 15 million years, when our ancestors left the water to live as land-dwellers, has existed for decades. Until now, palaeontologists had just two fossilised examples of animals from this interval – one from Scotland, the other from Nova Scotia.
'Discoveries like this open things up, like breaking up a log-jam.'
Professor Jennifer Clack, University of Cambridge
'The two countries were in the same locality 350 million years ago,' says Clack.
Such a scarcity of fossils from a seminal moment in evolutionary history had led researchers to come up with a bunch of possible explanations.
These include suggestions that it was indeed real, reflecting the actual pattern of evolution. Other researchers wondered if maybe a mass extinction at the end of the Devonian, 360 million years ago, might have suddenly changed the environmental conditions and given rise to land animals. Still others concluded that there must have been low levels of oxygen at the time which would have limited evolution on land.
The breakthrough implies that the apparent multimillion-year gap in the fossil record merely reflected an incomplete fossil collection rather than a lack of terrestrial animals. Not only that, but it seems that many tetrapods walked the Earth much earlier than previously suspected.
Reconstruction of pedes of various taxa. Pederpes, Greererpeton, Silvanerpeton and Proterogyrinus show asymmetrical metatarsals compared with those of the Devonian forms Acanthostega, Ichthyostega and Tulerpeton.
'These fossils are already revealing that the pentadactyl limb, which features five fingers or five toes, evolved a good 20 million years earlier than we thought,' Clack says.
'This suite of fossils from Scotland now gives us a fuller picture of events at the start of the Carboniferous, when animals left the water and started colonising the land,' says Clack, lead author of the study published in Proceedings of the National Academy of Sciences and member of the four-man research team.
Indeed Clacks says the find will almost certainly reveal how primitive animals at the end of the Devonian evolved into complex Carboniferous animals.
'The sequence of fossils over the crucial period should be able to say something about the rate at which animals evolved at the end of the Devonian mass extinction,' she says.
Now these fossils have discovered, it's likely many more will crop up, because fossil-hunters will now know where to look.
'Discoveries like this open things up, like breaking up a log-jam,' explains Clack.
Timothy R. Smithson, Stanley P. Wood, John E. A. Marshall, and Jennifer A. Clack, Earliest Carboniferous tetrapod and arthropod faunas from Scotland populate Romer's Gap, Proceedings of the National Academy of Sciences, published 5 March 2012, www.pnas.org/cgi/doi/10.1073/pnas.1117332109
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