This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: For nearly 30 years, a small fossil from Scotland has been celebrated as one of the earliest known reptiles. A new study out today in Nature and led by researchers at the American Museum of Natural History and the University of Oxford, in collaboration with the European Synchrotron Radiation Facility, is rewriting that story.
Discovered in 1984, the 345-million-year-old fossil of Westlothiana lizziae, nicknamed "Lizzie" for its lizard-like appearance, was originally described mostly from what is visible on the surface of its two crushed slabs: a nearly complete skeleton with four limbs, with fingers and toes bearing superficially claw-like tips. These features set Westlothiana apart from many of its contemporaries, which had more fin-like limbs. The fossil is deformed and difficult to interpret, but it was widely cited as the earliest land-adapted amniote—the group of vertebrate animals that includes reptiles, birds and mammals.
Since it was described in 1990, Westlothiana has been frequently used by researchers as a landmark fossil in discussions of the origin and timing of amniotes. "Lizzie has been an icon for the early evolution of amniotes for decades," said Xavier Jenkins, a postdoctoral fellow in the museum's Division of Paleontology and co-lead author of the study. "But when we were finally able to see inside the fossil, we found an animal that looked very different from what we expected." The research team used X-ray scanning at the European Synchrotron Radiation Facility in Grenoble, France, to see through the rock and reconstruct anatomical structures that could not be seen from the fossil's surface.
The scans revealed a primitive skull as well as ossified internal gills and a fish-like mouth lined with thousands of tiny teeth—features that rule out the possibility that Westlothiana was an ancestor of reptiles. The findings suggest that this animal was aquatic or amphibious, not terrestrial. "The surprisingly primitive features of the skull show us that Lizzie was not only not a reptile but belonged to a much more ancient lineage.
It turns out looks can be deceiving, and Lizzie lived a completely different lifestyle than we previously thought," said Ben Igielman, co-lead author of the study, who started this research during his graduate work at the University of Oxford. As a result of the study, researchers now conclude that the anatomical traits of Westlothiana that were once assumed to be exclusive to land-adapted amniotes, including its claws and foot structure—five digits on its hands and feet, a feature found in mammals and reptiles—had already evolved among earlier, more aquatic members of the stem lineage of tetrapods, the larger group of four-limbed animals that includes amniotes plus amphibians. "This is a big surprise," said study co-author Roger Benson, the museum's Macaulay curator of paleontology.
"It shows that, although we think of amphibians as being more 'primitive' than reptiles today, some of the traits of reptiles were already there in the ancestor of both amphibians and reptiles." The findings also bear on interpretations of fossil trackways that have recently been used to show that amniotes originated during the Devonian Period, more than 359 million years ago. Those trackways preserve impressions of an animal walking on four limbs with claw-like digits. If early tetrapod relatives such as Westlothiana also had claws, the presence of clawed tracks alone can no longer establish that the animal that made them was an amniote.
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