This fossil of Diadectes, a land-adapted tetrapod that lived in North America about 290 million years ago, is on view in the Hall of Vertebrate Origins. Westlothiana is from an older, aquatic lineage.Denis Finnin/© AMNH
For more than 30 years, a Scottish fossil named “Lizzie” was considered one of the earliest known reptiles. Now, a new look inside the 345-million-year-old fossil is revealing a very different story.
Discovered in Scotland in 1984, Westlothiana lizziae—nicknamed “Lizzie” for its lizard-like appearance—was described in 1990 as the earliest land-adapted amniote, the group that includes reptiles, birds, and mammals. Though the fossil was crushed and difficult to interpret, its four limbs and claw-like digits made it an important reference point in studies of early amniote evolution.
“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.”
Using x-ray scans at the European Synchrotron Radiation Facility in Grenoble, France, researchers were able to see through the rock and reconstruct parts of the animal that could not be seen from the surface. What they found changed the picture completely: a primitive skull, internal gills, and a fish-like mouth filled with thousands of tiny teeth.
These features show that Westlothiana was not a reptile at all and likely lived in or around water.
“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.
The discovery also reshapes scientists’ understanding of how reptiles and other land-adapted tetrapods evolved. Features such as five-toed feet and claw-like digits—once thought to be hallmarks of early amniotes—were already present in more aquatic members of the tetrapod lineage, the group that includes both amphibians and amniotes.
“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 may even change how scientists interpret ancient fossil trackways. Clawed footprints have sometimes been used as evidence that amniotes originated more than 359 million years ago. But if animals like Westlothiana already had claws, those tracks may have been made by more primitive amphibious relatives instead.
“The broader lesson is that terrestrial-looking traits, such as a reptile-like foot, a weight-bearing forelimb, and claw-like phalanges were not unique features of tetrapods, but rather accumulated piecemeal in their close relatives, in animals that were still living in and out of the water,” Jenkins said.