New Evidence from Scotland Narrows a Key Gap in Early Land Animal Evolution

What is it about?

Our group from the University of Texas at Austin and the University of Houston is trying to figure out exactly when some of the earliest land-walking animals lived. This is important because there is a big gap in the fossil record (called Romer’s Gap, around 360–345 million years ago) that makes it difficult to understand how vertebrates transitioned from water to land. One important fossil site for these early animals is the East Kirkton Quarry in Scotland. It has fossils of several early tetrapods, including species like Westlothiana, Silvanerpeton, and Eucritta. But until now, the age of the rocks where these fossils were found had only been estimated based on indirect methods, such as fossils from plants or tiny fossils nearby. Garza and colleagues directly dated the rocks containing these fossils using a method called U-Pb (uranium-lead) dating on zircon crystals, which are tiny minerals that can preserve geological time like a clock. Their analyses showed that the rocks are about 341 ± 3 million years old—a bit older than previously thought. This new age puts these fossils right in the middle of Romer’s Gap, suggesting that these animals help fill in that important missing chapter in vertebrate evolution.

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Why is it important?

This study presents a new radiometric age of 341 ± 3 million years, offering a more precise and reliable date than earlier fossil-based estimates. It confirms that key early tetrapod fossils from the East Kirkton Quarry fall within Romer’s Gap, rather than just after it, providing a clearer timeline for when vertebrates began transitioning from water to land. Geochemical analyses also reveal that these animals lived in a dynamic environment—a volcanic, tropical lake system shaped by hot springs and occasional marine incursions. Together, these findings help fill a major gap in the fossil record and shed new light on where and when early land animals lived.