Ancient Scottish Fossils Push Back Tetrapod Timeline

New research from the University of Texas at Austin reveals that the Westlothiana lizziae and similar ancient salamander-like creatures are 14 million years older than previously believed. The revised age-346 million years-places them in a little-understood period of evolutionary history called Romer's Gap.

Hector Garza, who recently completed his doctorate at UT's Jackson School of Geosciences, led the study published in PLOS One. Garza used a high-risk geochemical approach called uranium-lead dating on zircon crystals to determine the fossils' oldest possible age. Volcanic basalt rock, common in the area, rarely preserves zircons, so colleagues doubted he'd succeed.

But Garza's gamble paid off. Volcanic mudflows had carried zircons into the limestone beds at East Kirkton Quarry, entombing the ancient creatures alongside. X-ray analysis at the Jackson School revealed zircon grains in six of the samples, and laser dating at the University of Houston confirmed the fossils' maximum age.

Previously, scientists thought the fossils were no older than 331 million years, comparable to similar finds globally. This older age, however, positions them within Romer's Gap-a 360- to 345-million-year window marked by scarce fossil evidence of land animals. During this time, fish-like creatures developed lungs and legs to adapt to life on land.

"Better constraining the age of these fossils is key to understanding the timing of the emergence of vertebrates on to land," said Julia Clarke, Jackson School professor and study co-author. "Timing in turn is key to assessing why this transition occurs when it does and what factors in the environment may be linked to this event."

The East Kirkton Quarry remains one of the richest sites for early tetrapod fossils, including seven specimens of stem tetrapods like Westlothiana lizziae. Once a lush tropical forest with active volcanoes and a toxic lake, the site preserves evidence of the dramatic environmental conditions that shaped the evolution of life on land.

Research Report:New U-Pb constraints and geochemistry of the East Kirkton Quarry, Scotland: Implications for early tetrapod evolution in the Carboniferous