In 1984, an amateur palaeontologist found a small, nearly complete fossil in a quarry in West Lothian, Scotland. The creature, later named Westlothiana lizziae, is about 20 centimetres long and resembles a salamander. It is one of the earliest known examples of a four-limbed animal that had made the transition from water to land, a stem tetrapod: a common ancestor of every amphibian, reptile, bird, and mammal alive today, including humans. Despite its significance, no one had managed to accurately date it.

New research from The University of Texas at Austin, published in PLOS One, places Westlothiana lizziae and several similar creatures from the same Scottish site at a maximum age of around 341 million years — roughly 10 million years older than the previous estimate of 331 million. The figure of 346 million years, cited in some accounts of the research, represents the oldest individual zircon grain recovered; the study’s central estimate, derived from seven dates, is 341 ± 3 million years. That revision of 14 to 15 million years matters because it moves the specimens into a specific and poorly-understood period in the fossil record called Romer’s Gap.

This is one study, not settled consensus, and the dating method produces a maximum age rather than a precise one. What it offers is a better constraint on timing than anything previously available for these particular specimens.

What Romer’s Gap is

Romer’s Gap refers to the period from roughly 360 to 345 million years ago. It is named after the American palaeontologist Alfred Romer, who noticed in the mid-twentieth century that the fossil record from this window is strikingly sparse. The transition from water-dwelling fish to four-limbed land animals is thought to have happened during or around this period, but the fossils that should document that process are largely absent. Whether the gap reflects a real collapse in animal populations, a geological accident that destroyed the record, or simply a research blind spot is still debated.

The East Kirkton Quarry, where the Scottish fossils were found, sits in what was, hundreds of millions of years ago, a tropical landscape with active volcanoes and a toxic lake. Seven stem tetrapod fossils have been recovered there, including Westlothiana lizziae. The site is one of the better-preserved early tetrapod records in the world. What it had lacked was a reliable date.

Why dating the site was difficult

The standard technique for dating ancient rocks is uranium-lead radiometric dating, which relies on zircon crystals. Zircons form in certain rock types, particularly those that cool slowly from molten material. The East Kirkton site sits near ancient volcanoes whose flows hardened into basalt, a rock type where zircons do not typically form. Colleagues warned Hector Garza, the doctoral student at the UT Jackson School of Geosciences who led the study, that trying to extract datable zircons from these rocks was likely to produce nothing.

He tried anyway. The key turned out to be that as material eroded from the volcanic surroundings, sediment containing zircons washed into a lake where limestone was forming. That limestone entombed the early tetrapods, and with it came the zircons Garza needed. He X-rayed 11 rock samples and extracted zircons from six of them. He then conducted uranium-lead laser dating on those zircons at the University of Houston.

The result is a maximum age of 346 million years, placing the specimens inside Romer’s Gap.

What the new age does, and does not, establish

“Better constraining the age of these fossils is key to understanding the timing of the emergence of vertebrates on to land,” said Julia Clarke, professor at the Jackson School and a co-author of the paper. “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.”

That framing is careful, and it is worth taking seriously. The new dating does not explain why the transition from water to land happened when it did. It does not fill Romer’s Gap; it places specific specimens within it. The significance is that it becomes harder to argue these creatures lived outside the Gap, which means they were alive during the period palaeontologists most want to understand.

What drove animals from water to land, whether climate, competition, the availability of food, or some combination, remains an open question. Having better-dated specimens from within the Gap gives researchers a fixed point to work from when trying to connect evolutionary timing to environmental conditions.

Other co-authors on the paper include Associate Professor Elizabeth Catlos and Michael Brookfield of the UT Jackson School, and Thomas Lapen of the University of Houston. The National Museum of Scotland provided rock samples surrounding the fossils for analysis.

The next step, according to the paper’s authors, is to use the more precise age estimate as a reference point for understanding what was happening to the environment around the time these animals were alive, and whether that context helps explain the transition the fossils represent.

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