Land-walking ancestor is older than thought

A significant fossil puzzle piece in the evolution of the first vertebrate animals to leave Earth’s ancient seas and walked on land was discovered in Scotland more than 40 years ago. But it has only just been accurately aged – and the results have left palaeontologists stunned.

Grey fossils of lizard on black background
The fossil of the Westlothiana lizziae, which was found in the East Kirkton Quarry in West Lothian, Scotland. Credit: National Museums Scotland.

The creature was dubbed Westlothiana lizziae and is one of the oldest examples of a 4-legged animal, also known as a tetrapod, walking on Earth.

These pioneering tetrapods are the ancestors of all land-based vertebrates including dinosaurs, birds and humans, as well as those which went back into the water, like whales.

W. lizziae looked like a lizard or salamander and was only about 20cm in length. Its fossil was found by an amateur palaeontologist in 1984 at the East Kirkton Quarry in the small town of Bathgate just west of Edinburgh. The fossil specimen is affectionately known as “Lizzie” although it was not a reptile, and its sex hasn’t been determined.

Palaeontologists previously assumed that Lizzie was the same age as similar fossils from around the world – dating to about 331 million years ago (mya).

A stroke of luck led to new research which accurately aged Lizzie to 346 mya.

The results are  published in the journal PLOS One.

The best way to date such ancient rocks is to use zircon crystals. Geoscientists can determine the age of zircons based on the ratio of uranium to lead in the crystals. Older zircon has more lead which is the by-product of uranium’s radioactive decay.

In rocks without zircon, dating is much more difficult.

Zircon doesn’t often form in basalt rock which is the hardened lava flow of ancient volcanoes. This is exactly the kind of rock in which Lizzie and 6 other early tetrapods were found. The region 350 mya was a tropical forest with active volcanoes, a toxic lake, and diverse plant and animal life including giant insects and other arthropods.

Drawing of a lizard on a rock by a lake with volcanoes in the background
An environmental reconstruction of the East Kirkton area 346 million years ago, depicting Westlothiana lizziae resting on a rock. Credit: Hector Garza.

“I think that was one of the reasons why no one tried to go into them before,” says first author Hector Garza, a recent PhD graduate at the University of Texas at Austin in the US. “Because of all the time and effort that it takes to obtain the zircons and then taking the risk of not finding any.”

But the palaeontologists’ fortunes turned when they realised that the mud cascade and lava flow which fossilised the ancient creatures like Lizzie also included sediment that contained zircon.

Garza X-rayed 11 rock samples from the layers around 6 of the fossils to get the revised age of 346 million years.

This places Lizzie and the other stem tetrapods found at the quarry in a time period from 360–345 mya. This is known as Romer’s Gap – a period which, for unknown reasons, has a particularly low number of fossil discoveries. Romer’s Gap spans the beginning of the Carboniferous period which lasted 359 to 299 mya.

Romer’s Gap annoyingly lines up with the exact period when tetrapods first walked on land. Another fossil found from this time range in Victoria, Australia was earlier this month revealed to be a trackway from an early reptile-like creature – a discovery which suggests that the first land-walking vertebrates might have been even older than previously thought.

But lack of further evidence from Romer’s Gap places even more importance on existing fossils such as Lizzie and the other tetrapods found at East Kirkland.

“I can’t overstate the importance of the iconic East Kirkland tetrapods,” says co-author Julia Clarke, a professor at the University of Texas at Austin. “Better constraining the age of these fossils is key to understanding the timing of the emergence of vertebrates on to land. 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.”

Sign up to our weekly newsletter

Please login to favourite this article.