Australasian Science Magazine

By John Long

Tetrapod trackways are helping to decode the behaviour of these Devonian creatures.

I write this column from the blustery wind-swept coast of Valentia Island, a small island off the west coast of Ireland. I’m here studying some of the oldest known undisputed tetrapod trackways in the world, dating back to Middle Devonian times about 385 million years ago. The tracks are beautifully preserved, and show evidence of a large newt-like animal, just under a metre long, that has left large feet and small hand impressions.

Up until a month ago there was only one trackway known from the region, and it has become a real tourist attraction on the island. There is an arrow sign saying “Tetrapod Tracks” from the main road, and when you arrive another sign announces you are in the “Tetrapod Carpark”. Then another sign written in English and Gaelic directs you to the tetrapod track site. A beautifully crafted interpretation sign tells you about the significance of the site.

Finally you arrive at the beach and a metal railing separates you from the large flat bed of rock containing hundreds of individual prints of animals that once swam in shallow water through a river, their limbs gently caressing the ripple-marked bottom of the stream bed to leave a regular pattern of traces. Here you can see large foot traces interspersed with smaller hand traces. In one place the tracks are cut across by another animal, heading at a right angles to the main protagonist.

I’m here with Dr Grzegorz Niedzwiedzki from Uppsala University, one of the world’s leading experts on early tetrapod trackways. In January 2010 he rocked the scientific world by announcing trackways of early tetrapods dating back to the base of the Middle Devonian of Poland (c. 390 million years old). The paper made the cover of Nature, and caused quite a controversy at the time. Recent works have debated the authenticity of these tracks, but Grzegorz has many new finds, unpublished, that he feels will show them to be undisputed tetrapod trackways.

This week, on Valentia Island, we are making a new study of the trackways, searching for new data on their digit patterns and body movements. We have already uncovered a lot of new data about them, and even found a number of significant new trackways.

We know a lot about the skeletons of the Devonian tetrapods, especially Ichthyostega and Acanthostega from East Greenland, which lived about 365 million years ago. However, we know relatively little about their behaviour.

Trackways are special because we get a rare chance to witness how animals once moved and behaved. Some dinosaur tracks show hunting behaviour, or group dynamics, with the young being protected in the centre of the herd. It’s exciting to study these really ancient tetrapod tracks as they can tell us about the daily life of such animals, a small window into how they moved and interacted.

Another part of the puzzle about the early tetrapod life history was recently solved by Sophie Sanchez and colleagues when they revealed the growth history of Acanthostega. A synchrotron scan was able to identify growth lines inside their bones to determine how slowly they grew. This revealed they had a long juvenile phase of perhaps 8–10 years before reaching maturity.

This discovery was a huge surprise to biologists, as up until now we thought we were dealing with adult individuals found from these deposits. Instead we now learn that the vast majority found so far are not adults, so it implies that much larger animals could still be found in these deposits.

Australia has some fascinating early tetrapod trackways. In 1972 the discovery of trackways in the Genoa River region of eastern Victoria was then the oldest evidence of tetrapods in the world. These are now dated to the Frasnian age about

380 million years ago. However, in 1986 a set of trackways from the Grampians in western Victoria was described, suggesting that tetrapods appeared in the late Silurian or Earliest Devonian! The dating was uncertain, but the sequence likely has to be in the Early Devonian about 419–410 million years ago.

Armed with a new Australian Research Council grant we hope to re-examine these tracks and their geological context to once and for all resolve the enigma of the Grampians tracks. To do this we are bringing in world experts like Grzegorz to help discover new information about their hand and feet structure, and how these early four-limbed beasts once moved around.

All we need next is more body fossils of Devonian tetrapods in Australia to nail the problem. So far we have just one jaw dated at about 365 million years old called Metaxygnathus. We know they are here, so the search goes on.

Keep watching for new updates.