Scientists led by Trinity College Dublin have resolved the mystery of why a series of amphibian bones appeared to have been cooked after the animals died.

Scientists have solved a decades-long mystery as to why ancient tetrapods – amphibian-like creatures that lived over 300 million years ago and preserved in one of Ireland’s most important fossil sites, seemingly had their bones cooked after they died.

The Jarrow Assemblage, including Keraterpeton galvani, is one of Ireland’s most important fossil sites and preserves some of the oldest amphibians to live in terrestrial environments. These fossils are found in a coal seam in County Kilkenny, southeast Ireland.

Fossils from this site have one unique feature; their original internal bone morphology has been altered so that now it is difficult to make out detail from the fossils. The cause of this alteration has baffled scientists, with explanations for this alteration usually thought to be due to acid dissolving the bones when the animals were first buried. That is until now.

A team of scientists from Trinity College Dublin, University College Dublin, National University of Ireland, Galway, (now known as the University of Galway), the University of Birmingham (UK), and the Gemological Institute of America used a combination of computed tomography to produce X-ray images of the fossil, and laser ablation to analyse the chemistry of the bones, to investigate the causes of this alteration.

See also: Weird wonder fossils from Wales add to evolutionary puzzle

internal bone morphology

Lead author Dr Aodhán Ó Gogáin, from Trinity’s School of Natural Sciences, said: “Normally in fossil bone we see that the internal original structure is preserved. But when we looked at the X-ray images of fossils from Jarrow, we see that no internal bone morphology has been preserved and that bones have been partially replaced by the surrounding coal.”

The team also found apatite preserved in the bones. Trinity’s Dr Gary O’Sullivan, a co-author in the study, said: “The chemistry of the apatite crystals can tell us a lot about how it formed, whether it grew organically in the animal, formed when the animal was being buried, or whether some other factors influenced its growth.

“Apatite is a major constitute of living bone, so it is no surprise we find some preserved in these bones. However, when we look at the chemistry of apatite in the bones from Jarrow, we find that this apatite was formed by heated fluids within the Earth.”

Ó Gogáin added: “We have also been able to radiometrically date the apatite which shows it formed during a time when all the continents on Earth were coming together and colliding to form the supercontinent Pangaea.

“As these continents collided, they formed mountain belts with super-heated subterranean fluids flowing in them. It is these super-heated fluids, which flowed throughout Ireland, that cooked and melted the bones of these fossils causing the alteration we see today.”

And Trinity’s Dr Patrick Wyse Jackson, another co-author, stated: The Jarrow assemblage is of major scientific importance and is a significant element of Ireland’s geoheritage.

“It is great that finally the question of what altered the fossil bones of these animals has been resolved.”

Image: A photograph of the Jarrow amphibian Keraterpeton galvani on top and a CT-image of K. galvani below showing the alteration in the bones. © Dr Aodhan O’Gogain/ Trinity College Dublin.