The preservation of ancient bones is a wonderful phenomenon. These remains can be preserved so exceptionally that their internal structure remains intact, especially in a type of sedimentary fossil bed called a Lagerstätte.
But one particular Lagerstätte has puzzled scientists for decades.
The Jarrow Assemblage in Ireland features a bone-filled coal seam with internal structures so altered that it is difficult, and in some cases impossible, to make out the details of the fish and amphibian-like tetrapods that left them.
Scientists had previously attributed the altered structure of the bones to the acidic swamp or lake waters in which the bones were first deposited, conditions that meant the specimens “achieved fossilization, but only just.”
This, however, is not consistent with bones recovered from other Lagerstätten Coal Swamps that had similar paleoenvironmental settings.
Now scientists from Ireland and the UK have examined the Jarrow bones in exhaustive detail and determined the cause of the altered bones: they were literally cooked by superheated fluids that seeped into the rock as a result of the activity tectonics
“This study demonstrates that the alteration observed at Jarrow is in fact largely due to hydrothermal fluids during deep burial,” writes a team led by paleontologist Aodhán Ó Gogáin of Trinity College Dublin in Ireland, “and not is a direct product of environmental conditions during the initial stages of burial or early diagenesis.”
To get to the bottom of the bones’ unique preservation, the researchers used several techniques to reveal their chemistry. They combined high-resolution imaging with techniques to analyze the elemental composition and structure of materials and discovered that what is in the bones is not what we normally see in bones.
CT scans of some fossils showing internal alterations. (Ó Gogáin et al., Paleontology, 2022)
Rather, the bone has been partially replaced by coal and sphalerite (a mineral that can form under hydrothermal conditions), and the apatite crystals found there were a different form of the apatite crystals commonly found to the bones
“The chemistry of the apatite crystals can tell us a lot about how it formed, whether it grew organically in the animal, whether it formed when the animal was being buried, or whether some other factor influenced its growth.” , says Trinity geologist Gary O’Sullivan. Dublin College.
“Apatite is a major component of living bone, so it’s not surprising that we find some preserved in these bones. However, when we look at the chemistry of apatite in Jarrow’s bones, we find that this apatite was formed by heated fluids within the Earth.”
The chemistry of the bones suggests that hydrothermal fluids (superheated water from beneath the Earth’s crust) seeped into the rock, heating it to temperatures between 300 and 350 degrees Celsius (572 and 662 degrees Fahrenheit) and melting the apatite present in bones. As the bones cooled, the apatite recrystallized into the tabular form the scientists observed.
Radiometric dating based on the decay of uranium isotopes dated these crystals to approximately 302 million years ago, confirming the theory.
“We have also been able to radiometrically date the apatite which shows that it was formed during a time when all of Earth’s continents came together and collided to form the supercontinent Pangea,” explains Ó Gogáin.
“As these continents collided, they formed mountain belts with superheated underground fluids flowing from them. It is these superheated fluids, which flowed across Ireland, that cooked and melted the bones of these fossils causing the ‘alteration we see today”.
The team proposes that after the fish and tetrapods died, their bones were transported a short distance and quickly buried, preserving the almost fully articulated skeletons and scaly skins. As they were buried, the bones were compressed, resulting in fractures. This fracture allowed hydrothermal fluids to seep into the bones, altering their chemistry and structure.
As far as we know, this is unique to Jarrow and helps contextualize the history of the formation and its bones.
“The Jarrow assemblage is of significant scientific importance and is an important element of Ireland’s geographical heritage,” says palaeontologist Patrick Wyse Jackson of Trinity College Dublin. “It’s great that the question of what altered the fossil bones of these animals has finally been resolved.”
The research has been published in Paleontology.