Preservation of ancient bones is a remarkable phenomenon. So exceptionally these remnants can be saved that its internal structure is still intact, especially in a type of sedimentary fossil called Lagerstätte.
But one Lagerstätte left scientists scratching their heads for decades.
The Jarrow Assemblage in Ireland features chars strewn with bones with internal structures that have changed so much that it is difficult – and in some cases impossible – to determine the details of the fish and amphibian-like tetrapods that left them.
Previously, scientists had attributed the altered structure of the bones to the acidic waters of the swamp or lake where the bones were first deposited—conditions that meant the specimens “fossilized—but only.”
However, this is inconsistent with bones recovered from another coal bog Lagerstätten which had similar ancient environmental settings.
Now scientists from Ireland and the United Kingdom have examined the Jarrow bones in exhaustive detail and determined what caused the bones to change: they were literally cooked by superheated fluids that seeped into the rock as a result of tectonic activity.
“This study demonstrates that the change 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, rather than a direct product of environmental conditions during the initial stages of burial. or premature sweating.
To get to the bottom of the bones’ unique preservation, the researchers used a number of techniques to uncover the chemistry in them. They combined high-resolution imaging with techniques for analyzing the elemental composition and structure of materials, and discovered that what’s in bone isn’t what we normally see in bone.
Instead, the bone had been partially replaced with charcoal and sulfur (a mineral that can form in hydrothermal conditions), and the apatite crystals in it were of a different shape than the apatite crystals normally found in bone.
“The chemistry of the apatite crystals can tell us a lot about how they formed, whether they grew organically in the animal, formed when the animal was buried or whether some other factor influenced its growth,” says geoscientist Gary O’Sullivan of Trinity. Dublin College.
“Apatite is one of the main components of living bones, so it’s not surprising that we find some preserved in these bones. However, when we look at the chemistry of the apatite in the Jarrow bones, we find that this apatite is formed from hot fluids inside 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 of 300 to 350 degrees Celsius (572 to 662 degrees Fahrenheit) and melting the apatite in the bones. As the bones cooled, the apatite crystallized into the tabular shape the scientists observed.
Radiometric dating based on the decay of uranium isotopes dates these crystals to about 302 million years ago, which settles the theory.
“We were also able to radiometrically date the apatite, which indicates that it formed during a time when all the continents on Earth were coming together and colliding to form the supercontinent Pangea,” Gauguin explains.
“When these continents collided, they formed mountain belts with superheated subterranean fluids flowing from them. It was these superheated fluids, that flowed all over Ireland that cooked and melted the bones of these fossils causing the change we see today.”
The team suggests that after the fish and tetrapods died, their bones were transported a short distance and buried quickly, with fully articulated skeletons and scaly skin preserved. As they were buried, the bones became compressed, causing them to fracture. This fracture allowed hydrothermal fluids to infiltrate the bone, changing its chemical composition.
To our knowledge, this is unique to Jarrow, and helps contextualize the history of formation and its bones.
“The Jarrow Group is of great scientific interest and is an important component of Ireland’s geographical heritage,” says paleontologist Patrick Wise Jackson of Trinity College Dublin. “It is wonderful that the question of what changed the fossil bones of these animals has finally been resolved.”
Research published in Paleontology.
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