The bones of an ancient tetrapod that lived 300 million years ago were found in a coal seam by experts, and the bones were said to have been cooked at 350 degrees.

Ancient bone preservation is an amazing occurrence. These remains can be preserved so exceptionally well that their internal structure is unaltered, especially in a specific kind of sedimentary fossil bed known as a Lagerstätte.

Lagerstätte

However, one Lagerstätte in particular has baffled scientists for decades.

A coal seam in Ireland called the Jarrow Assemblage is full of bones whose internal structures have been so altered that it is difficult, if not impossible, to reconstruct the details of the tetrapods that left them.

The bones' altered structure was previously attributed by scientists to the acidic swamp or lake where they were originally deposited, which meant that the specimens barely managed to become fossilized.

However, this is at odds with bones found in other coal swamp Lagerstätten with comparable palaeoenvironmental conditions.

The Jarrow bones were literally cooked by the superheated fluids that percolated into the rock as a consequence of tectonic activity, according to researchers from Ireland and the UK who examined the bones in great detail.

According to a team led by Aodhán ó Gogáin, a paleontologist from Trinity College Dublin in Ireland, the research reported in Geological Society Publications shows that the alteration seen at Jarrow is not a direct result of environmental conditions during early diagenesis or early stages of burial, but rather is primarily a result of hydrothermal fluids during deep burial.

Unique Preservation and Hydrothermal Conditions

The researchers used a variety of techniques to unveil the chemistry in the bones to understand the unique preservation of the bones. They used methods to examine the elemental structure and composition of materials along with high-resolution imaging to find that the bones' contents differ from what is typically seen in bones.

Instead, the bone was partially replaced by coal and sphalerite, a mineral that can form under hydrothermal conditions, and the apatite crystals were not the typical skeletal apatite crystals.

Gary O'Sullivan, a geologist from Trinity College Dublin, explained that The chemistry of the apatite crystals can tell us a lot about how they formed, including whether they developed naturally inside the animal, during burial, or as a result of other circumstances.

It is not surprising that apatite, a key component of living bone, has been preserved in these bones. However, we discover that the apatite in the Jarrow bone chemistry was produced by heated fluids deep inside the Earth.

350 Degrees Celsius from 300 Million Years Ago

According to the chemistry of the bones, superheated water from beneath the Earth's crust may have seeped into the rock and heated it to temperatures between 300 and 350 degrees Celsius, melting the apatite that makes up the bones. The apatite crystallized again into the tabular form that the scientists saw as the bones cooled.

The theory was confirmed by radiometric dating, which used the decay of uranium isotopes to date these crystals to about 302 million years ago.

According to Gogáin, researchers were also able to radiometrically date the apatite, which demonstrates that it formed when the Earth's continents were colliding and merging to form the supercontinent Pangaea.

These continents formed mountain belts as they collided, with superheated fluids flowing out of them. These extremely hot fluids, which permeated all of Ireland, cooked and melted all the bones of these fossils, resulting in the change visible today.

Mystery Solved.

The team suggests that after fish and tetrapods perished, their bones were quickly moved a short distance and buried, preserving nearly fully articulated skeletons and scaley skins. The bones were fractured because of the compression that occurred during burial. Hydrothermal fluids were able to penetrate the fractured bones and change their structure and chemistry.

This is distinctive to Jarrow as far as we can tell and puts the formation's and its bones' past into historical perspective.

Patrick Wyse Jackson, a paleontologist from Trinity College Dublin, said that the Jarrow assemblage is a significant component of Ireland's geoheritage and major scientific significance. Jackson says that it is wonderful that the mystery of what changed the fossilized bones of these animals has finally been answered, as Science Alert reports.

The research by ó Gogáin, Sullivan, Jackson, and their colleagues has recently been published in Palaeontology.