Friday, 30 March 2012

The Death Of The Exploding Ichthyosaur Hypothesis

The early Jurassic Posidonia shales near the village of Holzmaden, Germany, have yielded, over the years, many incredibly well preserved fossils of ichthyosaurs, plesiosaurs, pterosaurs, fish and invertebrates. Some of the creatures are well known both in the fossil record and today, as their descendants have barely changed in hundreds of millions of years. Yet one such puzzle is why ichthyosaur skeletons are often found complete but disjointed, with some of the bones spread over a metre?

One particularly famous specimen was of a female with a still undeveloped embryo outside of the body cavity. A popular theory to explain the scattered nature of the remains was that putrefaction gases were created as the animals decayed. Until the carcass literally burst due to the immense pressure inside. Yet palaeontologists from the Institute of Forensic Medicine in Frankfurt, Germany, may have a different theory. 

The 'exploded' pregnant ichthyosaur discovered
 at the Posidonia shale in Germany 
As the Holzmaden ichthyosaur species and humans have a similar size range, the team, led by Swiss palaeontologist Christian Klug, used rotting human cadavers to measure how much gas built up in a decaying corpse and how much would be needed to cause a pressure-driven explosion. They found that the internal pressures only reached 0.35 on average, nowhere near enough to burst through the body. What is more, the corpses were at normal atmospheric pressure.

They would have required between 5 and 15 bar in order to explode. The ichthyosaur skeletons, covered partially by sediment and 150 metres of water would only have popped at far greater pressures still. The team believe that this result can be extended to all vertebrates. Klug and his colleagues believe that the scattered remains is to do with putrifaction gas. Yet instead of causing the carcasses to explode, it caused them to float.

As they decomposed, parts would dropped off, creating something similar to the aftermath of an explosion. Many skeletons are often found without the head. Palaeontologists used the floating, decomposition hypothesis to account for this, which explains the disjointed nature of ichthyosaur remains. It is important to be able to reconstruct how a fossil formed as it can often be misleading. Dinosaurs were once thought to have been preserved in their death throes, but now this is put down to the post mortem increase of elastic tension in the tendons.