Exploding Ichthyosaurs?

Sometimes truth can be grosser then fiction. Take the fossil record of the Posidonienschiefer Formation of Germany for example. Its best known for its beautiful preservation of many early Jurassic aged vertebrates, particularly a large number of Ichthyosaurs. The particularly striking thing about some of the formation’s Ichthyosaurs though is that many are apparently females, seemingly near-perfectly intact but surrounded by the smaller bodies of Ichthyosaur embryos scattered beyond their body cavities. Without evidence of predation these Ichthyosaurs seemed to cry out for a unique explanation. Today one of the prevailing theories is that these Ichthyosaurs actually exploded as they decomposed, scattering the embryos (and presumably some Ichthyosaur guts) onto the sediments around the mother. So is this really what happened?

1200px-Fischsaurier_(Stenopterygius_quadricissus)_-_Unterer_Jura_-_Holzmaden,_Baden-Württemberg
One of many Ichthyosaur fossils from the Posidonienschiefer Formation in Germany. Source: Wikimedia Commons

The Ichthyosaur fossils at Posidonienschiefer have been known for over a century. In 1905 an early study suggested the seemingly reasonable idea that ocean currents had acted on the bodies after they decomposed, strong enough to disturb the fragile embryos but not powerful enough to move the adults. This explanation seems sensible but there is a problem with it and this problem rests on the type of sediment found at the site. Posidonienschiefer is what palaeontologists call a Lagerstätte, meaning an area of truly exceptional fossil preservation. There are many Lagerstätte in the world and all have profoundly influenced palaeontology in some way by providing exquisite fossils for study. One reason that many of these sites demonstrate this unusual level of preservation is that the sediments there are very fine grained and most were once highly anoxic environments – places with low oxygen where scavengers were rare. These conditions tend to preserve fossils more effectively and retain more of the details of the original animals, partially by limiting decay. At Posidonienschiefer the sediments are so fine grained that the conditions they formed under have been likened to a ‘soup’. Such sediments would be very easily disturbed by even the gentlest currents and therefore evidence of such movement would be expected to be preserved in the rocks. There is no sign of such currents and so in the 1970s a new idea emerged about what scattered the embryos. Instead of currents these papers focused on the decomposition of the Ichthyosaurs themselves for their explanation.

It isn’t hard to find videos of whales exploding. The internet is rife with them and many people probably remember last year’s furore about what to do about a large whale carcass that washed up in Newfoundland and threatened to blow. In the past there have been disastrous attempts to negate the dangers by deliberately detonating these rotting carcasses and so these whales are a very highly profile example of what can happen as a corpse decomposes. The explosions are caused by the build up of gases, released internally by fermentation or purification of the organs. Usually these gases are harmlessly released, either through natural orifices or holes made by predators but sometimes the gases build up and up and up without being able to escape. Eventually there will be a catastrophic release of pressure. Some researchers speculated that perhaps a similar process was at work in Jurassic Germany. The idea goes that when these Ichthyosaurs died they floated to the surface and began to decompose. Eventually the pressure built up and their carcasses exploded, scattering the embryos but leaving no obvious damage on the skeleton of the adult. The bodies then sank to the bottom and were preserved. It seems possible and for a long time was accepted as the most likely explanation for what had happened to the Posidonienschiefer Ichthyosaurs.

Then in 2012 the plot thickened. An experiment was carried out by researchers from several German and Swiss universities to test whether such an explanation was physically possible. In order to do this they measured the abdominal gas pressure in decomposing human carcasses as a proxy for Ichthyosaurs and also took another look at the fossil record itself to see if they could find any new information one way or the other. They concluded that the idea of exploding Icthyosaurs was totally impossible. Based on their calculations the Ichthyosaurs would have sunk at which point the external pressure of the water would have prevented any explosion. They did note that at certain water depths and temperatures Icthyosaurs might have floated but concluded they would probably have degassed naturally long before exploding. Although the paper didn’t go as far as suggesting their own explanation of what did happen to these animals dismissing the exploding corpse scenario left palaeontologists with only the ocean current idea to fall back on.

For those of you disappointed at this lack of gory explosions never fear, all is not lost. In 2013 another paper was published in response to the 2012 study which suggested a third alternative. They agree that the explosion hypothesis is not particularly well supported by the evidence, especially if the Ichthyosaurs really did sink straight to the bottom when they died. But they also argued that there is little reason to suspect ocean currents either. Instead they suggest that the bodies may have imploded. In their scenario as the body began to decompose it did indeed build up some gas pressure, albeit not enough to explode. At some point, either through a natural weakness in the body or by the act of some scavenger during one of the brief moments when the sea bed was less anoxic, the body became thinner and vulnerable. Instead of being destroyed by internal pressure though it was external water pressure to which the corpse would succumb. This pressure would have caused the body wall to partially collapse and allowed the embryos to come spilling out.

Of course this explanation itself attracted criticism, notably from the authors of the 2012 study. The truth seems to be that there was something unique about the conditions at this place and at this time which caused these strange fossils but at the moment exactly what those conditions were is still unclear. I wouldn’t expect either of these papers to be the final word on the subject.

References:

  1. Residorf, A.G., et al. 2012. Float, explode or sink: postmortem fate of lung-breathing marine vertebrates. Palaeobiodiversity and Palaeoenvironments. 92(1). pp.67-81. DOI: 10.1007/s12549-011-0067-z
  2. Loon, van A.J., 2013. Ichthyosaur embryos outside the mother’s body: not due to carcass explosion but to carcass implosion. Palaeobiodiversity and Palaeoenvironments. 93(112). DOI: 10.1007/s12549-012-0112-6

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