A Palaeontological Puzzle Solved

The fossils of terrestrial vertebrates are often found in very life-like positions. Some have their legs stretched out as if they were running, while others are curled up in foetal postures. However one of the most iconic postures, seen in skeletons from everything from velociraptors to Archaeopteryx, is of the creature lying on its side. The spinal column is arched backwards and the legs are splayed. The overall effect is that of the creature still preserved in its death throes.

Yet we had no idea why they took on this final position before preservation. The shape, dubbed the opisthotonic posture or 'bicycle position' on account of the arrangement of the legs, arises due to poisoning, vitamin deficiency or damage to the cerebellum. This is a region of the brain which controls the 'anti-gravity' muscles, so-called because they keep the head, neck, spine and tail upright. These muscles contract, pulling everything backwards into the curved position.

The Compsognathus longipes specimen from the Solnhofen Plattenkalk

This theory seemed plausible, despite the fact that it did not explain the different positions of the limbs across various fossils. Yet traumas are quite specific and therefore cannot account for many of the fossils which display opisthotonic posture. The real problem arises from the fact that, for the death throes of the animal to be preserved, it would have to have been completely buried after death in a very short time.

Two scientists from universities in Switzerland and Germany believe that they have found a more plausible theory. Their study began by looking at a fossil of the chicken-sized dinosaur Compsognathus longipes from the 155 million year old Solnhofen Plattenkalk in Bavaria.

As the Solnhofen area was once a tidal lagoon, the Compsognathus carcass would have been jostled about, destroying the opisthotonic posture before burial and fossilisation. Instead something else must have been at work after death to create the position across a vast range of specimens. They suggested that the biomechanics of the carcass were ultimately responsible. They experimented by placing freshly killed chickens into a vat of water.

Subsequently, the necks arched backward by up to 90 degrees. Further decay over a period of months accentuated the angle. Dissection revealed that this was due to contractions within a vertebra-connecting ligament called the Ligamentum elasticum, vital for all long-necked, long-tailed dinosaurs.

'The preloaded ligament helped them saving energy in their terrestrial mode of life,'said palaeontologist Michael Wuttke from the Section of Earth History in the General Department for the Conservation of Cultural History Rhineland Palatinate in Mainz, Germany. 'Following their death, at which they were immersed in water, the stored energy along the vertebra was strong enough to arch back the spine.' This increased as muscles and soft tissue decayed.

The opisthotonic posture is especially prevalent in marine fossils or fossils of terrestrial vertebrates which fell into bodies of water, as in the case of the Compsognathus skeleton, as the buoyancy of the water greatly reduces the effects of gravity.