The famous feathered dinosaur archaeopteryx seems to have had a penchant for fossilising in painful positions,with its head cranked backward at a severe angle. The contorted posture is so common in dinosaur fossils that it has its own name: opisthotonus,from the Greek tonos,meaning tightening,and opistho,behind.
Since the 1920s,paleontologists have debated how these dinosaurs came to have such grotesque final resting positions. Some theorised that water currents moved the bones into formation,or that the muscle contractions of rigor mortis pulled the head backward. Others thought the animals must have died in pain.
New research proposes a simpler explanation. In a paper published last month in the journal Palaeobiodiversity and Palaeoenvironments,Achim G. Reisdorf of the University of Basel in Switzerland writes that the trouble with the death-throe hypothesis is that carcasses are flexible. To fossilise in the traumatic death position,a carcass would have to be quickly buried in the exact spot where it died,without any transportation.
But that is unlikely,Reisdorf wrote. Many of the dinosaurs found in opisthotonic posture are land animals that fell into sediment at the bottom of bodies of water,and probably had to settle before reaching their final resting place.
Reisdorf thought water might be the key. So he and a colleague,Michael Wuttke,decided to try some kitchen science. They bought fresh chicken necks from a butcher and plunged them into water buckets.
Immediately,the necks bent backward by 90 degrees. After three months and significant rotting,they had twisted further backward–to 140 degrees. These results were verified at Brigham Young University by a paleontologist,Brooks B. Britt,and an undergraduate student,Alicia Cutler.
When you hold a carcass in your hand,its like a limp rag,you can move it anywhere, Britt said. But as soon as we put the first dead chicken in the water,we realised,Holy smokes,this is amazing. The head immediately curved backwards.
To test whether muscle contractions caused the spasms,both groups of researchers removed the skin and muscle from the birds,and got the same result. Only by cutting the ligaments between vertebrae could they prevent the necks from bending backward in water.
The teams independently concluded that the ligaments in chicken necks were like rubber bands–bendable,but contracted by default to hold the birds head upright against gravity. In the dead chicken,those ligaments still want to return to their natural,unstretched position,but the dead weight of the bird fights against it. In water,however,buoyancy and lack of friction allow the ligaments to contract into their natural shape,cranking the neck backward as they go.
The observations have been replicated in other bird species,in saltwater and fresh water,and at various depths. Because birds are living dinosaurs,the researchers think the same phenomenon may have caused opisthotonic posture in nonavian dinosaurs as well.
Kevin Padian,a paleontologist at the University of California,Berkeley,is not entirely convinced. Because similar contortions of the spine and neck occur when modern animals experience trauma,he has argued that a fossils opisthotonic posture could indicate that the animal died of suffocation,starvation,poisoning or other brain injuries.
Reisdorf may be onto something,but it remains to be seen, Padian said. The new observations dont explain the opisthotonic fossils found in dry,non-aqueous sediments. Nor do they explain how mammals–which have a different ligament anatomy–also fossilised with these postures.
I dont think the two theories are mutually exclusive, he continued. The question is,how do you tell whats really going on? Like a 150-million-year-old crime scene,ancient fossils offer few clues to help paleontologists reconstruct the past.
But Reisdorf and Britt say their research reprieves most of these dinosaurs from the agonising deaths that had been proposed for them. A longstanding debate has been solved, Britt said,and it turns out to be pretty doggone simple: Just add water.