Why Are So Many Skeletons Arched Backwards? The Pose Fossils Keep Turning Up In
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An enormous number of articulated fossil skeletons are found with the head thrown back, the tail curved up and the limbs drawn in. The pose is so common that it demands an explanation, and the explanations have changed more than once.
What the pose looks like
The characteristic arrangement has the neck hyperextended so the skull is pulled back towards the hips, the tail arched upward, and the limbs flexed and drawn towards the body. It appears in fossils across an enormous range of vertebrates, including dinosaurs, birds, pterosaurs and mammals, and in specimens from many different depositional settings, which means any explanation has to account for its occurrence in water, in wind-blown sand and in volcanic ash alike. It is sufficiently common in well-preserved articulated specimens that it is arguably the default condition rather than an oddity, and it is striking enough to have been noticed and remarked on since the earliest days of the subject.
The explanations offered
Several mechanisms have been proposed and each has evidence and difficulties:
- •Water currents dragging a carcass and pulling the flexible neck backwards, which was the traditional explanation
- •Desiccation of ligaments, which shrink as they dry and pull the vertebral column into the arch
- •Muscular contraction during death from certain causes including asphyxiation or poisoning, which was proposed in 2007
- •The stiffening of the body after death, which fixes it in whatever position it held
- •Buoyancy in water, where a gas-filled body cavity rotates the carcass and the head hangs
- •The elastic ligament that holds the head up in life, which is under tension and contracts once the opposing muscles relax
The current view
The explanation with the most support is the simplest. A strong elastic ligament runs along the top of the neck in many animals, holding the head up without continuous muscular effort, and it is under tension in life balanced by the muscles beneath. When those muscles relax at death the ligament contracts unopposed, pulling the neck back, and the effect is stronger in animals with longer necks and more substantial ligaments. Experiments with bird carcasses demonstrated the pose developing within minutes in air and continuing in water, which addresses the range of settings it appears in. Drying of the ligaments afterwards reinforces the position. That account is mechanically straightforward, requires no particular cause of death and predicts the distribution across animals, which the alternatives do less well.
What a carcass goes through
The pose belongs to a wider set of changes a body undergoes before burial, and knowing the sequence is what allows a fossil to be read. Soft tissue decays, generally beginning with the gut, and the gases produced can bloat and eventually rupture a carcass, which affects whether it floats and how it comes apart. Joints disarticulate in a predictable order, with the head and the extremities separating first and the hips and vertebral column holding longest, so how much of a skeleton remains connected indicates how long it lay before burial. Scavengers remove and scatter elements. Currents sort bones by size and shape. Trampling fragments them. A specimen found articulated and arched therefore records rapid burial before any of that proceeded far, which is why such specimens come from particular kinds of deposit.
Why it was argued over
The dispute is a useful example of how such questions get settled. The proposal that the pose indicated death throes from a specific cause attracted attention because it appeared to make fossils testify about how animals died, which would be a substantial addition to what a skeleton can tell you. Critics responded with the experimental work on carcasses showing the pose developing without any such cause. Others noted that a hypothesis attributing the pose to a particular kind of death cannot explain its occurrence across every kind of animal in every kind of deposit. The exchange proceeded in journals with direct experiment rather than by assertion, and the more parsimonious explanation prevailed without the question being closed absolutely, which is the usual shape of a resolved dispute in the field.
The takeaway
Head thrown back, tail arched and limbs drawn in occurs across vertebrates in every kind of deposit, which any explanation must account for. The favoured account is that an elastic neck ligament under tension in life contracts once the opposing muscles relax, which experiments with carcasses reproduced within minutes. The proposal that it indicates a particular cause of death does not fit the distribution.