What Was Deinonychus? The Fossil That Changed How We See Dinosaurs
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Before 1969 the standard picture of a dinosaur was a cold-blooded, tail-dragging, slow-witted reptile that had failed and deserved to. Then John Ostrom described a three-metre predator from Montana with a lightly built skeleton, a stiffened tail held out behind for balance, long grasping arms and an enormous sickle claw on the second toe of each foot, and argued that this animal had been fast, agile and warm-blooded, and that birds were its descendants. Almost everything in the modern understanding of the group starts from that description.
The find in Montana
Ostrom, of Yale, collected the material in 1964 from the Cloverly Formation of southern Montana, rocks of the early Cretaceous about 115 million years old, and named the animal in 1969 from a name meaning terrible claw. The first site produced an unusual assemblage: the remains of at least four individuals of the predator alongside one large plant eater, Tenontosaurus, of a size no single one of them could plausibly have killed. That association has been read ever since as evidence of pack hunting, though it is disputed, since a shared carcass attracts scavengers and a trap can accumulate bodies over time. What the bones themselves showed was not ambiguous. The skeleton is light, the limb proportions are those of a runner, the wrists have a hinge allowing the hands to fold, and the tail carries bundles of bony rods that turn it into a rigid counterweight rather than a dragging appendage.
The claw
The second toe of each foot bears a curved claw up to thirteen centimetres long on the bone, longer in life with its horny sheath, and it is held clear of the ground by a joint that hyperextends, so the animal walked on two toes and kept the weapon sharp. What it was for has been reconsidered more than once. Ostrom pictured a slashing kick that disembowelled prey, an image that entered popular culture immediately. Later experiments with a mechanical leg and a pig carcass found the claw poor at cutting and good at puncturing and gripping, and a 2011 study by Denver Fowler and colleagues proposed a different model drawn from modern birds of prey: the predator leaps onto an animal smaller than itself, sinks the claws in to hold it, keeps its balance by flapping its arms, and eats it while it is still alive, which is precisely what a hawk does to a rabbit.
Why it changed the field
Ostrom used the anatomy to argue against the whole prevailing picture, and the resulting shift is known as the dinosaur renaissance:
- •Posture: the skeleton only works horizontally, with the tail out behind, which forced the reconstruction of every theropod away from the upright kangaroo pose used since the nineteenth century
- •Activity: the limb proportions, the stiffened tail and the light build indicate a fast, agile animal rather than a sluggish one
- •Metabolism: Ostrom argued that an animal built like this must have had a high, bird-like metabolic rate, which opened a debate on warm-bloodedness that is still running and has mostly gone his way
- •Bird origins: he revived the nineteenth-century idea, out of favour for fifty years, that birds descend from small theropods, by cataloguing detailed similarities in the wrist, shoulder, hand and pelvis with Archaeopteryx
- •The wider effect was cultural, since Ostrom's student Robert Bakker popularised the new picture in print and illustration, and a generation of research followed
The feathers it must have had
No feathers have been found on this species, and it is now taken as near-certain that it had them. Its close relatives from the Yixian beds of Liaoning in China, preserved in volcanic ash of extraordinary fineness, are covered in them: Sinornithosaurus, Microraptor with flight feathers on all four limbs, and the larger Zhenyuanlong with well-developed wings it could not fly with. Since feathered relatives sit on both sides of it on the family tree, the simplest reading is that the Montana animal had them too, and a 2007 study found quill knobs, the attachment points for wing feathers, on the forearm of the closely related Velociraptor. The arm-flapping in the prey-holding model then acquires a function, since feathered forelimbs would help a predator stay balanced on a struggling animal, and the same motion has been proposed as a step towards powered flight.
Its afterlife on screen
The raptors of Jurassic Park are this animal under another name. Michael Crichton drew directly on Ostrom's work, visiting him to discuss it, and then used the name Velociraptor because he found it more dramatic, while describing an animal of the size and build of the Montana species; the real Velociraptor was about the size of a turkey. The film's creatures are consequently a size match for Deinonychus, scaled up further for effect, and they are famously scaly, because the first feathered dinosaur fossils from China were published in 1996, while the film came out in 1993. The result is that the single most influential image of a dinosaur in popular culture is based on the specimen that overturned the old picture, named after a different animal, and drawn without the feature that its own family tree now demands.
The takeaway
The three-metre predator described by John Ostrom in 1969 from Montana had a light skeleton, a tail stiffened into a counterweight, folding wrists and a hyperextensible sickle claw on each second toe, and it forced a rethink of dinosaurs as fast, agile and probably warm-blooded animals from which birds descend. The claw is now read as a gripping tool for pinning prey rather than a slashing blade, feathered relatives on both sides of it make feathers near-certain, and the raptors of Jurassic Park are this animal wearing another name.