Did Dinosaurs Live in Herds? Bone Beds, Trackways and the Social Lives of Giants

A mass death assemblage is a deposit in which many individuals of one species are buried together, and dinosaurs left many. The Centrosaurus beds of Alberta, the Maiasaura beds of Montana with an estimated 10,000 animals, the Edmontosaurus beds of the Dakotas and the Protoceratops and Psittacosaurus groups of Mongolia all contain one species in numbers, with juveniles and adults mixed, which is the signature of a herd caught by a single event, a flood, a drought, a volcanic ashfall, rather than of animals dying one by one over years. The pattern is commonest in the plant-eaters, the horned ceratopsians and the duck-billed hadrosaurs, and it appears more often in the Late Cretaceous than earlier, which may reflect the animals or the rocks.

Footprints record the living. Trackway sites on every continent show multiple dinosaurs of one kind moving in the same direction across the same surface, and some show more. At Davenport Ranch in Texas, 23 sauropod trackways run parallel, with the smaller animals' prints among the larger, which was read as young in the centre of the herd, though a re-examination found the pattern less tidy. In the Yanguoxia site in China and along the Purgatoire River in Colorado, sauropods walked in groups whose members turned together. Theropod tracks are more often solitary, and a set of parallel tracks of the large carnivore Acrocanthosaurus at Glen Rose in Texas, running beside sauropod tracks, has been read as a hunt and as a coincidence of animals using the same shore. The evidence in brief:

• •Single-species bone beds with mixed ages: strong evidence of herds in ceratopsians, hadrosaurs, Psittacosaurus and some sauropods
• •Parallel trackways with consistent spacing: groups moving together, mainly in sauropods and ornithopods
• •Nesting colonies: hadrosaurs and titanosaurs laying in the same ground in numbers, with nests spaced as in a bird colony
• •Juvenile-only groups: clusters of young Sinornithomimus and Psittacosaurus, suggesting that the young of some species stayed together apart from adults
• •Solitary finds: most large theropods and the armoured ankylosaurs are found alone, which may mean solitary lives or may mean preservation

The pack-hunting raptor of the films rests on one site: a Deinonychus bone bed in Montana where several of the sickle-clawed predators lie with a large plant-eater, Tenontosaurus, which was read in the 1960s as a pack that had brought down prey and died in the attempt. The reading has weakened. The isotopes in the teeth of Deinonychus of different ages show that the young ate different food from the adults, which in modern animals is a sign that the young were not fed by a pack; the crocodile and Komodo dragon model, in which several predators converge on a carcass and squabble over it, fits the bone bed as well. Tyrannosaurus bone beds in Alberta and Utah contain several individuals of different ages together, which is suggestive, and some workers argue for family groups; the honest answer is that the evidence for coordinated hunting in any dinosaur is thin, and that gathering at a kill is not the same thing.

Birds are dinosaurs and crocodiles are their closest living cousins, and both bracket the possibilities. Crocodiles gather in numbers, guard nests and carry their young, but do not cooperate in any organised way; birds flock, colony-nest, form family groups and, in a few species, hunt together. A behaviour found in both, such as nest guarding, was probably present in the dinosaurs between them, and the brooding oviraptorosaurs and the crocodile-style buried nests of sauropods fit that. Beyond it, each species is its own case. The herbivores that lived in numbers were probably doing what herbivores do now, finding safety and finding each other; the predators were probably doing what large predators mostly do now, which is hunting alone or in loose company, and the raptors of the films remain a hypothesis with one bone bed behind it.

The fossil record shows where animals died, not how they lived, and a mass death tells of one bad day rather than of daily habit; animals that lived alone can die together at a shrinking waterhole. Trackways record a few minutes, and a group walking together may have been a herd or a queue at a river. The colours, calls, displays and courtship that the crests of the hadrosaurs and the frills of the ceratopsians imply, and that would have been the fabric of any social life, are inferred and not observed. The dinosaurs that lived in herds are those whose bones say so, the ones that lived in packs are those whose bones might, and the rest are silent.

Many plant-eating dinosaurs lived in groups, as single-species bone beds of thousands of horned and duck-billed dinosaurs, parallel sauropod trackways and nesting colonies show, and some young stayed together apart from adults. Evidence that predators hunted in coordinated packs rests on one Deinonychus site and has weakened, with gathering at a carcass fitting the bones as well; birds and crocodiles bracket what is likely, and for most species the record of a single death or a few minutes' walking cannot say more.