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Life in a crowd - plant-eaters
Many plant-eating dinosaurs lived, walked (and stepped on each others toes and tails), and died together. The evidence for this can be seen in communal nesting sites, bone injuries, trackways and bone-beds. What does this mean for our understanding of dinosaur life?
We know from research that living in herds is complex. Animals need to recognise who’s who, fight for social positions and compete for mates. Success at these requires strength or size, display features like horns or antlers, or adopting certain behaviours. If we can detect any of these in dinosaur fossils, it suggests they faced similar concerns – and that we need to take another look at their spikes, horns and fancy headgear.
The advantages of herding can be seen with modern animals. Large numbers reduce the risk of attack by predators and help with raising young. A bone-bed of ceratopsians (Centrosaurus apertus) in Alberta, Canada, that drowned about 75 million years ago, contains the remains of over 300 individuals from a range of age groups - good evidence that they lived in herds.
Living together - meat-eaters
Our picture of the lone predator is changing. Recent fossil discoveries suggest some lived in co-operative packs, raised their young and were probably quite social. This helps shatter the ‘built purely to kill’ stereotype, but creates a whole new set of questions. Were groups family-based, male- or female-dominated, or something else? Did they hunt in packs or compete for social positions and mates? Fossils, although intriguing, cannot provide clear answers. Our only insights come from living predators – which simply tell us that theropods could have interacted in any number of ways.
In what ways do modern predators interact?
- Lions live in male-dominated groups of between 4–40 lions, but the females are the main hunters.
- Komodo dragons are largely solitary, coming together only to breed and eat. However, the dominant male eats first.
- Tasmanian devils tend to live and hunt alone. However, they often gather together in large groups when feeding and will share the spoils of the hunt.
- Wolves live and hunt in family groups consisting of a single breeding pair, non-breeding adults and immature wolves.
- Crocodiles are normally solitary, although hatchlings tend to stay together and some species will continue to live in family groups. Food brings congregations of large numbers and some species have been known to hunt cooperatively.
Evidence for living in groups
- Fossils from Montana of Deinonychus antirrhopus provide some evidence for pack-hunting, as all the dinosaurs apparently died at the same time. However, it is also possible that their deaths were due to fighting over the recent kill, rather than co-operative hunting.
- Monolophosaurus jiangi was an unusual-looking theropod that lived in Asia about 170 million years ago. The bony crest on its head may have been used as a display feature to attract mates or to amplify calls.
- Giganotosaurus belongs to the same group as the North American Allosaurus and has three fingers on each hand, typical of allosaurs. It displaced Tyrannosaurus rex as the largest meat-eating dinosaur when its discovery was announced in 1995. Giganotosaurus possibly lived in family groups, as at least four of these dinosaurs of varying ages were found in a fossil bone-bed in Argentina.
- Cryolophosaurus had a crest over the eyes and facial bumps and horns. These were relatively fragile so they were most likely used in courtship displays. Cryolophosaurus was discovered in Antarctica. Although the Antarctic was warmer when it lived, it was still cool with long winter nights.
- Dilophosaurus had two prominent head-crests that may have been used for courtship displays or to help identify other members of the same species. Three individuals were found in a fossil bone-bed, indicating that Dilophosaurus lived in small groups. As a medium-sized theropod, hunting in groups would allow it to bring down bigger prey.