Dinosaurs living together

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?

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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 . 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.

Around 66 million years ago, a 14km-wide asteroid smashed into our planet. An estimated 15 billion tonnes of soot spread through the atmosphere, creating one long night that lasted several years and made photosynthesis all but impossible. It heralded an endless winter that saw average temperatures fall by as much as 28oC. These are the conditions that the few wretched creatures that survived the initial impact had to endure &#; not to mention the earthquakes, tsunamis, wildfires and volcanic eruptions that swiftly followed in its wake.

Around three-quarters of all species went extinct and no animal bigger than a Labrador dog survived. But according to researchers at the University of Texas, things could have been very different. They reported findings that had the asteroid struck Earth just a few minutes earlier, it would have hit the deep ocean rather than the shallow sea of the Yucatan Peninsula in present-day Mexico.

Had that been the case, then the damage would have been more localised. Some of the dinosaurs far from the impact site might have survived, and the world would be a different place today. In our own history, only the feathered theropod dinosaurs (a group of bipedal dinosaurs) we know as birds made it through the calamity, but how would things have turned out if their larger relatives had joined them? Would dinosaurs still be alive today and could mammals such as humans have evolved? What would our world look like if we shared it with the descendants of animals like T. rex and Triceratops?

&#;I&#;m sure a fairly nice diversity of non-avian dinosaurs would still be here,&#; says Dr Stephen Brusatte, a palaeontologist at the University of Edinburgh. &#;If there was no sudden, catastrophic shock of the asteroid, I really don&#;t see anything that&#;s happened since &#; whether it was the spread of grasslands; changing ocean currents; the separation of Antarctica from South America, which caused a cold snap; or the more recent Ice Ages &#; that would have knocked off the dinosaurs.&#;

Over the years many have tried to imagine what kind of creatures dinosaurs might have evolved into had they survived. The most famous attempt is a book called The New Dinosaurs: An Alternative Evolution, by Scottish geologist and author Dougal Dixon. For this magnificent work of speculative zoology, Dixon conjured up creatures such as the &#;cutlasstooth&#; &#; a pack-hunting, sabre-toothed predator from South America; the &#;cribrum&#; &#; a flamingo-like, filter-feeding theropod from Australia; and the &#;gourmand&#; &#; a relative of T. rex that lost its front limbs entirely and developed a distensible jaw to allow it to rapidly swallow prey whole, much like a snake.

Perhaps this last idea isn&#;t entirely wide of the mark. Dr Tom Holtz, an expert on theropod dinosaurs at the University of Maryland in the US, says that both tyrannosaurs and abelisaurs, the two types of big meat-eater present in the Late Cretaceous, are notable for their tiny forelimbs. &#;Given that arms were non-critical for hunting, it&#;s possible that a Cenozoic [current geological era] tyrannosaur could have been armless,&#; says Holtz.

The beginning of the Cenozoic Era (which spans the period from 66 million years ago until the present day) might essentially have been an ecological extension of the Late Cretaceous. Various creatures such as titanosaur sauropods (huge, long-necked dinosaurs like Argentinosaurus), hadrosaurs (duck-billed dinosaurs like Edmontosaurus), ceratopsians (horned, beaked dinosaurs like Triceratops), and predators such as the tyrannosaurs would still have remained common.

But as we head further from the Cretaceous towards the present day, there would likely have been significant changes, says Dr Andy Farke at the Raymond M. Alf Museum of Paleontology in Claremont, California. &#;If dinosaurs were still around today they&#;d be pretty different to what we think of at the end of the age of the dinosaurs &#; things like T. rex and Triceratops,&#; he argues. &#;You might still recognise them as a dinosaur, but who knows what kind of body shapes and body plans might have come up in the past 66 million years.&#;

Many of the mammals with which we&#;re familiar might not have had the opportunity to evolve.

&#;You can&#;t underestimate the importance of that extinction 66 million years ago in really hitting the reset button for mammals and clearing the playing field,&#; adds Farke.

Tree huggers

Already in the Cretaceous there were numerous fluffy, feathered theropods scampering in the trees. Assuming that flowering plants continued to spread and thrive as they did in our history, then could primate-like dinosaurs have specialised to take advantage of the fruit they produced? Prof Matthew Bonnan, a palaeobiologist at Stockton University in New Jersey, argues that the evolution of primates' large, forward-facing eyes with colour vision was primarily to forage for fruit.

&#;Is there a connection between being frugivorous [fruit-eating] and having a larger brain? We don&#;t know, but one could imagine arboreal dinosaurs that formed a co-evolutionary relationship with flowering plants by eating their fruits and dispersing the seeds,&#; he says. &#;Whether these fruit-eating dinosaurs would have evolved complex social groups like primates is pure speculation.&#;

Other ecological spaces little explored by dinosaurs were aquatic environments. &#;In mammals we&#;ve seen a return to the sea, in several different iterations,&#; says Farke. &#;We&#;ve had things like whales and manatees that have gone back into the oceans, and things like otters that spend a lot of time in the water. It&#;s cool to think about what dinosaurs could have looked like if they&#;d gone in a cetacean direction.&#;

But if their giant marine reptile relatives &#; the mosasaurs and plesiosaurs &#; had survived, then dinosaurs might have found it hard to get a foothold.

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There could also have been other consequences of dinosaurs and their reptilian relatives, such as the flying pterosaurs, not petering out at the end of the Cretaceous. Although birds co-existed with dinosaurs for a long time in the Cretaceous, their diversity was low compared to today. &#;Modern bird groups underwent an explosive radiation after the mass extinction, maybe because pterosaurs went extinct and opened up new niches,&#; says

Dr Victoria Arbour, a palaeontologist at the Royal Ontario Museum in Toronto. &#;Without the mass extinction, maybe birds wouldn&#;t be as diverse and successful as they are today, and maybe we wouldn&#;t have things like songbirds, parrots, hawks, or hummingbirds at all.&#;

Most experts seem to agree that the largest land mammals such as elephants, mammoths, giant relatives of rhinos and sloths, and perhaps even horses and giraffes, probably couldn&#;t have evolved if large dinosaurs had remained to occupy the niches they came to fill.

But perhaps smaller mammals such as rodents, bats and primates would have been just as successful. If that had been the case, then some of those primates could have climbed down from the trees onto the grasslands and savannahs that eventually replaced the thick forests of the Cretaceous, and evolved into hominids, as our ancestors did.

&#;If we speculate that humans had evolved alongside dinosaurs, then they probably would have been able to co-exist,&#; says Farke. &#;Humans already evolved in ecosystems that had large land animals and predators. We probably would have done okay.&#;

&#;Unarmed, solitary humans are still easy targets for large predators like bears and lions,&#; agrees Arbour. &#;But overall humans are pretty good at surviving alongside large, dangerous animals.&#;

Cenozoic extinction

Dinosaurs might not have been so lucky though, as humans seem to have a special skill for killing off large animals. Perhaps the biggest dinosaurs would have gone the way of the mammoth and the dodo. &#;Humans are really good at extinguishing megafauna &#; through hunting, climate change or habitat destruction,&#; Arbour says. &#;Dinosaurs in the 21st Century, just like modern animals, would probably have reduced populations and face the threat of extinction.&#;

Big dinosaurs would perhaps only persist in protected reserves, such as national parks and wildlife refuges &#; modern-day equivalents of Jurassic Park. Smaller dinosaurs that infringed on crops or livestock would probably be hunted as &#;nuisance&#; animals, as wolves and dingoes are today, adds Arbour. &#;It would be really hard for large sauropods to survive alongside us. They&#;re so big and would require so much food, that I doubt we could set aside enough wild spaces for them to thrive.&#;

City dwellers

The dinosaurs that might do particularly well in the modern era are those that could learn to live and thrive alongside people. In our world today, the vast majority of animal biomass is made up of the species that we farm or have domesticated, or those that live around our cities and developments &#; and so it would also have been in a reality where humans and dinosaurs co-existed. There might have been dinosaur equivalents of seagulls, pigeons, rats, raccoons and foxes &#; all very well adapted to take advantage of the resources available in urban environments.

&#;Small, scrappy dinosaurs might have been able to eke out a living on the margins of housing developments,&#; suggests Farke. You can just imagine little beaked herbivorous dinosaurs nibbling at the roses and hydrangeas in your garden.

&#;Animals that do well in urban environments today tend to be those that are good at eating whatever we&#;re throwing away, and making use of the structures we build,&#; agrees Arbour. &#;Small omnivorous or predatory theropods would perhaps have been lurking around garbage cans.&#;

Obviously, we might have domesticated dinosaurs to exploit for meat and eggs or agricultural labour, and we would very likely have taken them into our homes as pets &#; the feathery or scaly equivalents of dogs and cats.

Perhaps, though, the idea that humans could have evolved in a world filled with dinosaurs is simply too far-fetched. &#;I have no doubt that we would not be here,&#; says Brusatte. &#;The asteroid was one of those dominoes that set in motion a chain of events that led to us. Without the dinosaurs disappearing, mammals would not have had the same opportunity.&#;

He argues that mammals had already existed with dinosaurs for 160 million years or more when the asteroid struck. But they were mostly &#;marginal, shadowy little creatures&#; and &#; had the asteroid not caused a mass extinction &#; would likely remain that way today.

As Brusatte points out: &#;What&#;s another 66 million years when it had already been like that for 160 million years already.&#;

This is an extract from issue 318 of BBC Focus magazine.

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