What might have happened if that asteroid missed? Huge dinosaurs would have likely persisted. In fact, as Earth’s temperature has decreased since they roamed, it’s possible that – like other animals in colder conditions – dinosaurs would have got bigger.

It’s important to remember here that dinosaurs were already massive. For instance, Tyrannosaurus rex was the largest pure predator that ever lived on land: it was the size of a double-decker bus, about 13 metres long and weighing eight tonnes. Some long-necked sauropods like Argentinosaurus and Patagotitan were more than 30 metres (100 feet) long and weighed in excess of 60 tonnes – heftier than a Boeing 737. They were the biggest animals to ever walk Earth.

Palaeontologists have long wondered why dinosaurs got so large in the first place. Early scientists suspected it might be related to the physical environment: maybe gravity was weaker during the Age of Dinosaurs, or the atmosphere was swamped with oxygen.

Yet we now know this was not the case: dinosaurs were big for their own intrinsic, biological reasons. The largest sauropods, for example, had a perfect combination of features that allowed them to supersize. Their long necks gave them access to a never-ending buffet of plants other animals couldn’t reach. They had efficient lungs that took in more oxygen than mammal lungs. And those lungs were connected to air sacs that cooled their bodies.

Although they died when the Chicxulub asteroid hit 66 million years ago, it’s likely these sauropods were already close to their biological limit. Any larger and it’s possible they wouldn’t have been able to move, breathe, or hold up their bodies.

But while this is speculation, one thing is clear: if huge dinosaurs survived today, big mammals like elephants or rhinos wouldn’t have the space to co-exist. Mammals would still be in the shadows.

Asked by: Joshua Irwandi, Leeds

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Authors

Dr Stephen Brusatte

Steve is a professor and palaeontologist at the University of Edinburgh and the author of the book The Rise And Reign Of The Mammals (£20, Picador), a 325-million-year odyssey of mammalian evolution and the people who study mammal fossils.

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