- They were better than their competitors at surviving the cold.
Dinosaurs bestrode the Mesozoic era like colossi. Literally. The larger sauropods (think Brontosaurus, but five times the mass) were the biggest animals ever to walk on land. Even in Mesozoic times, though, it was not always thus. The Mesozoic comprises three periods: the Triassic, the Jurassic and the Cretaceous. Colossus-like bestriding was restricted to the Jurassic and Cretaceous. So how did it come about?
When they first appeared, in the middle of the Triassic, there was nothing obviously special about dinosaurs. Other groups of largish reptiles jostled with them for space on planet Earth. Their supremacy, like that of the mammals which eventually superseded them, was based on surviving a mass extinction that took out the competition. Paul Olsen of Columbia University, in America, and Sha Jingeng of the Nanjing Institute of Geology and Palaeontology, in China, now think they know what gave them their edge when this happened. It was, they write in Science Advances, an ability to withstand cold.
Most people agree that the event which annihilated dinosaurs (apart from the winged and feathered varieties known, these days, as birds) was a collision between Earth and a large space rock—either an asteroid or a comet. It was this that marked the end of the Cretaceous. The end of the Triassic was more long-winded. It seems to have been caused by a series of massive volcanic eruptions associated with the separation of the Old and New Worlds as the Atlantic Ocean opened and the supercontinent of Pangaea broke apart.
Volcanoes dump carbon dioxide into the atmosphere, which tends to warm the climate. The late Triassic was already a period of high CO2 levels and high temperatures (there are, for example, no signs from the period of polar ice caps). A widespread assumption is therefore that the extinction was caused by yet further warming.
Dr Olsen, Dr Sha and their colleagues disagree. They observe that volcanoes also emit a lot of sulphur dioxide, which goes on to form aerosol particles that reflect sunlight back into space and so cool things down. Modelling suggests pulses of sulphur-dioxide emission might have led to falls in average temperatures around the world of as much as 10°C. At the time of the mass extinction the Arctic would thus have had freezing winters. And the team have now shown that it did.
Their evidence comes from lake deposits in the Junggar basin of Xinjiang. These straddle the boundary between Triassic and Jurassic. Many of the rocks in question are composed of grains of two different sizes, indicating those grains have different origins. Today, this is particularly characteristic of sediments in places like the Sea of Okhotsk, where the smaller grains are carried to their resting places by currents in the water while the larger ones hitchhike on the undersides of ice rafts that have broken away from the shore. The researchers’ hypothesis is that the Junggar-basin lakes got cold enough in winter for surface ice to form along their shorelines.
Years of excavations of Triassic strata around the world show that reptiles of the period were already stratified by latitude. Dinosaurs were abundant and diverse near the poles. Other groups, particularly pseudosuchians (represented today by crocodiles and their kin), preferred more tropical climes. Dr Olsen and Dr Sha suspect this reflects an underlying difference in cold tolerance. The feathers of modern birds, it is now believed, are hypertrophied and specialised versions of much simpler heat-retaining filaments that were as ubiquitous in dinosaurs as hair is in mammals. Possession of these would have kept dinosaurs living near the poles warm. There is no evidence that ancient pseudosuchians had anything equivalent, and modern crocodilians certainly do not.
If sulphurous emissions caused by the Atlantic rifting had the cooling effect Dr Olsen and Dr Sha suspect, it is clear from the strata in the Junggar basin is that dinosaurs shrugged it off, for their footprints, complete with imprints of feathers, are found on the shorelines of the Junggar-basin lakes. The pseudosuchians, however, could easily have been a goner as a result. Then, when the eruptions ceased, the dinosaurs were able to move into the now-vacated lower latitudes and evolve into the behemoths beloved of natural-history museums and film directors alike.