How long did dinosaurs rule the earth

The cold and sea level fluctuations wiped out crocodiles

Compared to Sarcosuchus imperator Most of the crocodiles still alive today look like lapdogs: Even the large estuarine crocodiles in Australia do not come close to the twelve meters length and eight tons that gave the primeval crocodile the English nickname SuperCroc. And Sarcosuchus imperator was just one of very many species that existed during the Cretaceous and Jurassic Periods that even survived the mass extinction 66 million years ago when it completely wiped out the dinosaurs. Today, on the other hand, only 23 species survive worldwide, several of which are critically endangered. Philip Mannion from Imperial College in London and his research group wanted to know what changes in the history of the earth ultimately decimated the once successful reptile group.

To do this, the researchers compared the entire fossil record of the crocodile-like and current representatives with climate data from the history of the earth. This resulted in several decisive events over the past millions of years, which ultimately decimated biodiversity. In the northern latitudes, falling temperatures at the beginning of the Ice Age ensured that many species became extinct: Crocodiles are ectothermic, so they cannot regulate their body temperature themselves like mammals, for example - so cold brought them death. The northernmost recent species are the alligators in China and the USA; they survive freezing temperatures even for a short time. In Africa, around ten million years ago, numerous species disappeared when the Sahara expanded at the expense of the extensive wetlands that existed at the time. In contrast, the formation of the South American Andes caused the end of most of the species that were found in the Amazon basin at the time: the mountain range caused the mega-swamps of the region to be drained.

For oceanic crocodile species, however, sea level fluctuations played a decisive role: When the oceans retreated during the ice ages because water was bound in the ice, many species also died out. On the other hand, they benefited from warm periods: Then the seas expanded further and flooded larger regions on the mainland. The newly created habitats and the richer food spectrum encouraged the evolution of new species, says Mannion. It remains unclear why the crocodiles survived the mass extinction on the Cretaceous-Tertiary border. Then they seized the opportunity and conquered numerous free ecological niches that the dinosaurs had previously occupied. Since the Pleistocene, however, crocodiles have been mainly limited to tropical regions, so global warming could offer them new opportunities - if humans would let them and stop chasing them.