New fossil analysis has highlighted two key developments in the evolution of Earth’s largest land animals. First, a burst of global warming 300 million years ago devastated tropical forests, triggering rapid evolutionary development of reptiles – and paving the way for the eventual rise of dinosaurs. Then, 235 million years later, the extinction of the dinosaurs enabled mammals to emerge from their shadow and grow spectacularly in size and diversity.
The earlier event – the intense warming near the end of the Carboniferous period – is less well known, though a study by scientists at Bristol University and Royal Holloway, University of London shows how important it was for the development of vertebrate life.
For mysterious reasons, the climate suddenly became hotter and dryer 300 million years ago, wiping out almost all the tropical forests on Earth. While amphibian populations suffered severe losses along with the forests, fossil analysis shows that reptiles – better able to adapt to dry habitats – underwent an evolutionary explosion.
The increasing reptilian diversity was a response to habitat fragmentation, the researchers say, as new species arose in the isolated fragments of surviving forest and the new arid habitats. The study appears in the journal Geology.
“It is fascinating that even in the face of devastating ecosystem collapse, animals may continue to diversify through the creation of endemic populations,” says Sarda Sahney of Bristol. “Life may not be so lucky again in the future, should the Amazon rainforest collapse.”
The extinction of the dinosaurs (successors to one reptilian line that arose after the Carboniferous forest collapse) 65 million years ago is one of the best-known episodes in geological history. Everyone has assumed that the dinos’ disappearance opened the evolutionary door to the mammals that survived the asteroid impact but only now, in an international study published in the journal Science, have researchers proved the point.
“The dinosaurs disappear and all of a sudden there is nobody else eating the vegetation,” says Jessica Theodor of the University of Calgary. As well as confirming the rapid growth in mammalian size after the dinosaurs, the study shows that the ecosystem can reset itself quickly. “You lose dinosaurs 65 million years ago, and within 25 million years the system is reset to a new maximum for the animals that are there in terms of body size. That’s a pretty short time-frame, geologically speaking,” she says.
The biggest mammal ever to walk the Earth was Indricotherium transouralicum, a herbivore that weighed about 17 tonnes. It lived in Eurasia 34 million years ago.