The largest land mammals that ever lived, Indricotherium (37 to 23 million years ago) and Deinotherium (8.5 to 2.7 million years ago), would have towered over the living African Elephant.
Credit: IMPPS
SYDNEY: The extinction of the dinosaurs paved the way for a rapid increase in the size of the world's mammals, from a maximum size of 10 kg about 200 million years ago to well over 10 tonnes.
Mammals grew as they diversified to fill the vacant niche left by their larger dinosaur predecessors, according to a new study published in the journal Science.
"Mammals originated about 210 Ma years ago and yet didn't diversify greatly in size or ecological role until 65 million years ago. It wasn't until non-avian dinosaurs were wiped out that there was sufficient ecological space for mammal diversification to occur," said evolutionary biologist Felisa Smith from the University of New Mexico, lead author of the study.
Body size estimates from fossils
There has been much speculation over the role of the end-Cretaceous extinction event on mammalian evolution, but this is the first study to explain how it had such a swift impact on body size.
To investigate changes in mammalian body size following the demise of the dinosaurs, an international research team compiled data on the largest mammal species in each taxonomic order over their entire evolutionary history on the four largest continents.
For large animals without an existing body size estimate in the literature, the researchers made new estimates by extrapolating body size from measurements of fossilised bones and teeth.
Size increased over 20 million years
The data from each continent pointed to a startlingly rapid increase in overall mammal size that lasted for about 20 million years before reaching a plateau around 42 million years ago.
Not only does it suggest that mammal size grew in response to the absence of the dinosaurs, but also that some kind of universal constraint eventually kicked in to place an upper limit on maximum body size.
"The study tells us a lot about the generality of the factors that set the maximum size of land mammals," Monash University biologist Alistair Evans, a member of the research team, told Cosmos Online.
"Because the patterns are so similar on the different continents, it seems to indicate that there are similar constraints due to the way animals make a living that mean that they can't get any bigger."
