Researchers have demonstrated that the extinction of dinosaurs 65 million years ago paved the way for mammals to get bigger - about a thousand times bigger than they had been.

The study, released today in the prestigious journal Science, is the first to quantitatively explore the patterns of body size of mammals after the demise of the dinosaurs.

The research, funded by a National Science Foundation Research Coordination Network grant, was led by scientists at the University of New Mexico and brought together an international team of palaeontologists, evolutionary biologists, and macroecologists from universities around the world.

To document what happened to mammals after the extinction of dinosaurs, researchers collected data on the maximum size for major groups of land mammals on each continent, including Perissodactyla, odd-toed ungulates such as horses and rhinos; Proboscidea, which includes elephants, mammoth and mastodon Xenarthra; the anteaters, tree sloths, and armadillos; as well as a number of other extinct groups. The researchers spent three years compiling the large database.

Dr Alistair Evans, a co-author on the paper and an Australian Research Fellow at Monash University, said the database is unique.

"The database allows us for the first time to compare maximum size of mammals on different continents since the extinction of the dinosaurs."

The goal of the research was to revisit key questions about size, specifically in mammals.

"Size affects everything about an animal, from feeding and reproduction to extinction. We really wanted to know how big the many different groups of mammals evolved to be, and how quickly they got there - we were looking for the biggest of the big," said Dr Evans.

Researchers found that mammals grew from a size of about ten kilograms when they were sharing the Earth with dinosaurs to a maximum of 17 tonnes afterwards. Moreover, the pattern was surprisingly consistent across space, time, feeding groups and lineages. The maximum size of mammals began to increase sharply about 65 million years ago, peaking in the Oligocene Epoch (about 34 million years ago) in Eurasia, and again in the Miocene Epoch (about 10 million years ago) in Eurasia and Africa.

The largest mammal that ever walked the Earth - Indricotherium transouralicum, a hornless rhinoceros-like herbivore that weighed approximately 17 tonnes and stood about five and a half metres high at the shoulder - lived in Eurasia almost 34 million years ago.

"The remarkable similarity in the trajectory of mammalian body size evolution on different continents suggests that there were similar ecological roles to be filled by giant mammals across the globe," said Dr Evans.

"The consistency of the pattern strongly implies that biota in all regions were responding to the same ecological constraints."

The results give clues as to what sets the limits on maximum body size on land: the amount of space available to each animal and the climate they live in. The colder the climate, the bigger the mammals seem to get, as larger animals conserve heat better. It also shows that no one group of mammals dominates the largest size class - the absolute largest mammal belongs to different groups over time and space.

"The results were striking. Global temperature and land area set constraints on the upper limit of mammal body size, with larger mammals evolving when the Earth was cooler and land area was greater. We were able to show that biological processes were similar across different groups of mammals, and independent of the history of each continent," said Dr Evans.

Dr Evans' research at Monash University involves investigating diet and body size in mammals, and how these are influenced by their evolutionary history.

Source: Monash University