Pre-historic mammals sized up by new research

Researchers at the University of Calgary have discovered that after dinosaurs went extinct, mammals quickly got bigger before eventually reaching their maximum size.

“You go from about ten kilos to about ten tonnes in about 25 million years, so that’s pretty rapid,” said researcher and co-author of the paper Jessica Theodor, a U of C biology associate professor.

“We figured out which species had the biggest teeth, for a given group for a given time interval, and made an estimate of their mass based on equations with their living relatives,” said Theodor. “We compiled this data set of the maximum size of mammals for each group through the Cenozoic, so basically from when dinosaurs were around in the Mesozoic into the Cenozoic, and took a look at the pattern.”

Mammals have many different ways of processing food so researchers can identify many mammal species by their teeth. Tooth size in mammals is very strongly correlated with overall body size, which began to increase 65 million years ago.

“Around 40 million years ago, you basically hit the maximum size that mammals ever get and then it stays pretty close to steady throughout the rest of the time interval,” explained Theodor.

Colder temperatures and larger land masses seemed to encourage the growth rate of mammals after the dinosaurs.

“People have looked at overall body size evolution in mammals, but they’ve never looked at what happened to the maximum limit of body size in mammals,” said Theodor. “That’s the difference between what we did and what has already been done.”

“This project started as the brain child of the first author of the paper, Felisa Smith, at the University of New Mexico,” said Theodor. “We’ve met twice a year for the last four or five years in New Mexico to work on projects related to mammal body size.”

There are about 20 authors on the paper and the group is still working on a number of different projects. This was the first to be published.

The research attempts to answer the ecological question as to why land mammals never reached the size of marine mammals or dinosaurs.

The largest mammals are about 10 tonnes, where as the biggest herbivore dinosaurs reached 110 tonnes.

The high metabolism of mammals, which is generally considered higher than dinosaurs, places a constraint on size. In mammals, reproductive ability is scaled to body size, so small mammals like mice can produce more offspring than an elephant.

“It takes a lot longer to grow an elephant,” said Theodor.

Theodor said while there may not be any practical application to the research, it is still worthwhile.

“Some things are just interesting because the world is an interesting place,” said Theodor. “One thing I tell my students is that the past isn’t now with extra dinosaurs. There were no grasses as we know them today in the age of dinosaurs.”

Theodor’s main research focuses on the evolution of hoofed mammals, ungulates, which are very common in the fossil record.

“Today they form a pretty important part of the ecosystem for humans,” said Theodor.

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