Mammals waited until dinosaurs went extinct 65 million years ago to make their move.

A new study in the Nov. 26 issue of the journal Science has revealed that in the 20 million years after the extinction of dinosaurs, mammals grew 1,000 times larger.

Alison Boyer, a postdoctoral student in the Yale Department of Ecology and Evolutionary Biology, 20 paleontologists, evolutionary biologists, and macroecologists from around the globe worked on the project which was funded by the National Science Foundation. The researchers explained this rapid evolution as part of the diversification that occurred as mammals filled the ecological niches left behind by the extinct dinosaurs. While other evolutionary biologists questioned the originality of the study’s conclusions, Boyer said she is excited about the opportunities for future research that the study presents.

“I think our study illustrates how much evolution is dependent not only on the environment, but on other organisms,” lead author Felisa Smith said, who is also a professor at the University of New Mexico.

The scientists collected data on the size of various groups of large mammals living on each continent between 65 and 40 million years ago. They found that land mammals grew to astonishingly large sizes, with the largest species reaching a weight of 34,000 pounds, twice the size of the average African Elephant today.

Although researchers said there was a clear association between the extinction of the dinosaurs and the evolution in size of the land mammals, Brown University professor Christine Janis, who did not participate in the study, said it is difficult to determine whether the results reflect a causation between the two factors, or simply a correlation.

The research team did consider the possible influence of abiotic factors by comparing physiographic histories of each continent.

“When we examined the evolution of mammals on the different continents, we found they all displayed the same pattern,” Smith said. “This suggested universal drivers or constraints that were largely independent of the geologic history of each continent.”

The study found that the size of the land mammals reached a plateau around 40 million years ago, an event the scientists largely attribute to the biologically constraining abiotic factors global temperature and terrestrial land mass.

Smith said that both temperature and land area appeared to constrain the upper limit of the mammals’ size, which she said was particularly evident when fluctuations in size mirrored fluctuations in these abiotic factors.

Boyer said she expects the study to spark a renewed debate over “what sets the limits” of the size of land mammals and affects their evolution.

“I hope that this study will also encourage others to gather and collate large data sets on fossil vertebrates, and contribute them to repositories such as the Paleobiology Database,” she said. “Studies such as ours would be impossible without the many researchers who published their data.”

But other evolutionary biologists expressed concern about the study’s originality.

“The study doesn’t tell us much new about evolution, but it is nice to see [the evolution of giant mammals] well documented,” Brown University professor Christine Janis said. “What was known simply from hand waving was put into a provocative context.”

Despite this criticism, Walter Jetz, head of the Jetz Lab for Global Biodiversity, Ecology and Conservation of Terrestrial Vertebrates, praised the study’s interdisciplinary nature.

“The study offers a neat integration of biogeographic and paleontological knowledge to help understand the evolution of a key animal attribute,” Jetz said.

According to the study, the largest know mammal is the Indricotherium transouralicum, a 34,000- pound rhinoceros-like mammal that lived in Eurasia approximately 35 million years ago.