Mammals聽started聽living large聽shortly聽after聽the聽extinction of the dinosaurs聽66 million years ago. Before聽the asteroid impact that sealed the dinosaurs鈥� fate, the largest mammal was about the size of a聽domestic聽cat. Four million years聽later,聽a relative blink of an eye in evolutionary terms,聽mammals such as the herbivorous聽Pantolambda were about the size of sheep. Precisely what spurred this growth spurt had been unclear, but now palaeontologists have found new clues locked in fossil teeth that suggest Pantolambda鈥榮 fast life style helped it evolve a bigger body.
Pantolambda didn鈥檛 look quite like any mammal alive today. The creature belonged to an extinct group called pantodonts that thrived 62 million years ago during the early part of the Paleocene.聽 at the University of Edinburgh, UK, and his colleagues have found that Pantolambda young spent seven months in the womb and were weaned quickly, before undergoing rapid growth until they perished at around 10 years of age.聽鈥淲e were floored by the results,鈥� says Funston, with the mammal鈥檚 life history seemingly perfectly suited to 鈥渁n empty, unstable ecosystem鈥�.
The critical clues came from聽fossilised聽Pantolambda teeth. Mammal teeth record specific moments of their life in their chemistry, such as when they were born or stopped suckling from their mothers. By looking at these clues, the researchers were able to outline the life of this previously enigmatic mammal.
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Overall, the creature鈥檚 development was most similar to that of modern placental mammals that carry their offspring for a long time and wean them quickly. But Pantolambda also seemed to perish earlier than expected for a creature of its size, having what the researchers have deemed a 鈥渇ast鈥� life history. 鈥淕iving birth to well-developed young gives them the best shot at survival, and maturing fast means as many young as possible get a chance to reproduce,鈥� says Funston.
By comparing fossil clues with modern mammals, the team found that Pantolambda mothers gave birth to single offspring that already had their eyes open and had full sets of teeth. The young grew fast and were quickly able to fend for themselves, allowing them to proliferate and fill the landscape relatively rapidly. Being born at an advanced stage of development and growing fast allowed Pantolamba to proliferate through ancient ecosystems, with competition for resources driving mammals to evolve larger sizes and more complex behaviours.
鈥淭his discovery shows that only a few million years after the mass extinction, there was already diversity in placental life history strategies approaching what we see today,鈥� says at Stony Brook University in New York.
Nature
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