Early mammals lived faster and died younger than modern ones

The extinction of the dinosaurs left an ecological hole that was quickly filled by mammals, which rapidly developed larger bodies. Now it seems this evolutionary growth spurt was a result of living life in fast forward

By Riley Black

An artist’s impression of Pantolambda

Florilegius / Alamy

Mammals started living large shortly after the extinction of the dinosaurs 66 million years ago. Before the asteroid impact that sealed the dinosaurs’ fate, the largest mammal was about the size of a domestic cat. Four million years later, a relative blink of an eye in evolutionary terms, mammals such as the herbivorous 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‘s fast life style helped it evolve a bigger body.

Pantolambda didn’t 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. Gregory Funston 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. “We were floored by the results,” says Funston, with the mammal’s life history seemingly perfectly suited to “an empty, unstable ecosystem”.

The critical clues came from fossilised 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.

Overall, the creature’s 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 “fast” life history. “Giving 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,” 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.

“This 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,” says Natasha Vitek at Stony Brook University in New York.

Journal reference: Nature, DOI: 10.1038/s41586-022-05150-w

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