Mammals That Lived Before The End-Cretaceous Extinction, About 62 Million Years Ago

About 62 million years ago only 4 million years after an Everest-size asteroid hit Earth and ended the age of dinosaurs fuzzy creatures with finger-like digits on their feet emerged as some of the first large mammals to ever roam the planet.

These animals, about the size of a big dog, towered over the shrew to possum-size mammals that existed before the space rock struck, and now, scientists think they know how the critters outgrew their diminutive mammal cousins.

In a new study, published Wednesday (Aug. 31) in the journal Nature, researchers analyzed the fossilized teeth and bones of Pantolambda bathmodon, a stocky, now-extinct mammal that weighed roughly 92 pounds (42 kilograms) when fully grown.

“They probably got a little bit bigger than the analyzed specimens, so that’s pushing 100 pounds (45 kg), which is pretty large when you think about the fact that this is a mammal that lived only four million years after T. rex went extinct,” said lead author Gregory Funston, who was a research fellow at the University of Edinburgh during the study.

“Mammals hadn’t gotten bigger than a badger for the whole Mesozoic (252 million to 66 million years ago), so Pantolambda was two or three times that size,” said Funston, who is now at the Royal Ontario Museum in Toronto, Canada.

But what was the secret to their impressive size?

According to the new study, P. bathmodon likely evolved to birth big, highly developed babies that, similar to the newborns of modern giraffes and hippos, popped out of the womb ready to walk.

To be able to hit the ground running, P. bathmodon babies likely first gestated in their mothers’ wombs for about seven months, nourished by a placenta.

“Today, placentals seem to be unique among mammal groups in having long gestation periods, resulting in larger and more developed young,

but it is not clear when in their evolutionary history placental mammals evolved to have a longer gestation,” said Gemma Louise Benevento, a postdoctoral researcher in macroevolutionary palaeobiology at the Senckenberg Biodiversity and Climate Research Centre (SBiK-F) in Germany, who was not involved in the study.

The new research provides evidence that 62 million years ago, P. bathmodon was capable of carrying months-long pregnancies, and suggests that this reproductive strategy could have helped diverse placental mammals explode in size following the extinction of nonavian dinosaurs, Benevento told Live Science in an email.

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