Slow Death of Dinosaurs Gave Way to the Size Growth of Mammals

Following the extinction of the dinosaurs, the size of mammals exploded. A fresh examination of fossils provides clues on how this progresses.

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Dinosaur-Killing Asteroid

Fewer than 4 million years after an asteroid the size of Everest struck Earth and ushered in the end of the dinosaur era, some 62 million years ago, fuzzy animals with finger-like digits on their feet appeared as some of the first huge mammals to ever wander the world.

Scientists believe they have discovered the mechanism by which these creatures, which were about the size of a large dog, outgrew their small mammal relatives before the impact of the space rock.

Researchers examined the fossilized teeth and bones of Pantolambda bathmodon, a stocky, now-extinct mammal that weighed around 92 pounds (42 kilograms) when fully grown, in a recently published study Wednesday (Aug. 31) in the journal Nature.

According to lead author Gregory Funston, a research fellow at the University of Edinburgh at the time of the study, "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."

Pantolambda was two or three times that size, according to Funston, currently working at the Royal Ontario Museum in Toronto, Canada. "Mammals hadn't gone bigger than a badger for the whole Mesozoic, 252 million to 66 million years ago," Funston said.

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How Did Mammals Grow?

(Photo : Image from The Wonderful Paleo Art of Heinrich Harder in the public domain)

What was the secret to their astonishing size, though? The new research suggests that P. bathmodon evolved to produce large, highly developed offspring who, like modern giraffes and hippos' infants, burst from the womb prepared to walk.

P. bathmodon embryos probably spent the first seven months of their development in their mothers' wombs, where they were fed by the placenta and prepared to hatch.

It is unclear when placental mammals evolved to have a longer gestation in their evolutionary history, according to 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

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

Understanding a Prehistoric Mammal

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According to Benevento, the current study shows that P. bathmodon was capable of carrying pregnancies lasting several months 62 million years ago. This reproductive approach may have enabled a diversity explosion in placental mammal size after the demise of nonavian dinosaurs.

P. bathmodon resembled a variety of contemporary mammals in terms of appearance. Funston told Live Science that it might have appeared bear-like in certain aspects and canine-like in others. The animal's feet, which included fingers and nails, resembled human hands and had a long, thin tail.

In addition, P. bathmodon's absence of a proportionately large head to match its bulky body suggests that its ancestors' body size increased before their brain size did. According to recent research, many placental mammals that appeared after the end-Cretaceous extinction exhibit this "brawn before brains" pattern of evolution.

Key To Understanding

If this odd dog-bear hybrid carried its young for a long time before giving birth to big offspring, other placental mammals might have been doing the same thing. According to him, this could explain why mammals' sizes suddenly increased when the dinosaur-killing asteroid collided with Earth.