These mammals have large muscle attachments and generally hefty bones, and among living animals they have the greatest similarity to those ground-dwelling and burrowing species, says Shelley. “So the hypothesis that came out of this was the animals that survived the extinction preferentially survived because they were able to dig to get underground, survive that immediate impact period and the fires, the nuclear winter, and just hunker down for a bit.”
Because the survivors were – there’s no other way to put it – hench, their descendants too inherited their robust body shape. “You can see it for that 10-million-year period during the Paleocene,” says Shelley. “Even if you’re an animal that is living in trees, they’re still really chunky.”
If mammals did indeed take to life underground, either by burrowing themselves or making use of others’ subterranean shelters, Bertrand suspects this might be reflected in their agility too – or a lack of it. “We know that there was a collapse of the forest and so all those animals living in trees didn’t have a habitat anymore,” she says. “And so, one of the hypotheses would be that there were fewer animals able to engage in very agile behavior.”
Bertrand plans to investigate the inner ear bones of mammals of this era to see whether they back up the idea of an underground turn after the asteroid. The inner ear is crucial for balance, so if an animal is adapted to make finely tuned, agile movements this is sometimes reflected in the structure of these delicate bones. However, if they were burly diggers, such agility would not have been necessary. “It could give us more clues,” she says. That said, she points out the drawbacks of relying too heavily on bones to infer how an animal moved around, something that struck her while watching the recent Commonwealth Games.
“I was watching the gymnasts doing crazy things and I was like, that’s funny – we have the same skeleton and I can’t do any of that,” Bertrand laughs. “I thought, well, that’s really interesting because maybe having that capability can help you survive, but from the bones you wouldn’t know.”
The asteroid destroyed most living plants, the first link of many food chains on land. But it didn’t destroy all their seeds – and these may have been vital sustenance for the disaster fauna. Generalist mammals with the ability to turn their palates to seeds as well as insects, and anything else going, did better than those with more particular diets.
“The animals that got through the extinction survived basically just by not being too specialized,” says Shelley. For instance, the Didelphodon (the cat-sized carnivorous marsupial relative) preyed on animals that were few and far between after the extinction. “It specialized too much and lost its niche,” says Shelley. “Whereas if you’re a small animal you can adapt your diet and your lifestyle more rapidly.
“That’s a good way to survive the extinction.”
As well as those that could generalise, there were a few specialisms that would have done well, says Brusatte. In particular, seed-eaters were in luck. “Seeds were a food bank that was just there available to any animal that already had the capacity to eat them,” he explains. “So if you were something like a T. rex you were out of luck, evolution did not bestow you with the ability to eat seeds. But for birds with beaks and some mammals who were specialized seed eaters then whoa, what kind of good turn of fate is that?”
Beyond sustaining the disaster fauna, seeds helped re-establish forests and other vegetation when the nuclear winter faded. “Those seeds survived in the soil and then, when the sunlight came down again those seeds started to grow,” says Brusatte.
As the Paleocene wore on, ecosystems recovered and mammals began to fill the niches left empty by the non-avian dinosaurs. “Mammals started to diversify right away after the dinosaurs went extinct, and they started to become very diverse in every possible way,” says Bertrand.
For one thing, bodies got larger quickly. But for a time, the Edinburgh team have found, the the size of mammals’ brains didn’t keep pace.
“I think that’s very important, because we might think intelligence is what makes us survive and be so able to dominate the planet,” says Bertrand. “But, from the data it’s not large brains that made animals survive after the asteroid.”