Sunday 26 June 2016

New Research Highlights Mammal Extinction During The KT Boundary

The classic image of the KT extinction: plucky mammals
emerging from burrows amidst a wasteland of dead dinosaurs
The classic image of the KT extinction event is the dinosaurs dropping like flies in a landscape filled with tree stumps and charred vegetation. The herbivorous species declined quickly while the carnivorous species clung on for a little longer, initially on the dwindling populations of prey and then as ever more desperate scavengers, until their final demise. Yet when the dust settled, the humble yet triumphant mammals emerged from their burrows to claim the new world, scampering through the skulls of their reptilian predecessors. Mammals are the dominant terrestrial vertebrate group today. Most of their diversity evolved soon after the KT extinction, but it was assumed that they had survived the extinction itself with little decline in their species numbers compared to the more unfortunate dinosaurs.

A recent study, however, has shown that they actually suffered extremely heavy losses. Analysis of the mammalian fossil record was conducted in western North America on a window from 2 million years prior to, to 300,000 years after the asteroid collision with Earth which marks the KT boundary. 'The species that are most vulnerable to extinction are the rare ones, and because they are rare, their fossils are less likely to be found,' said Dr Nick Longrich from the University of Bath. The species that tend to survive are more common, so we tend to find them. The fossil record is biased in favour of the species that survived. As bad as things looked before, including more data shows the extinction was more severe than previously believed.'

The red line marks the KT extinction. The rapid
recovery of the modern mammal groups is apparent
The analysis showed that an astonishing 93% of mammal species went extinct across the KT boundary. Intriguingly this is in excess of other groups which successfully traversed the extinction boundary also, such as birds, crocodiles and amphibians. This begs the question of why mammals then managed to become the dominant terrestrial vertebrate group. The answer, however, was also made apparent by the study. While mammals were hit hard, they recovery time was much faster compared to other surviving groups, doubling the number of species prior to the extinction in just 300,000 years.

'It wasn't low extinction rates, but the ability to recover and adapt in the aftermath that led the mammals to take over,' said Longrich. 'You might expect to see the same few survivors all across the continent. But that's not what we found. After this extinction event, there was an explosion of diversity, and it was driven by having different evolutionary experiments going on simultaneously in different locations.. This may have helped drive the recovery. With so many different species evolving in different directions in different parts of the world, evolution was more likely to stumble across new evolutionary paths.'

North America was closer to the epicentre of the impact event than other continents and so may have suffered a slightly higher rate of extinction than say Asia. The study, however, is based on a much larger data set than previous ones and so is certainly a more faithful representation of the change in mammal diversity across the KT boundary. Future studies may well confirm similarly high magnitudes of mammal extinction around the globe along with their phenomenally rapid recovery and subsequent ecological expansion