Palaeontologists explore ‘survival of the simplest’

When a meteor hit Earth near the end of the Cretaceous, around 66m years ago, the planet lost nearly three-quarters of its species. The dinosaurs were famously wiped out yet plenty of turtles, crocodiles, birds and mammals survived. How some groups managed to limp on just as others disappeared of the face of the planet has long stumped palaeontologists. New work presented by Denver Fowler, a PhD student at Montana State University, to the annual meeting of the Society of Vertebrate Paleontology, held in Los Angeles, suggests that the number of niches that species occupied during their lives played a big role.

Pachycephalosaurus [Credit: Holly Woodward]

In recent years palaeontologists have realised that as dinosaurs matured, they changed, often in dramatic ways. Indeed, just before Mr Fowler spoke, Mark Goodwin, a palaeontologist at the University of California, Berkeley, revealed that Dracorex hogwartsia, an ostensibly new species that was first described in 2006, was in fact a juvenile form of the better-known thick-skulled Pachycephalosaurus. “They didn’t just look different, they also had different teeth and likely different diets at the various stages of their lives,” explains Dr Goodwin.

Aware of these findings, Mr Fowler realised dinosaurs had to have occupied different environments and depended upon different resources as they grew. He also knew that the ancestors of mammals, which shared the planet with dinosaurs in the late Cretaceous were different. Rather than occupy multiple niches as they developed, young mammals were raised on milk until they were fully mature. They subsequently exploited the single niche to which they were well adapted. All this led Mr Fowler to suspect that dependence on multiple niches actually made a species more vulnerable to extinction.

To test his idea Mr Fowler constructed a simple mathematical model in which real species from a late Cretaceous ecosystem, like Tyrannosaurus, Triceratops and numerous mammals, were assigned to between one and five niches, depending on their body size and hypothesised life history. He then let loose a virtual meteor, randomly removing niches from the ecosystem, tallied up how many species lost niches that they occupied and struck those from the model as extinct.

He found that when half the niches in the simulation were removed, 78% of the dinosaurs in the model died out. In contrast, just 40% of the mammals did. Clearly, a 22% survival rate is more than zero, as actually experienced by the dinosaurs. (Indeed, some animals which occupy multiple niches during their lives, like amphibians, survived the Cretaceous deep impact.) But it goes a long way towards that dismal figure. A few other contributing factors, Mr Fowler points out, could easily have done for the survivors. In good times, taking advantage of multiple niches means more resources and less intra-specific competition. In bad times, though, it can be a curse.

Source: The Economist [November 10, 2013]