Feature

When the Dinosaurs Died

The southern hemisphere may have played an important role in animal evolution following the dinosaur die-off.

About 65 million years ago, some catastrophic event on Earth -- probably a huge meteor impact in Mexico -- wiped out the dinosaurs. This scenario is widely accepted by scientists, who also generally believe that the great majority of bird and mammal species then living bit the dust at the same time.

But this picture has been radically challenged in a paper by Drs Alan Cooper and David Penny, who use DNA evidence and fossils to argue that a relatively large number of birds and mammals survived the catastrophe -- and that much of the important evolution of these species occurred in the southern hemisphere, which has been largely overlooked by scientists in the north.

Cooper is a molecular evolutionist, formerly of Victoria University's Institute of Molecular Systematics. He collaborated with theoretical biologist Penny, who works at Massey University and is also a research associate at Victoria, in the development of the new approach.

It has been thought that modern bird orders appeared after the Cretaceous-Tertiary (K/T) boundary 65 million years ago, because, apart from northern hemisphere loons, no bird fossils have been found from before that time. Cooper and Penny took a different approach, examining the DNA of a number of bird and mammal species.

Comparison of DNA from two different species of birds reveals differences, with the DNA being more different as the birds are more distantly related. If you know the average rate at which birds' DNA changes, then the amount of similarity or difference between a pair of modern birds can be used to estimate how long ago these two birds diverged from a common ancestor. Correlating the DNA analysis with fossil remains, which can be dated quite accurately by well-established means, helps establish this rate of evolution.

The researchers examined the DNA of 16 orders of birds and also some mammals. While they accept that different species may have evolved at different rates, they believe their statistical analysis over a range of species gives a reliable average rate of evolution.

Using these results they constructed a robust family tree which suggests that modern birds as a whole first appeared in the Early Cretaceous, about 130 million years ago. They argue that at least 21 lineages, including parrots, wrens and ducks, survived the event at the K/T boundary. So why are there so few fossils of these early birds?

"The fossil record for birds is fairly poor prior to the K/T boundary and is primarily concentrated in the northern hemisphere," Cooper says. "Most geologists have been from the northern hemisphere and most of their field areas have been in the north. Perhaps, therefore, the difference between the fossil record -- poor as it is -- and our data is that they have been looking in the wrong place."

There is supporting evidence for this claim in that many bird groups have their earliest fossil records, most basal (fundamental) members, or greatest biodiversity today in the the land fragments formed by the break-up of the ancient southern super-continent Gondwana: Africa, Australia, South America, India, Madagascar and New Zealand. Examples of birds meeting at least one of these criteria include ratites (such as moas, kiwis, emus), passerines (songbirds, including almost all our common garden birds), parrots, ducks, pigeons, penguins, pheasants, seabirds and loons.

"It means that there may be a northern hemisphere bias in the study of evolution for this period," Cooper says. "There are a few sites starting to turn up in South America and Madagascar, but the balance is still very strongly skewed."

As well as the bird species studied by Cooper and Penny, it is known that many other animals also came through the K/T period. These include mammals, crocodiles and frogs.

"The general idea has been that the asteroid that hit Mexico 65 million years ago, or the extensive period of volcanism in India at the same time, somehow caused changes in the global ecology that caused the dinosaurs, pterodactyls and many marine taxa such as ammonites to die off," Cooper says. "But if so many land animals survive -- including the frogs, which are very sensitive to changes in environment -- then the idea of a single ecological event does not fit the pattern of extinction of dinosaurs, but survival of crocodiles, frogs, mammals and birds.

"Perhaps a more reasonable approach might be to say that if the mammals and birds are increasing in diversity during this time, then biological competition with dinosaurs -- for example, egg predation by tree-living primitive mammals with keen senses of smell -- might be a contributing factor. The deathknell of many taxa does appear close to 65 million years ago, so it is probable that the meteorite impact had something to do with it, but it is likely that something else had already put them in a weakened state, ready to be eliminated by something that many others survived," Cooper says.

He would like to see more investigation of southern hemisphere dinosaurs, mammals and birds.

"This could drastically improve our ideas of major evolutionary events such as the K/T mass extinction, and I would like to see more scientists being able to pursue these questions," he says. "There is exciting work to be done in our backyard."

Bernard Carpinter is a journalist at Victoria University.