Fossil Record: Dinosaur Family Line Traced to Limited Area in Africa

Researchers have announced fossils dating back to about 10 million years after the mass extinction of the dinosaurs suggest a line of animals considered the biological predecessors of the dinosaurs lived in what are now the African countries of Tanzania and Zambia many millions of years before and after the great die-off.

The research is included in a paper appearing this week in the Proceedings of the National Academy of Sciences.

The new insights come from seven fossil-hunting expeditions held since the early 2000s in Tanzania, Zambia and Antarctica and funded through the National Geographic Society and National Science Foundation.

Also reviewing pieces from existing fossil collections, study researchers created two profiles of four legged-animals existing in the region about 5 million years before and again about 10 million years after the extinction event, during which nine out of the 10 dominant dino species disappeared about 252 million years ago.

"The fossil record from the Karoo of South Africa remains a good representation of four-legged land animals across southern Pangea before the extinction event. But after the event animals weren't as uniformly and widely distributed as before. We had to go looking in some fairly unorthodox places," said Christian Sidor, a University of Washington professor of biology and the lead author for the study paper.

Prior to the extinction event, for example, the pig-sized Dicynodon - believed to have resembled a fat lizard with a short tail and a head resembling that of a turtle - was a dominant plant-eating species across the southern areas of Pangea, the super continent composed of what are all of the world's continents today.

Southern Pangea included the landmasses that evolved into Africa, South America, Antarctica, Australia and India.

But, "groups that did well before the extinction didn't necessarily do well afterward," said Sidor, who also is the curator of vertebrate paleontology at the UW's Burke Museaum of Natural History and Culture. "What we call evolutionary incumbency was fundamentally reset."

So, while the Dicynodon disappeared after the mass extinction and other related species were greatly diminished in numbers, other animals, such as the archosaurs, apparently flourished.

Findings from 10 million years after the extinction showed archosaurs, which scientists say led to Nyasasaurus parringtoni, a dog-sized creature with a five-foot tail announced last year as the earliest known dinosaur, were in the Tanzanian and Zambian basins exclusively, not distributed across all of southern Pangea, as other four-legged animals had been before the extinction event.

Archosaurs are the group of reptiles that includes crocodiles, dinosaurs, birds and a variety of extinct forms.

"Early archosaurs being found mainly in Tanzania is an example of how fragmented communities became after the extinction event," Sidor said.

A new framework for analyzing species distributions developed by a UW graduate student revealed that prior to the mass extinction, 35 percent of all four-legged species were found in two or more of the five areas studied, with some species having ranges that stretched 1,600 miles (2,600 kilometers), encompassing the Tanzanian and South African basins.

But, 10 million years after the extinction, there was clear geographic clustering and just 7 percent of species were found in two or more regions, the research authors said.