Washington: Many scientists believe that predecessors to dinosaurs missed the race to fill habitats emptied when nine out of 10 species disappeared during Earth`s largest mass extinction 252 million years ago.
That thinking was based on fossil records from sites in South Africa and southwest Russia.
It turns out, however, that scientists may have been looking in the wrong places.
Newly discovered fossils from 10 million years after the mass extinction reveal a lineage of animals thought to have led to dinosaurs in Tanzania and Zambia in the mid-Triassic period, millions of years before dinosaur relatives were seen in the fossil record elsewhere on Earth.
"The fossil record from the Karoo of South Africa, for example, is a good representation of four-legged land animals across southern Pangea before the extinction," said Christian Sidor, a paleontologist at the University of Washington and lead author of a paper reporting the findings.
Pangea was a landmass in which all the world`s continents were once joined together. Southern Pangea was made up of what is today Africa, South America, Antarctica, Australia and India.
"After the extinction," said Sidor, "animals weren`t as uniformly and widely distributed as before. We had to go looking in some fairly unorthodox places."
The insights come from seven fossil-hunting expeditions in Tanzania, Zambia and Antarctica funded by the National Science Foundation (NSF). Additional work involved combing through existing fossil collections.
The researchers created two "snapshots" of four-legged animals about five million years before, and again about 10 million years after, the extinction 252 million years ago.
Prior to the extinction, for example, the pig-sized Dicynodon-said to resemble a fat lizard with a short tail and turtle`s head-was a dominant plant-eating species across southern Pangea.
After the mass extinction, Dicynodon disappeared. Related species were so greatly decreased in number that newly emerging herbivores could then compete with them.
The snapshot of life 10 million years after the extinction reveals that, among other things, archosaurs roamed in Tanzanian and Zambian basins, but weren`t distributed across southern Pangea as had been the pattern for four-legged animals before the extinction.
Archosaurs, whose living relatives are birds and crocodilians, are of interest to scientists because it`s thought that they led to animals like Asilisaurus, a dinosaur-like animal, and Nyasasaurus parringtoni, a dog-sized creature with a five-foot-long tail that could be the earliest dinosaur.
"Early archosaurs being found mainly in Tanzania is an example of how fragmented animal communities became after the extinction," Sidor said.
A new framework for analyzing biogeographic patterns from species distributions, developed by paper co-author Daril Vilhena of University of Washington, provided a way to discern the complex recovery.
It revealed that before the extinction, 35 percent of four-legged species were found in two or more of the five areas studied.
Some species` ranges stretched 1,600 miles (2,600 kilometers), encompassing the Tanzanian and South African basins.
Ten million years after the extinction, there was clear geographic clustering. Just seven percent of species were found in two or more regions.
The findings appeared in this week`s issue of the journal Proceedings of the National Academy of Sciences.