Seven Fossil-hunting Expeditions in Tanzania, Zambia Yield Surprising Results

Apr 30, 2013 by Sci-News.com

Newly discovered fossils from 10 million years after the end-Permian mass extinction reveal a lineage of animals thought to have led to dinosaurs in what is now Tanzania and Zambia.

10 million years after the mass extinction, members of the archosaur reptiles – such as the 10-foot-long Asilisaurus pictured – had more restricted geographic ranges compared to the communities of four-legged animals that existed before the extinction (Marlene Donnelly / Field Museum of Natural History)

10 million years after the mass extinction, members of the archosaur reptiles – such as the 10-foot-long Asilisaurus pictured – had more restricted geographic ranges compared to the communities of four-legged animals that existed before the extinction (Marlene Donnelly / Field Museum of Natural History)

The new data come from seven expeditions since 2003 in Tanzania and Zambia, along with work combing through existing fossil collections.

Prof Christian Sidor from the University of Washington, who is first author of a paper reporting the results online in the Proceedings of the National Academy of Sciences, explained: “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.”

Prof Christian Sidor and his colleagues headed by Dr Linda Tsuji, also from the University of Washington, created two ‘snapshots’ of four legged-animals about 5 million years before and again about 10 million years after the Earth’s largest mass extinction (about 252 million years ago).

Prior to the extinction event, for example, the pig-sized Dicynodon was a dominant plant-eating species across southern Pangea.

Pangea is the name given to the landmass when all the world’s continents were joined together. Southern Pangea was made up of what is today Africa, South America, Antarctica, Australia and India. After the mass extinction at the end of the Permian, Dicynodon disappeared and other related species were so greatly decreased that newly emerging herbivores could suddenly compete with them.

“Groups that did well before the extinction didn’t necessarily do well afterward. What we call evolutionary incumbency was fundamentally reset,” Prof Sidor said.

The pig-size Dicynodon was part of a large, dominant group of plant eaters found across the southern hemisphere until the mass extinction event weakened their numbers so that newly emerging herbivores could compete (Marlene Donnelly / Field Museum of Natural History)

The pig-size Dicynodon was part of a large, dominant group of plant eaters found across the southern hemisphere until the mass extinction event weakened their numbers so that newly emerging herbivores could compete (Marlene Donnelly / Field Museum of Natural History)

The snapshot 10 million years after the extinction event reveals that archosaurs were in Tanzanian and Zambian basins, but not distributed across all of southern Pangea as had been the pattern for four-legged animals prior to the extinction.

Archosaurs are the group of reptiles that includes crocodiles, dinosaurs, birds and a variety of extinct forms. They are of interest because it is thought they led to animals like Asilisaurus, a dinosaur-like animal, and Nyasasaurus parringtoni, a dog-sized creature with a five-foot tail that scientists in December 2012 announced could be the earliest dinosaur, or else the closest relative found so far.

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

“A new framework for analyzing biogeographic patterns from species distributions, developed by co-author Daril Vilhena, provided a way to discern the complex recovery.”

It revealed that before the extinction event 35 percent of 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. Ten million years after the extinction event, there was clear geographic clustering and just 7 percent of species were found in two or more regions.

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Bibliographic information: Christian A. Sidor et al. Provincialization of terrestrial faunas following the end-Permian mass extinction. PNAS, published online before print April 29, 2013, doi: 10.1073/pnas.1302323110