Dinosaur ancestors out of ancient Africa

Tuesday, May 7, 2013

Winners are grinners: After the ancient extinction, some animals, like Asilisaurus, had more restricted ranges (Source: Marlene Donnelly/Field Museum of Natural History)

Jennifer Viegas
Discovery News

Ten million years after the world's largest mass extinction, a lineage of animals thought to have led to dinosaurs took hold in what is now Tanzania and Zambia, according to new research.

The study, published in Proceedings of the National Academy of Sciences , reveals how the end-Permian mass extinction 252.3 million years ago permitted a significant reorganisation of terrestrial animals living in the southern part of the supercontinent Pangea.

Out of this chaos emerged the dinosaur predecessors, which likely ushered in the Dinosaur Age.
There were losers, such as fat lizard-looking Dicynodon, which sported a short tail and a turtle's head. It completely bit the dust after the huge extinction event, which led to the disappearance of 90 per cent of all life on the planet.

On the winners' side were silesaurs, which were plant-eaters very closely related to dinosaurs.

"In Tanzania, the main silesaur that we find is called Asilisaurus kongwe," says study co-author Kenneth Angielczyk.

"Asilisaurus was about the size of a medium dog, like a golden retriever, and they tended to have long thin limbs."

Yet another fossil find was Nyasasaurus parringtoni, a Labrador retriever-sized animal with a one-and-a-half-metre-long tail.

Nyasasaurus is either the oldest known dinosaur or the closest known relative of dinosaurs, but we can't completely rule out either option because the material is rather fragmentary," says Angielczyk, who is Associate Curator of Paleomammalogy at the Field Museum of Natural History.

Angielczyk and his colleagues unearthed these creatures over the course of seven fossil-hunting expeditions in Tanzania, Zambia and Antarctica.

The researchers created two "snapshots" of four-legged animals about five million years before the mass extinction event, and 10 million years after it.

Great Dying

The cause of the great die-off remains a mystery, with intense volcanic activity, a meteorite strike and extreme global warming all possible candidates.

Before the extinction event, 35 per cent of four-legged species were found in the majority of the areas studied, with some species having ranges that stretched 2600 kilometres.

Ten million years after the die-off, just seven per cent of species were found in the same number of areas.

In addition to the already mentioned animals, the survivors included other archosaurs, which is a group that includes modern crocodiles, modern birds and -- back in the day -- dinosaurs.

Cynodonts also lived through the onslaught. This group later evolved into mammals, so these were our very distant ancestors. Certain additional reptiles and amphibians survived too.

While the fossil discoveries would seem to suggest that the motherland for dinosaurs was Africa, the researchers point out that landmasses were configured very differently then.

What is now Africa was part of Gondwanaland, the southern portion of Pangea.

True dinosaurs first show up about 230 million years ago from what is now Argentina says co-author and geologist Sterling Nesbitt of the Field Museum of Natural History.

"We think that dinosaurs first evolved in Gondwanaland-including Africa, South America, India, Madagascar, Australia, and Antarctica," he says.

Bruce Rubidge, a dinosaur specialist at the University of Witwatersrand, says the expeditions by this team of researchers to little-explored Permian and Triassic-aged depositional basins in Africa and Antarctica, which form part of the supercontinent Gondwana, "has greatly enhanced our understanding of the distribution of land-living vertebrates that lived more than 200 million years ago."

"The results of this research provide documentation of distribution, both in time and space, of important land living vertebrates soon after the greatest extinction event of all times and indicate how the post extinction recovery fauna evolved and became distributed around the world," says Rubidge.
The study could serve as a warning to us, since humans are now contributing to yet another mass extinction event. The Triassic animals were only just starting to recover eight million years after the huge die-off.

"Eight million years is a long time to wait," says Angielczyk. "If we value the communities of organisms around us, conserving and protecting them is the main way we can ensure they will persist."

Megafauna victims of 'climate not humans'

Fierce debate: many of the 90 species of giant animals that roamed ancient Australia and New Guinea were wiped out before the arrival of Aboriginal people, say researchers. (Source: Peter Schouten/UNSW)

Stephen Pincock

 There is no evidence to support the idea that humans were primarily responsible for wiping out the extraordinary gigantic animals that once roamed Australia, says a group of Australian and US scientists.

The claim is the latest salvo in a longstanding and ferocious scientific disagreement over what killed animals such as 100 kilogram marsupial lions, rhinoceros-sized marsupial Diprotodons and goannas as big as large saltwater crocodiles.

On one side of the debate, including the authors of the latest paper, are researchers who suggest megafauna gradually died out as climate change altered the landscape of Sahul, the single Pleistocene-era landmass that combined Australia and New Guinea.

On the other side, are those who argue that humans hunted the animals out of existence, who point to evidence suggesting the megafauna extinction occurred within 10,000 years of people arriving in Australia for the first time.

The latest paper, published in the Proceedings of the National Academy of Sciences , reviews published evidence on the topic and argues that the finger of blame points more definitively toward climate change.

The paper's authors say there is only firm evidence for about 8 to 14 megafauna species still existing when Aboriginal people arrived. Another 50 species were completely absent from the fossil record of the past 130,000 years.

On the other hand, they say, as more fine-grained environmental data has been gathered, it has become apparent that the climate of Sahul was increasingly arid and erratic climate during the past 450,000 years.

"There is very strong evidence that climate had a major role in the extinction process, and we have no evidence that humans had a major impact," says study author Judith Field, an archaeologist from the University of NSW.

"You have difficulty making the argument that humans had a major role in the extinction of the megafauna in Australia unless you can put humans and megafauna in the same place at the same time.

"Only one site on continental Australia does that," Field says referring to Cuddie Springs in south eastern Australia.

"What you can say is that there is now strong evidence pointing to climate as being the major driver."
Lead author, Associate Professor Stephen Wroe, also from the University of NSW, says the interpretation that humans drove the extinction rests on assumptions that have increasingly been shown to be incorrect.

"There has never been any direct evidence of humans preying on extinct megafauna in Sahul, or even of a tool-kit that was appropriate for big-game hunting," Wroe says.

"It is now increasingly clear that the disappearance of the megafauna of Sahul took place over tens, if not hundreds, of millennia under the influence of inexorable, albeit erratic, climatic deterioration," he says.

Scientific disagreement

But Christopher Johnson, professor of wildlife conservation from the University of Tasmania argues that the new paper adds little to the debate.

"It contains no new evidence. It is a commentary paper, and what it mainly does is re-state an interpretation of megafaunal extinction in Australia that these same authors have presented in many previous similar publications."

Johnson, together with climate change scientist Professor Barry Brook from the University of Adelaide, dispute the authors' interpretation of the evidence on climate change.

Johnson also says they misinterpret the fossil evidence of the extinction of the megafauna.

"It is well known that even when a whole group of species goes extinct simultaneously, the fossil record will give the appearance of a staggered series of extinctions because fossils don't actually date the extinction of species, and the latest fossil for a species will often have been formed long before the species actually went extinct," he says.

"This is a well-known problem in interpretation of the fossil record, and there are well established methods available to deal with it."

Responding to this criticism, Field says the most recent fossil evidence for some megafauna species dates from more than 100,000 years before humans arrived on Sahul.

"We actually discuss this in the paper.

"The question you must ask is how many thousands of years do you allow after the last appearance date of a species before accepting an animal is now extinct?"