THE EXTINCTION BEFORE THE AGE OF DINOSAURS



Edaphosaurus. Exhibit Museum of Natural History, University of Michigan, 1109 Geddes Avenue, Ann Arbor, Michigan, USA.
　

FROM: NATIONAL SCIENCE FOUNDATION

What Happened to Dinosaurs' Predecessors After Earth's Largest Extinction 252 Million Years Ago?
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.

Or did they?

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.

That's still 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," says Christian Sidor, a paleontologist at the University of Washington.

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," says Sidor, "animals weren't as uniformly and widely distributed as before. We had to go looking in some fairly unorthodox places."

Sidor is the lead author of a paper reporting the findings; it appears in this week's issue of the journal Proceedings of the National Academy of Sciences.

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.

"These scientists have identified an outcome of mass extinctions--that species ecologically marginalized before the extinction may be 'freed up' to experience evolutionary bursts then dominate after the extinction," says H. Richard Lane, program director in NSF's Division of Earth Sciences.

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.

"Groups that did well before the extinction didn't necessarily do well afterward," Sidor says.

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 says.

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 technique--a new way to statistically consider how connected or isolated species are from each other--could be useful to other paleontologists and to modern-day biogeographers, Sidor says.

Beginning in the early 2000s, he and his co-authors conducted expeditions to collect fossils from sites in Tanzania that hadn't been visited since the 1960s, and in Zambia where there had been little work since the 1980s.

Two expeditions to Antarctica provided additional finds, as did efforts to look at museum fossils that had not been fully documented or named.

The fossils turned out to hold a treasure trove of information, the scientists say, on life some 250 million years ago.

Other co-authors of the paper are Adam Huttenlocker, Brandon Peecook, Sterling Nesbitt and Linda Tsuji from University of Washington; Kenneth Angielczyk of the Field Museum of Natural History in Chicago; Roger Smith of the Iziko South African Museum in Cape Town; and Sébastien Steyer from the National Museum of Natural History in Paris.

The project was also funded by the National Geographic Society, Evolving Earth Foundation, the Grainger Foundation, the Field Museum/IDP Inc. African Partners Program, and the National Research Council of South Africa.

-NSF-

NSF REPORTS TRIASSIC VOLCANIC ERUPTIONS CAUSED MASS EXTINCTION

Photo:  Volcanic Killer.  Credit:  NSF

FROM: NATIONAL SCIENCE FOUNDATION
Before Dinosaurs' Era, Volcanic Eruptions Triggered Mass Extinction

More than 200 million years ago, a massive extinction decimated 76 percent of marine and terrestrial species, marking the end of the Triassic period and the onset of the Jurassic.

The event cleared the way for dinosaurs to dominate Earth for the next 135 million years, taking over ecological niches formerly occupied by other marine and terrestrial species.

It's not clear what caused the end-Triassic extinction, although most scientists agree on a likely scenario.

Over a relatively short time period, massive volcanic eruptions from a large region known as the Central Atlantic Magmatic Province (CAMP) spewed forth huge amounts of lava and gas, including carbon dioxide, sulfur and methane.

This sudden release of gases into the atmosphere may have created intense global warming, and acidification of the oceans, which ultimately killed off thousands of plant and animal species.

Now, researchers at MIT, Columbia University and other institutions have determined that these eruptions occurred precisely when the extinction began, providing strong evidence that volcanic activity did indeed trigger the end-Triassic extinction.

Results of the research, funded by the National Science Foundation (NSF), are published this week in the journal Science.

"These scientists have come close to confirming something we had only guessed at: that the mass extinction of this ancient time was indeed related to a series of volcanic eruptions," says Lisa Boush, program director in NSF's Division of Earth Sciences.

"The effort is also the result of the EARTHTIME initiative, an NSF-sponsored project that's developing an improved geologic time scale for scientists to interpret Earth's history."

The scientists determined the age of basaltic lavas and other features found along the East Coast of the United States, as well as in Morocco--now-disparate regions that, 200 million years ago, were part of the supercontinent Pangaea.

The rift that ultimately separated these landmasses was also the site of CAMP's volcanic activity.

Today, the geology of both regions includes igneous rocks from the CAMP eruptions as well as sedimentary rocks that accumulated in an enormous lake. The researchers used a combination of techniques to date the rocks and to pinpoint CAMP's beginning and duration.

From its measurements, they reconstructed the region's volcanic activity 201 million years ago, discovering that the eruption of magma--along with carbon dioxide, sulfur and methane--occurred in repeated bursts over a period of 40,000 years, a short span in geologic time.

"This extinction happened at a geological instant in time," says Sam Bowring, a geologist at MIT. "There's no question the extinction occurred at the same time as the first eruption."

In addition to Bowring, the paper's co-authors are Terrence Blackburn and Noah McLean of MIT; Paul Olsen and Dennis Kent of Columbia; John Puffer of Rutgers University; Greg McHone, an independent researcher from New Brunswick, N.J.; E. Troy Rasbury of Stony Brook University; and Mohammed Et-Touhami of the Université Mohammed Premier (Mohammed Premier University) Oujda, Morocco.

Blackburn is the paper's lead author.

More than a coincidence

The end-Triassic extinction is one of five major mass extinctions in the last 540 million years of Earth's history.

For several of these events, scientists have noted that large igneous provinces, which provide evidence of widespread volcanic activity, arose at about the same time.

But, as Bowring points out, "just because they happen to approximately coincide doesn't mean there's cause and effect."

For example, while massive lava flows overlapped with the extinction that wiped out the dinosaurs, scientists have linked that extinction to an asteroid collision.

"If you want to make the case that an eruption caused an extinction, you have to be able to show at the highest possible precision that the eruption and the extinction occurred at exactly the same time," Bowring says.

For the time of the end-Triassic, Bowring says that researchers have dated volcanic activity to right around the time fossils disappear from the geologic record, providing evidence that CAMP may have triggered the extinction.

But these estimates have a margin of error of one to two million years. "A million years is forever when you're trying to make that link," Bowring says.

For example, it's thought that CAMP emitted a total of more than two million cubic kilometers of lava.

If that amount of lava were spewed over a period of one to two million years, it wouldn't have the same effect as if it were emitted over tens of thousands of years.

"The timescale over which the eruption occurred has a big effect," Bowring says.

Tilting toward extinction

To determine how long the volcanic eruptions lasted, the group combined two dating techniques: astrochronology and geochronology.

The former is a technique that links sedimentary layers in rocks to changes in the tilt of the Earth.

For decades, scientists have observed that the Earth's orientation changes in regular cycles as a result of gravitational forces exerted by neighboring planets.

The Earth's axis tilts at regular cycles, returning to its original tilt every 26,000 years. Such orbital variations change the amount of solar radiation reaching the Earth's surface, which in turn has an effect on the planet's climate, known as Milankovich cycles.

This cyclical change in climate can be seen in the types of sediments deposited in the Earth's crust.

Scientists can determine a rock's age by first identifying cyclical variations in deposition of sediments in quiet bodies of water, such as deep oceans or large lakes.

A cycle of sediment corresponds with a cycle of the Earth's tilt, established as a known period of years.

By seeing where a rock lies in those sedimentary layers, scientists can get a good idea of how old it is. To obtain precise estimates, researchers have developed mathematical models to determine the Earth's tilt over millions of years.

Bowring says the technique is good for directly dating rocks up to 35 million years old, but beyond that, it's unclear how reliable the technique is.

He and colleagues used astrochronology to estimate the age of the sedimentary rocks, then tested those estimates against high-precision dates from 200-million-year-old rocks in North America and Morocco.

The geologists broke apart rock samples to isolate tiny crystals known as zircons, which they analyzed to determine the ratio of uranium to lead.

The technique enabled the team to date the rocks to within approximately 30,000 years--a precise measurement in geologic terms.

Taken together, the geochronology and astrochronology techniques gave the geologists precise estimates for the onset of volcanism 200 million years ago.

The techniques revealed three bursts of magmatic activity over 40,000 years--a short period of time during which massive amounts of carbon dioxide and other gas emissions may have drastically altered Earth's climate.

While the evidence is the strongest thus far for linking volcanic activity with the end-Triassic extinction, Bowring says that more work can be done.

"The CAMP province extends from Nova Scotia all the way to Brazil and West Africa," he says. "I'm dying to know whether those are exactly the same age."

-NSF-