2008-10-12 – South Dakota, USA: scoperto nuovo Plesiosauro (plesiosaur)

Fossil find puts river on scientific cutting edge

Discovery near Pickstown ‘special’

 

THOM GABRUKIEWICZ • tgabrukiew@argusleader.com • October 11, 2008

PICKSTOWN – Jim Martin probed the cliffside soil with a blue-handled rock hammer, pounding at times, then using the hammer’s chiseled end to scrape at a fine-grained, dark layer.

“There it is, there we go,” he said. “That’s exactly what we’re looking for.”

And with that, he picked up a handful of dark, gritty material from a 2-inch band just above the layer of hard shale that used to be the bottom of a great inland seaway covering what now is South Dakota.

Martin, executive director of the Museum of Geology at the South Dakota School of Mines in Rapid City, was on a “CSI”-style investigation on the shores of Lake Francis Case, in a bay across from the Fort Randall Dam. He was trying to find out what killed one short-necked plesiosaur, a 25-foot marine reptile that zoomed through the water with the aid of four paddle-like flippers while devouring most anything in its path.

“Think of it as the Loch Ness monster, although that’s simplistic,” Martin said. “The finest fisherman ever to live. It was agile, fast – it would have done well with just its front flippers, moving like a seal or a penguin – but this one had a super-charger on its rear end.”

The fossilized plesiosaur, only the second nearly complete skeleton ever found in the state, was discovered in July at Lake Francis Case by 11-year-old Devon Zimmerman of Sioux City, Iowa. Its discovery extends not only science’s knowledge of the beast but also of events that shook the Great Plains landscape of the late Cretaceous period, sometime between 72 million and 74 million years ago.

“I was out walking on the shore, tossing rocks,” Devon said. “I thought it was something, something really cool.”

His mother, DeeAnn Zimmerman, added: “He found it and said it looked like a spine. His dad (Duane) said it was probably a piece of wood. Really, we just sort of forgot about it. Then we went back and looked at it and said, ‘Oh, no, that really is a fossil.’ And we went to the visitors’ center to report it.”

Cody Wilson, lake manager with the U.S. Corps of Engineers in Pickstown, said the Zimmermans did the right thing. Instead of chipping it out and trying to take some of it away to sell on eBay, the Zimmermans preserved the find for the public, for science, for the rest of history, Wilson said.

When informed of the find, Martin was more than willing to drive from Rapid City to check it out.

“I think I was in the car the next day,” he said.

The fossilized remains curve in the rock, the long-billed skull facing away from shore. Bits of scale and small bones filled the area where the reptile’s fleshy stomach would have been.

“Something killed this critter pretty fast,” Martin said. “And that’s the thing about this particular specimen; we have its last supper in its stomach, the fossil remains of three different kinds of fish.”

The plesiosaur was found in a layer of silicified shale, a hard rock layer that’s like quartz.

“Hit it with a hammer, and it’ll bounce back,” Martin said. “We had to go back and get the jackhammer and saw.”

Volunteers – including Devon Zimmerman – carefully cut their way around the fossil, packed the rock in plaster and trucked it back to Rapid City. There, another volunteer will work with a small pneumatic air device to slowly separate the fossil from the shale.

“It’ll take another five years to get it out of the stone,” Martin said. “But I can wait. Plesiosaurs have been found on every continent, but this one’s special. We’ve found skulls, bones, but to get 60 percent of one fossil, I’d have been happy with 20 percent. And we could be talking about a new critter here.”

That’s what brought Martin back in late September to Lake Francis Case, to try to figure out what caused this plesiosaur to expire so suddenly, and to be preserved almost instantly after death.

The great inland seaway of the Cretaceous stretched from the Gulf of Mexico northwest toward what now is northern Alaska. The sea was as deep as 700 feet near the Gulf, and probably about 200 feet deep over South Dakota, Martin said.

It was rich with a variety of Mesozoic marine reptiles, including plesiosaurs, giant marine turtles and mosasaurs – basically a crocodile with paddles. Mosasaurs, called the tyrannosaurs of the sea, are the most commonly preserved marine reptiles in the state.

“Plesiosaurs – the pliosaurid, or short-necked version – is a different matter,” Martin said. “I wish people would find more so I could have a bigger sampling. But two in a lifetime – I’ll take it.”

The handful of dirt in his hand was compressed ash, and when it was deposited across the inland seaway, it settled into a layer that was more than 2 feet thick.

Above that layer, Martin pointed to layer of sand, and then a yellowish, lumpy layer filled with balls of pumice. Both the ash and pumice suggest a volcanic event.

“I would say this plesiosaur died during the Manson Impact,” Martin said.

The Manson Impact refers to a meteorite that struck what now is Pocahontas County, Iowa. Buried 100 to 300 feet below the town of Manson is the geologic record of the impact, which struck with the force of 10 trillion tons of TNT, said Ray Anderson, with the Iowa Department of Natural Resources.

The stony meteorite probably weighed about 10 billion tons, and its impact instantly ignited everything within 150 miles, toppled trees up to 300 miles away and most certainly killed every animal within 650 miles.

“Basically, this thing came up for air and got a lung full of hot glass shards,” Martin said. “And as it started to sink, choking, it got rolled in a giant wave. A tsunami, if you will, that swept across the seaway all the way from the impact crater in Iowa. Pretty much the most catastrophic day you could have.”

To be sure, Martin took samples of each layer, which he’ll study and date to confirm his hypothesis.

If it turns out to be a new species, there’s the distinct possibility that Martin will name it after Devon. Martin named the first nearly complete plesiosaur fossil found – a new species that dates back to 80 million years ago – after its finder, Paul Neumiller of Bonesteel.

“That would be really awesome,” Devon said.

Reach Thom Gabrukiewicz at 331-2320.

source (with video): http://www.argusleader.com/apps/pbcs.dll/article?AID=/20081011/NEWS/810110326/1001