Thomas R. O'Donnell

A huge Iowa impact’s role in a big, scary fossil discovery

In University research on September 14, 2015 at 9:17 pm
A conception of Pentecopterus, drawn by Yale University's Patrick Lynch.

A conception of Pentecopterus, drawn by Yale University’s Patrick Lynch.

Decorah, generally known for its Norwegian-American museum, Luther College and spectacular Iowa River scenery, became famous for another reason in the last couple of weeks.

The Winneshiek County seat community has lent its name to an ancient resident: an intimidating, potentially man-sized sea creature armed with terrifying tentacle-like appendages.

Scientists at Yale University and the Iowa Geological Survey (IGS), based at the University of Iowa Oakdale campus, published the research in a recent issue of BMC Evolutionary Biology. The creature is so strange and menacing – and the artist’s rendition so eye-catching – that websites and newspapers around the world picked up the story.

Lost in the hoopla, however, is the deeper (literally) discovery behind the now-famous fossil.

The scientists – Yale’s James Lamsdell and Derek Briggs, the IGS’s Huaiboa “Paul” Liu and Robert McKay, and Brian Witzke of the U of I’s Department of Earth and Environmental Sciences – named the creature Pentecopterus decorahensis in honor of where it was found. It’s the oldest eurypterid, or sea scorpion, ever found, going nearly half a billion years back to the Ordovician Period of the Paleozoic Era, when a sea covered what is now Iowa.

Eurypterids are called sea scorpions because of their resemblance to the stinging tropical arachnids, but are more closely related to horseshoe crabs, those throwbacks to ancient times. They’re ancestors to several creatures we see today, including lobsters. (“Red Eurypterid,” however, isn’t too appealing as a restaurant name.)

You would need a vat of drawn butter to eat a Pentecopterus. The fossils indicate they may have been as much as 1.7 meters (5.5 feet) long, with shield-like heads and spine-covered appendages adapted to grabbing prey. Dave Marshall, a writer for the Biomed Central blog network, gives a great description and an account of what makes this discovery so exciting for paleontologists.

A fossilized Pentecopterus leg. The image comes from the BMC Evolutionary Biology paper.

A fossilized Pentecopterus leg. The image comes from the BMC Evolutionary Biology paper.

In essence, Liu said, Pentecopterus and other fossils found near Decorah are leading scientists to think differently about those long-ago times.

The usual ancient marine fauna, like that found in other Iowa fossil deposits, aren’t in the area around Decorah. The standard trilobites, brachiopods and other creatures aren’t present. “That means the environment is different,” Liu said. Instead, the researchers find eurypterids, jawless fish, conodonts and other unusual ancient species. “That mix of fauna is so much different from the regular Ordovician strata,” he added. The fossils found near Decorah have “provided us a new picture to tell us what Ordovician life looks like.”

Why that mix of fossils is so different is due to that deeper discovery I mentioned.

For decades, geologists knew there was something different about the rock strata in part of Winneshiek County. The fossils were different and well drillers were hitting unusual rocks.

McKay, Liu and their IGS colleagues, with the late Decorah geologist Jean Young, gradually determined that the strange geology extended through a circular region roughly four miles in diameter. It was buried crater left behind by a huge asteroid that struck during the Ordovician period.

Liu and his colleagues published a report on what’s come to be called the Decorah Impact Structure in 2006 2009. The U.S. Geological Survey and a Smithsonian Institution researcher (as I noted in 2013) later verified the crater’s existence, with a near-exact match to the Iowa researchers’ calculated dimensions.

The geologists think that seawater rushed into the newly formed crater, pulling with it the blasted material and sea creatures. The low-oxygen atmosphere in the water found deep in the crater helped preserve the creatures’ remains, leaving a trove of unusual – and unusually well-preserved – fossils. Some even include signs of soft tissue, which usually isn’t preserved in rock. This IGS article gives the background.

The researchers called this the Winneshiek Lagerstätte, a term describing an area of especially well-preserved fossils.

Paul Liu (right) with colleagues Carrie Davis and Robert McKay of the Iowa Geological Survey in 2010, at the excavation pit in the diverted Iowa River. Image from the IGS website.

Paul Liu (right) with Carrie Davis and Robert McKay of the Iowa Geological Survey in 2010, at the excavation in the diverted Iowa River. Image from the IGS website.

The find led Liu and the IGS to seek a National Science Foundation grant and in 2010 they built a temporary dam to push aside a section of the Iowa River. It wasn’t the easiest place to work, but the river bed held the only outcropping of underground rock they could find. The team pumped water out of the area and dug a 20- by 30-foot hole to a depth of 12 feet, pulling out around 5,000 pounds of fossil-laden shale.

Over the last few years, U of I students have split the shale and searched for fossils. Besides seeking large-scale artifacts, they also examined the rocks under microscopes to find the remains of tiny creatures.

Some of the larger fossils were highly unusual. Liu initially thought they were plants, so he asked his wife Xiuying, a paleobotanist now working as a research assistant at University Hospitals and Clinics, to have a look.

“I don’t think it’s a plant,” she told her husband. “I didn’t see any plant structure.”

At about the same time, Liu’s Yale collaborator, Derek Briggs, came to the U of I to present a seminar. Liu showed Briggs some pictures of the unusual fossils.

H. Paul Liu with a partial fossil of a eurypterid. Image from the Iowa Geological Survey website.

H. Paul Liu with a partial fossil of a eurypterid. Image from the Iowa Geological Survey website.

“He was really interested,” Liu said, and wanted to see the actual fossils at the IGS rock library in Oakdale. But Briggs’ flight home was to depart soon. “Well, it’s probably the last time you’ll get to see it,” Liu told him.

“I have a half hour. That has to be enough,” Briggs replied. After examining the fossils, he concluded it was a eurypterid – a big one. The IGS researchers later found even better Pentecopterus fossils – around 150 in all, including a complete appendage – that helped confirm Briggs’s conclusion.

The fossils are located in the Oakdale rock library, Liu said, but eventually they’ll be in the paleontology repository on the U of I campus.

While Pentecopterus decorahensis is getting all the attention right now, Liu believes the entire Winneshiek fauna are the real stars. They’re drastically different from other collections and extraordinarily well preserved. The Decorah Impact Structure’s unusual nature also adds to the find’s reputation, he said.

“This research included two major discoveries,” Liu said. “One is the fossils. Another thing is the impact.” Both are exciting and engage scientists and the public, he added.

Beyond the huge sea scorpion, however, lie other strange fossils found in the 2010 dig. Liu and his colleagues will start to detail them in four papers awaiting publication. Each has its own surprises.

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  1. This is so cool! I hadn’t read abut this anywhere else. Wow!

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