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Next Stop: The Twilight Zone

Posted on July 25th, 2016

Researchers Uncover Extraordinary Fossil Deposit near Decorah

Something strange was happening near Decorah. Iowa Geological Survey (IGS) geologists had seen mysterious rocks brought up by well drillers in the area—samples so different from those collected just a few miles away. The Jordan aquifer, which commonly provides well water for residents in that part of the state, was strangely absent around Decorah. Locals had also been finding odd coal-like burnable rocks in the area for decades — rocks that turned out not to be coal after all.

What could be going on at Decorah? (Cue the Twilight Zone music.)

In early December 2004, IGS researchers Bob McKay and H. Paul Liu were doing fieldwork near the Upper Iowa River, in that “Twilight Zone” area near Decorah. McKay leaned over the water to pick up an intriguing piece of shale and tumbled into the chilly water. Liu tried to pull his colleague out, but McKay dove in and came out with a shale sample from an outcropping in the riverbed.


Strange ShalePaul near Decorah

Researchers were intrigued by the strange shale sample, which contained some extraordinary fossils. Liu was the first to suggest that Decorah could have been the site of a meteorite impact—basically, a crater.

Scientists have not only identified and confirmed the Decorah Impact Structure, but Liu and his team also found an astonishing collection of fossils there. The well-preserved fossils are numerous and unusual; the site represents one of only a few such “Lagerstätten” (the scientific term for a deposit of extraordinarily well-preserved fossils) from the Middle Ordovician period, roughly 465 million years ago.

National Science Foundation funding in 2010 made it possible for Liu and his colleagues to erect a temporary dam to divert the Upper Iowa River so they could excavate the only outcrop of the shale. They found more than 5,000 fossils—many unknown to scientists until now—in the slabs of shale.


Nightmare Fodder

One of the most dramatic finds was the Pentecopterus decorahensis, a giant sea scorpion. The creature, the earliest and largest such animal of that period, sported a long head shield, a narrow curving body, and huge scary claw-like limbs that could easily trap its unfortunate prey. The Pentecopterus’ modern relatives include spiders, lobsters, and ticks. Imagine a sixfoot tick (cue the scary music again).

Eurypterids-3-0-Vertical-Fullsize Patrick Lynch-Yale UniversityWorking with colleagues at Yale University, Liu has been able to form a picture of how this predator looked and what its life was like in the shallow and likely brackish waters that covered Northeast Iowa millions of years ago.

Besides Pentecopterus, a eurypterid, Liu and his colleagues discovered fossils (some with impressions of soft tissue) of other previously unknown creatures, including conodonts (extinct, eel-like creatures), phyllocarids (shrimp-like organisms), jawless fish, algae (quite rare), spores from some of the earliest known land plants, and other enigmatic fossils that have yet to be identified. It’s the kind of find that scientists who study these ancient creatures live for—“the discovery of the decade in early Paleozoic paleontology,” as one reviewer exclaimed.


A New Window

What made the Decorah crater such a good environment for fossil preservation? During the Middle Ordovician period, Liu says, shallow saltwater covered much of what is now the Midwest. The area around Decorah was near the coast, where the water was less salty. Thus, distinctive organisms evolved to exist in the conditions there. After the meteorite struck the Earth, the water near the crater’s seafloor became very still, brackish, and low in oxygen. Organisms in the water died and fell to the seabed, where they laid undisturbed.

“This opens a new window to tell us what Ordovician life was like,” Liu says. There’s much more work to be done, he adds. Liu and his colleagues can’t wait to get on with it.Twilight Zone_Shianne 3


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