Winneshiek Crater Creatures
he Upper Ordovician strata of the Cincinnati region boast one of the oldest known eurypterids (sea scorpions), the great Megalograptus. However, not the oldest. That honor belongs to the Winneshiek Shale, a lens of Middle Ordovician rock found only along the Upper Iowa River near Decorah, Iowa. Dated to the middle Darriwilian Stage, it was deposited roughly 464 – 467 million years ago, about 14 – 17 million years prior to the Cincinnatian.
Based on material found in the Winneshiek, Pentecopterus decorahensis was described in 2015 as the earliest known eurypterid, though the true origin of the group is likely older still, perhaps dating to the Early Ordovician or even Cambrian. Pentecopterus was a megalograptid, a close relative of our very own Megalograptus, and shares many similarities, with an elongated body, spiny appendages, and large size (nearly 1.7 m long).
In addition to giant eurypterids, the Winneshiek Shale preserves a host of other rare and unusual fossils: algae, phyllocarid crustaceans, possible jawless fish, and giant conodont animals. The latter were lamprey-like creatures typically known from their microscopic “teeth”, which are found throughout the marine rocks of the Paleozoic and Triassic. The Winneshiek conodonts are remarkable because they are common (up to 50% of specimens collected), often articulated in life position , and of an astonishing size (over a centimeter, suggesting animals surpassing half a meter in length). Curiously, more typical Ordovician marine fossils are rare or absent from the Winneshiek: no echinoderms, cephalopods, trilobites, articulate brachiopods, bryozoans, or graptolites.
Why such an unusual fauna? The Winneshiek was deposited in the basin of a ~5.6 km (~3.5 mile) wide meteorite impact crater, dubbed the Decorah impact structure. The precise dating of this impact is fuzzy, but likely earlier in the Middle Ordovician. A number of other impact craters of this age are known from around the world and recent research has suggested that the Earth may have had a sparse ring of asteroids during this time.
The Decorah crater may have been partially cut off from the surrounding seas, filled with shallow waters. Some researchers have suggested the water was brackish rather than fully saline. The deposited muds were organic-rich, possibly indicating a low-oxygen, lagoon-like environment, which would explain the lack of other fauna. However, given that it sheltered giant eurypterids and conodonts, the environment must not have been too inhospitable. More work is needed to investigate this exceptional deposit.