View of field area in Upper Missouri River Breaks National Monument, central Montana, where igloos were discovered. Igloos come from the light gray and dark gray banded rocks at the base of the exposures. Raymond Rogers, Macalester College

Scanning electron microscope view of parasite-induced igloos on Cretaceous clam shells. Jeff Thole, Macalester College

St. Paul, Minn. – The Cretaceous Judith River Formation of Montana is renowned for its remarkable fossil record, which includes organisms as diverse as crayfish, crocodiles, mammals, and dinosaurs. New research led by Macalester College professor Raymond Rogers published in Geology adds an invisible, but important player to this 76-million-year-old ecosystem. Curious igloo-shaped traces preserved on ancient clam shells reveal that tiny parasitic flatworms were infecting their hosts when dinosaurs ruled North America.

Small, soft bodied organisms like parasites are notoriously difficult to preserve in the fossil record. This means they are absent from most reconstructions of fossil food webs. But the fossils collected from the Upper Missouri River Breaks National Monument in Montana provided a rare opportunity to track parasite infestation. Macalester students joined Rogers and combed through thousands of fragments of ancient clams to collect a sample of shells marred by the minute igloo-shaped traces.

“These fossil igloos are nearly identical to traces found in living clams infected by trematodes (flatworms),” said Rogers. “The clams build the igloo to isolate the invading parasite.”

Clams served as just one host in the complex flatworm lifecycle.  Exactly who the definitive host was remains a mystery.  It could have been a shorebird that fed upon the infected clams, which is the case today, or it could have been another vertebrate host, possibly even a dinosaur that dined on clams.

Though the soft bodies of the parasites have long since decayed away, the tiny resilient traces tell an important ecologic story.  First, the igloos record the oldest known occurrence of this mode of flatworm parasitism on clams, extending it back 20 million years to the Cretaceous.  Second, they push the parasite-host association into freshwater environments; before it was only known in marine clams. Finally, these parasites have complex life cycles that connect different animal hosts within ecosystems, allowing researchers to refine connections in fossil food webs.

Rogers said the discoveries highlight the fact “that no fossil is too small or too obscure to reveal important, unexpected details about ancient ecosystems.”

By tracking the prevalence of igloos, the team can next investigate how rates of flatworm infection change across past events in earth history, including sea-level fluctuations and mass extinctions. But these findings also have implications for current climate change.

“Trematodes are important to the health of the environment, and ultimately, to us,” said Judith Skog, program director in the National Science Foundation’s Division of Earth Sciences. “Because these parasites have more than one host, they can affect an entire ecosystem. Understanding these interactions over a long time scale allows better predictability of environmental change.”

This research was made possible with support from the National Science Foundation, the Bureau of Land Management, and the David B. Jones Foundation.

“Pushing the record of trematode parasitism of bivalves upstream and back to the Cretaceous”

Authors:  Raymond Rogers1,2, Kristina Curry Rogers1,2, Brian C. Bagley2, James J. Goodin1, J. H. Hartman3, Jeffrey T. Thole1, and M. Zatón4.

1Geology Department, Macalester College, St. Paul, Minnesota 55105 U.S.A.
2Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota 55455-1333, U.S.A.
3Harold Hamm School of Geology and Geological Engineering, University of North Dakota, Grand Forks, North Dakota 58202
4University of Silesia in Katowice, Faculty of Earth Sciences, Będzińska 60, PL-41-200 Sosnowiec, Poland

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March 1 2018

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