EVIDENCE FOR TREMATODE PARASITISM IN LATE CRETACEOUS (CAMPANIAN) FRESHWATER BIVALVES FROM THE JUDITH RIVER FORMATION, MONTANA

Detecting parasitism in the fossil record is challenging, partly because parasites are often minute and generally lack biomineralized hard parts. In the case of bivalve mollusks, which today are known to host a diverse array of parasites, evidence of ancient parasite infestation usually comes in the form of shell malformations that reflect reaction of the host to parasitism. For example, distinctive igloo-shaped shell malformations related to gymnophallid trematode infestation have been documented in living marine bivalves (Neoleptonidae, Nuculanidae) from the Beagle Channel and Falkland Islands and from late Holocene marine bivalves (Cyamiidae) from Tierra del Fuego. These structures reflect the bivalve’s attempt to isolate and eliminate the parasite, with the trematode maintaining an opening (hence the igloo appearance). Here we report identical igloo-shaped structures on freshwater bivalves from the Upper Cretaceous (Campanian) Judith River Formation (JRF) of Montana. Four vertebrate microfossil bonebeds in the Coal Ridge Member of the JRF were bulk-sampled and sieved in an effort to recover small vertebrate fossils, including the remains of dinosaurs, crocodiles, turtles, amphibians, fish, and mammals. Abundant molluscan shell debris was recovered along with bones and teeth, and represent “Unio,” the “fingernail clam” Sphaerium, and at least two species of gastropod. Igloo-shaped malformations were documented on shell fragments of Sphaerium in all four sites, with the frequency of occurrence ranging from 3.3–11%. The igloo-shaped malformations occur exclusively on the inner surfaces of Sphaerium shells, and have not been identified on other mollusks in the sample. They range in size up to ~1.7 mm long axis, show no preferred location or orientation (igloo aperture direction) on affected valves, and generally occur as single isolated structures. Rarely two or three igloo-shaped malformations occur on a single shell fragment. The presence of igloo-shaped malformations on Sphaerium from the Judith River Formation pushes the first appearance of trematode-induced igloo-shaped malformations back ~76 million years into the Campanian, and demonstrate that this live-live host-parasite interaction, previously documented in marine settings, also transpired in freshwater Mesozoic ecosystems.