VALVE GROWTH TERATOLOGY IN AN ANOMIA ARGENTARIA SKELETOZOAN ASSOCIATED SQUATTERING IN A CRASSATELLA VADOSA FROM THE COON CREEK LAGERSTÄTTE (LATE CRETACEOUS) OF WESTERN TENNESSEE

The Coon Creek Lagerstätte (Late Campanian) type section consists of lower lithofacies ("Classic Coon Creek" beds) composed of micaceous, glauconitic, clayey-sand and sandy-clay overlain by a mold-rich condensed shell-bed horizon and an overlying trace fossil rich horizon. Invertebrate fossils within the classic “Classic Coon Creek” are essentially unaltered and preserve fine details of morphology and geochemistry within their shells. The fauna is diverse consisting of, foraminifera, nannofossils, ostracods, bryozoans, corals, bivalve, gastropod, cephalopod, and scaphopod mollusks, crabs, lobsters, shrimp, annelids, sea urchins, sharks, numerous fish taxa (bone, teeth, and otoliths), turtles, mosasaurs, plesiosaur, and the rare calcareous algae, sea grass, and fossil land plants (lignite and leaves). Reported herein for the first time is an unusual teratological occurrence of the bivalve Anomia argentaria that “squattered” within the inner surface of a disarticulated and partly bioeroded left valve of the bivalve Crassatella vadosa. The Anomia, acting as a skeletozoan on the Crassatella substrate, secreted a right “upper” valve only during its entire ontogeny (congenital) and suppressing growth of a left valve, instead, using the Crassatella as its left valve. The Anomia demonstrates clear xenomorphism of shape to the inner topography of the Crassatella, along with adoption of the general shell shape outline of the Crassatella thus making the Anomia outline more consistent to the Crassatella inner shape. The Anomia developed a geniculation that coincides with the pallial line of the Crassatella as it grew over that feature, thus showing topographic xenomorphism. Muscle scars from the Anomia are visible as slight indentations in the inner surface of the umbo region of the Crassatella and these correspond to muscle scar pattern of the Anomia right valve. The Anomia developed a thickened shell where it grew into and over the posterior adductor scar of the Crassatella.