Modern unionid bivalve from the CMNH collection:
Pyganodon grandis (Say, 1829), CM 61.6839.

The Hell Creek Formation was deposited by a system of Late Cretaceous rivers that flowed into an ancient inland ocean called the Western Interior Seaway. The Hell Creek represents a landscape of numerous rivers and lakes, which were home to a variety of freshwater bivalved molluscs. The dominant components of this freshwater bivalve assemblage were members of the mussel group Unionidae. Unionidae is the largest freshwater bivalve group, containing hundreds of genera and thousands of species including the familiar genus Unio. The geologic range of the Unionidae is from the Triassic (~240 million years ago) to the present.

Characteristics Unio shares with all other bivalves include a bilaterally symmetrical, two-valved shell, a hatchet-shaped foot, and sheet-like gills. All freshwater bivalves incubate their eggs during reproduction. They begin life as parasites of fish or amphibians. Freshwater bivalve larvae are called glochidia, and are equipped with a small triangular-valved shell and attachment organs consisting of one or more hinged spines on the shell or a long byssal thread. Young of Unio use their shell spines to attach themselves to the fins of their fishy host. Unio moves by using its pointed muscular foot to burrow shallowly or plough along a surface. In Unio, the margins of the mantle are only slightly fused to provide two posterior siphons, a wide inhalant siphon and a narrow exhalant siphon. These siphons scarcely project beyond the edge of the shell.

Photo: Cathy Klingler, CMNH.