unionid bivalve from the Carnegie Museum of Natural History collection:
Pyganodon grandis (Say, 1829), CM 61.6839.
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.
Cathy Klingler, Carnegie Museum of Natural History.