Page 334 - Carbonate Facies in Geologic History

The Rudist Bivalves 321

Maastrichtian

Fig.XI-2. Rudist distribution in time

frequent changes in salinity and temperature. Bob F. Perkins (personal communi-
cation) has also observed these same rudists in backreef bedded strata above
caprinid mounds where they seemingly occupied tidal pools or formed patches of
low relief on very shallow sea bottoms. Coogan cites other rudist groups which
may form biostromes in the Middle Cretaceous strata including Eoradiolites and
Coalcomana in the Texas Albian.
2. Monopleurid rudists are the most primitive of a second major group of
rudists. In these the left valve is a cap and has the two teeth and single socket; the
right valve is elongate, spiral, or conical and is attached. Monopleurid shells are
thin and cellular. The genus seems to have been tolerant of clay mud for they
are found in inverted conical clumps in marls generally to the exclusion of other
rudists. They commonly are found in biostromes in beds as old as Barremian.
3. The caprinid rudists have long, slender, twisted, right valves with canal-
bearing walls. They grew in abundance and formed moderately large mounds in
shallow back reef positions. Many specimens appear in growth positions in these
mounds, rooted in the ever-present mud matrix which accumulated in and around
the rudist patches. Many other individuals are overturned and lie in irregu-
lar positions in these patches. The rudists were not colonial and did not form a
framework by budding and branching. Their bulky, elongate, twisted, stem-like
forms intertwined with and were attached to each other as well as rooted in the
substrate. They must have formed a substantial baffie capable of growing in
quieter water, but also of resisting surf action. Caprinid rudist patches, with some
relief, are known to have also formed at shelf margins and in downslope positions
as well as in backreef areas. The shelf margin patches generally have a micritic
matrix and are surrounded by grainstone flank beds whose particles are exclu-
sively rudist fragments derived from the mound inhabitants. Apparently the cap-