Page 333 - Carbonate Facies in Geologic History

320 The Rise of Rudists; Middle Cretaceous Facies in Mexico and the Middle East

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Capralina Monopleu ra
pinguiscula Toucasia texano
Twi.led conical Subequally coiled valve.; .hell wilh
attached valve; a thin while inner layer and brown
a cap-like free ouler layer
valve
CAPROTINIO MONOPlEURIO REQUIENIO

CAPRINIOS
hick partitioned wall Thick vesicular wall

Caprinuloideo

Fig.XI-i. Major types of Cretaceous rudist bivalves after drawings compiled by A.J.Coogan

Bob Perkins, Alan Coogan and others have studied the ecology of Middle
Cretaceous rudists for many years. The observations below come mainly from
their detailed analyses of back reef mounds in strata on the Texas shelf and from
studies on shelf margins in central Mexico. Four ecologic assemblages are known.
These are only partly related to major taxonomic groups.
1. The interior beds of carbonate banks and shelves contain bedded limestone
formed in part by biostromes of Requienia and Toucasia whose left valves are
large and attached and whose shells are thin and laminate. Dentition is normal,
two teeth occurring in the smaller right valve. Both valves tend to coil. The
requienid group is long-ranging and primitive in that it is directly related to the
Jurassic ancestor, Diceras. It is found in a variety of environments and lived in
association with other rudist types. Alan Coogan (personal communication)
noted its occurrence with the radiolitid Sauvegesia in the El Abra of Mexico. But
biostromes of requienids may also occur to the exclusion of other organisms
except abundant miliolids and are interspersed with mm laminates, algal stroma-
tolites, intraformational flat pebble conglomerate, dolomite crusts, and hard-
ground surfaces. They thus form an important part of the restricted marine la-
goonal to intertidal environment of the Cretaceous, resisting successfully the