Page 353 - Introduction to Paleobiology and The Fossil Record

340 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 13.5 Rudists: bivalves disguised as corals

The rudists were aberrant heteroconch bivalves that range in age from the Late Jurassic to the Late
Cretaceous and occupied the Tethyan region. During a relatively short interval they developed a
bizarre range of morphologies, and although many groups apparently mimicked corals, the rudists
were probably not reef-building organisms. The rudists were inequivalved with a large attached
valve, usually the right valve of conventional terminology, and a small cap-like free valve. Virtually
all rudists had a single tooth fl anked by two sockets in the attached valve, and two corresponding
teeth and a socket in the free valve. The valves functioned with an external ligament and pairs of
adductors attached to internal plates or myophores. Three growth strategies have been identifi ed
(Fig. 13.10). Elevators had tall conical shells with a commissure raised above the sediment–water
interface to free the animal from the risk of ingesting sediment. The elevators were thus similar to
solitary corals, suggesting a possible reef-building strategy. Clingers or encrusters were fl at, bun-
shaped forms that usually adhered to hard substrates. The recumbents had large shells, extending

laterally extravagantly over the seafloor like large calcified bananas. The rudists occupied carbonate

shelves throughout the Tethys region, with their larvae island hopping around the tropics, often
growing together in a gregarious habit; clusters or clumps probably trapped mud in molluskan-rich
structures. As noted above it now seems likely that the rudists were never true reef-building organ-
isms although they came close to fulfilling that mode of life.

Thomas Steuber (University of Bochum) has developed a comprehensive database on rudist
bivalves together with spectacular pictures of rudist accumulations. Study of this comprehensive
database, and a smaller dataset that can be used to reconstruct ancient paleogeographic associations
at http://www.blackwellpublishing/paleobiology/, can be used for a variety of exercises. The small
dataset investigates the biogeography of Campanian rudists, emphasizing their relationship to the
paleotropics (Tethyan province) on http://www.blackwellpublishing.com/paleobiology/.

elevators
encrusters
D

C

B E
A
G
F

recumbents
I
H
Figure 13.10 Rudist growth strategies: encrusters (A, B, H and I), elevators (C, D and E) and
recumbents (F, G). (From Skelton, P.W. 1985. Spec. Pap. Palaeont. 33.)

through the exhalant slit in the outer lip. During The gastropod shell is normally oriented
ontogeny the inactive track of the slit is succes- with the aperture facing forward and the apex
sively overgrown with shell material to form facing upwards. If the aperture is on the right-

the selenizone, the calcified track of the slit hand side, the shell is coiled clockwise in a
band separating the siphons from the mouth. so-called dextral mode; sinistral shells have