The Rudists, object of Dr. Alencáster's paleontological interest constructive morphology of the Rudist Shell

Abstract

Rudists (Order Hippuritida Newell, 1965) developed a series of modi cations in the shell morphology distinguishing them from other bivalves and allowing the advantageous occupation of some particular ecological niches on the Tethyan margin shallow carbonate platforms, were they proliferated during the Cretaceous.

Their shell was formed by two layers. The outer shell layer, made of low Mg content calcite and simple prismatic microstructure, is frequently well preserved. The inner shell layer, made of aragonite, is normally replaced by spar calcite. Predominance of one layer on the other varies within the different rudist families.

Although some are more or less equivalve, most are very inequivalve. The form of the valves ranges from spirally coiled (prosogyrate, open, very high, and with few whorls) in primitive forms to conical (more or less high) in more modi ed forms, as a consequence of holo-periferal accretion growth (commissural margin far from the umbo), instead of hemi- periferal accretion growth (the umbo remains close to the dorsal commissural margin) as in most bivalves. They can reach big sizes.

They were attached to the substrate by one of the two valves. Attachment by left or right valve, besides other characters, has been used to distinguish two main rudist groups or super-families, the Requienioidea Kutassy, 1934 (poorly diversi ed, 2 families) and the Radiolitoidea d’Orbigny, 1847 (much more diversi ed, 12 families).

The hinge (teeth, sockets, and ligament), the attachment surface of the adductor muscles (myophores), as well as other structures consequence of the growth type adaptation (laminae, apophyses, accessory cavities), are the more distinctive characters respectively to other bivalves and those mainly used in the taxonomical distinction of families. Other characters are related to the structure of any of both shell layers in one or both valves (tabulae, dissepiments, canals, “cellular” structures). Some of these morphological characters, or combinations of them, appeared and are exclusive of one family, but others represent convergences occurring in different families. Identi cation of such convergences represented a great step forward in rudist taxonomy and phylogeny.

All this array of characters, with their variations, is responsible for the great diversity among rudists. Their functional and ecological interpretation, together with the study of the sediments where the different taxa occur, signi cantly contribute to understand the marine ecosystem in the shallow margins of the Tethys during the Cretaceous. Their geographical distribution also contributes to paleogeographic reconstructions. Moreover, as this great morphological and taxonomical diversity was originated during a relatively short time interval, rudists are very useful in the biostratigraphy of platform sediments.

The fact that the outer shell layer of rudists is currently preserved without mineralogical or microstructural changes due to diagenesis has favored its use in geochemical studies, to obtain both paleo-temperature data and absolute dating. This last calibrates the biostratigraphical proposals. In the same way, micro- and mega-rhythms observed in the growth lamellae of the rudists outer shell layer have been related to different astronomical and/or biological cycles.