Costellacesta, A New Subgenus of Lima from the Cretaceous of the Gulf and Atlantic Coast Province

Abstract

The new subgenus C ostellacesta is proposed for large Maestrichtian species of Lima with an opisthocline shell, large lunule and byssal gape, single (posterior) auricle, a resilifer situated posterior to the beak, and ornamentation consisting of fine costellae intercalated between prominent, equally developed costae. Three species of C ostellacesta are known and described: L. (C.) riddleiJ n. sp., the type species (oldest; Chiwapa Sandstone Member, Mississippi) , L. (C.) sayrei Stephenson (Corsicana Marl, Texas), and L. ( C.) insolita Stephenson (youngest: upper Peedee Formation, North Carolina). These represent a single line of descent without significant break. Successively younger species demonstrate the following evolutionary trends: change in marginal outline, particularly an increase in concavity of the dorsoanterior margin; increase in the relative size of the lunule and byssal gape; reduction in the strength of the costae and costellae; increase in number of costellae, and decrease in number of costae per unit length; possible decrease in maximum adult size. Costellacesta is possibly ancestral to plicate Cenozoic Limas of the subgenus Plicacesta Vokes. The habitat of C ostellacesta was probably analogous to that of living giant Limas ( Acesta, Plicacesta)-cold, deep waters of the continental slope and abyss, rarely ranging into warm, shallow shelf environments. This might explain the uncommon occurrence of C ostellacesta in shallow water Cretaceous deposits of the Atlantic and Gulf Coastal Plain Province. The subgenus is known only from this area, and probably represents an endemic group.