Functional morphology and taxonomic significance of a novel cuticular Structure in Cretaceous Raninid crabs (Decapoda: Brachyura: Raninidae)

Abstract

Fossil raninid crabs,Cretacoranina punctata(Rathbun, 1935), from the Pawpaw Formation (Upper Cretaceous) of Fort Worth, Texas, exhibit an unusual cuticular morphology. Comparison of the cuticle of extantRaninoides louisianensisto that ofC. punctatareveals general similarities in endocuticular, exocuticular, and epicuticular ultrastructure; however, their gross morphology is strikingly different. The surface of the carapace ofC. punctataappears pebbled, much like the surface of a basketball, with closely packed, hexagonal caps. In cross-section, these caps are the upper portion of fungiform structures within convoluted exocuticle. Along the anterior margin, anterolateral margins, and pterygostomial region of the carapace ofC. punctataeach cap dips slightly posteriorly, creating a series of tiny terraces. In contrast to terrace lines, questa lines, spines, and nodes that provide frictional resistance in interactions with coarse-grained sediments, the fine relief and contouring of the pebbled surface of the carapace ofC. punctataprovides frictional resistance in interactions with fine-grained sediments.Cretacoranina dichrous, C. trechmanni, C. testacea,andC. schloenbachi,as well asEucorystes carteriwere all found to possess variations of the exocuticular structures seen onC. punctata.This pebbled surface has not been recognized in any other decapod taxon, nor has its structure and function been described previously.