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 extant Raninoides louisianensis to that of C. punctata reveals general similarities in endocuticular, exocuticular, and epicuticular ultrastructure; however, their gross morphology is strikingly different. The surface of the carapace of C. punctata appears 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 of C. punctata each 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 of C. punctata provides frictional resistance in interactions with fine-grained sediments. Cretacoranina dichrous, C. trechmanni, C. testacea, and C. schloenbachi, as well as Eucorystes carteri were all found to possess variations of the exocuticular structures seen on C. punctata. This pebbled surface has not been recognized in any other decapod taxon, nor has its structure and function been described previously.