No abstract

In a preliminary cladistic analysis of the bivalve family Cardiidae (Schneider 1992), members of the subfamilies Protocardiinae, Lahilliinae, and Laevicardiinae, plus the genus Nemocardium, were found to be the least derived taxa of cardiids. A cladistic analysis is undertaken of the genera and subgenera of these cardiid taxa, plus several Mesozoic taxa which have never been assigned to any subfamily. The Late Triassic Tulongocardium, which is placed in Tulongocardiinae subfam. n., is the sister taxon to all other cardiids. Protocardiinae is restricted to the genus Protocardia. Most other Mesozoic taxa which have been placed in the Protocardiinae are found to be members of the Lahilliinae. Nemocardium is placed in the Laevicardiinae. Incacardium, Pleuriocardia, and Dochmocardia form a monophyletic group, Pleuriocardiinae subfam. n. Pleuriocardiinae, Laevicardiinae, and the remaining members of the Cardiidae (herein informally termed “cucardiids”) form a monophyletic group.

The shell microstructure of Carboniferous and Triassic permophorids; Triassic and Recent carditids; Devonian, Carboniferous, and Triassic crassatelloideans; and Jurassic through Recent cardioideans is examined in a phylogenetic context, using separate microstructural and morphologic data sets, as well as a combined data set. The microstructural and morphologic data sets are significantly incongruent, but the combined data set suggests that modiomorphoideans (modiomorphids and permophorids) are basal to crassatelloideans; crassatelloideans are basal to carditids (includingSeptocardia), and carditids are basal to cardiids. On the other hand, the possibility of direct permophorid ancestry for the carditid-cardiid clade cannot be excluded, as suggested by the retention of permophorid-like matted (transitional nacreous-porcelaneous) structure in some early carditids and cardiids. In the absence of stratigraphic data and other evidence for phylogenetic relationships, shell microstructure offers limited potential for assessing subfamily-level phylogenetic relationships within the Cardioidea. This is because of microstructural convergences reflecting biomechanical adaptations for fracture control and abrasion resistance, and possibly also selection for metabolic economy of secretion in tropical, oligotrophic habitats. General evolutionary trends in cardiid shell microstructure are nevertheless apparent: Cretaceous cardiids completely replaced an ancestral laminar, matted structure in their inner shell layer with non-laminar porcelaneous structures; evolved better defined CL structure, stronger reflection of the shell margins, and increased thickness or secondary loss of the ancestral prismatic outer shell layer; and, inProtocardia(Pachycardium)stantoni, added inductural deposition. Some Cenozoic cardiids then evolved wider first-order crossed lamellae, non-denticular composite prisms, composite fibrous prisms, ontogenetic submergence of a juvenile non-denticular composite prismatic outer shell layer into the CL middle shell layer, or ontogenetic submergence of the inner part of a juvenile fibrous prismatic outer shell layer into the CL middle shell layer.The shell microstructure ofHemidonax donaciformisis unusual for a cardioidean, and suggests closer affinities with the superfamily Tellinoidea than with the superfamily Cardioidea.Extensive inductural deposits inProtocardia(Pachycardium)stantoniraise the possibility that photosymbiosis evolved among some Mesozoic members of the Protocardiinae, thereby increasing the likelihood that this feature has evolved several times independently in the Cardiidae.Cemented, calcareous periostracal granules or spines are known to occur in modiolopsoideans, mytiloideans, modiomorphids, permophorids, trigonioids, astartids, cardiids, myoids, pholadomyoids, and septibranchoids. Consequently, the presence of these structures is not necessarily indicative of close anomalodesmatan affinities.

The microstructure of the non-window portions of the shell of Corculum cardissa resembles other Fraginae, with predominantly fibrous prismatic outer, branching crossed lamellar middle, and complex crossed lamellar inner layers. Both the anterior and posterior windows in its shell reflect reduced pigmentation and incursion of the outer shell layer, but the posterior windows involve deeper incursion plus reduction of the outer and middle sublayers of the outer shell layer and microstructural modification of the middle shell layer to enhance light transmission. The planoconvex shape of the posterior windows has more likely evolved to direct and focus light toward the deeper, zooxanthellae-rich gills and anterior mantle, than to merely disperse light.