Carditidae

Abstract: The heteroconch bivalve fauna of the deep-water (>200 m) Keasey Formation in northwestern Oregon records the Eocene–Oligocene climatic transition and replacement of tropical widely-distributed taxa by the cryophilic taxa that dominate modern high-latitude faunas of the North Pacific. Low-diversity assemblages occur in tuffaceous mudstone and siltstone facies of a deep nearshore basin at the onset of subduction on the Cascadia Margin. Six species of anomalodesmatan heteroconchs have been treated separately, and the remaining Keasey heteroconchs treated here include one basal archiheterodont and 13 imparidentian euheterodonts. The families represented are Carditidae, Thyasiridae, Lucinidae, Lasaeidae, Cardiidae, Tellinidae, Basterotiidae, Mactridae and Veneridae. New taxa include the genus Anechinocardium and seven new species: Cyclocardia moniligena, Conchocele bathyaulax, Conchocele taylori, Kellia saxiriva, Kellia vokesi, Moerella quasimacoma and Saxicavella burnsi. Three species lacking adequate material for formal description are treated in open nomenclature. New features useful for taxonomic discrimination include heretofore unrecognized differences in micro-spines and lamellae on the posterior slope in cardiid bivalve genera and presence of a faint subumbonal ridge in three genera of basterotiid bivalves. Four of the new species are in families in which thiotrophic and methanotrophic chemosymbioses have evolved. Specimens are typically articulated. Shell material is well preserved in the massive units but often chalky or highly altered at cold-seep sites. Fossils are never abundant except for local concentrations in carbonate mounds and associated carbonate lenses, blebs and nodules at cold-seep localities. Multiple lines of evidence of both diffuse and robust flow of fluids rich in reduced compounds are reviewed for three sites where the fauna includes an inner core of chemosymbiotic taxa and peripheral zone of taxa that are opportunistic and tolerant of hypoxia.

Abstract: Carditamera bajaensis , new species, is described from semi-infaunal specimens collected in the intertidal zone in the Golfo de California, Baja California Sur, Mexico. The new species resembles Carditamera affinis (G. B. Sowerby I, 1833), the only valid Carditamera species known from within the Golfo de California, with which it has been mistaken, but it differs in shell structure and most conspicuously in life mode — semi-infaunal for C. bajaensis versus byssally attached to hard substrata for C. affinis. Haplotype networks constructed from two mitochondrial genes (16S rRNA and cytochrome b) and one nuclear gene (internal transcribed spacer 2) indicate a clear genetic break between C. affinis and C. bajaensis , as suspected initially due to their different modes of life and shell morphology. This pair of species, C. affinis and C. bajaensis , overlapping in distribution yet genetically distinct, possibly indicate ecological speciation.

Abstract: The carditid genus Cyclocardia is currently the most diverse genus of the family, including nearly 180 nominal species encompassing wide stratigraphical (Cretaceous–Recent) and geographical (Antarctica, South and North America, Europe, Africa, Alaska, Russia, Japan and New Zealand) ranges. Due to the lack of autapomorphies in the diagnosis of the genus and its large account of species, we re‐evaluate the systematic and phylogenetic status of Cyclocardia. We applied three approaches: bibliographic revision, phylogenetic analysis and an exploration of morphological disparity. We used a shell–character matrix comprising 65 taxa (2 outgroups, 29 non‐Cyclocardia carditids and 28 species of Cyclocardia) for phylogenetic and disparity analyses. Maximum Observable Rescaled Distances was used to construct a distance matrix to compare Cyclocardia species and other carditid groups. According to our results, Cyclocardia represents a non‐monophyletic taxon and is thus a ‘wastebasket taxon’, chiefly because its diagnosis was based mainly on plesiomorphic characters. The European species C. kickxi and C. chameformis are placed within Scalaricardita, and the previously proposed genus Crassicardia is monophyletic (including C. crassidens, C. crebricostata, C. isaotakii and C. rjabininae). Three new genera are proposed for new groups identified by the phylogenetic analysis: South American Oesterheldia gen. nov. (including O. cannada and O. dalek), western North American Coanicardita gen. nov. (including C. ventricosa and C. occidentalis), and North Pacific Hippocampocardia gen. nov. (including H. barbarensis, H. hamiltonensis and H. yakatagensis). The newly defined monophyletic Cyclocardia is restricted to the Atlantic Ocean species C. borealis, C. novangliae and C. compressa.

Abstract: . Bivalve classification has suffered in the past from the crossed-purpose discussions among paleontologists and neontologists, and many have based their proposals on single character systems. More recently, molecular biologists have investigated bivalve relationships by using only gene sequence data, ignoring paleontological and neontological data. In the present study we have compiled morphological and anatomical data with mostly new molecular evidence to provide a more stable and robust phylogenetic estimate for bivalve molluscs. The data here compiled consist of a morphological data set of 183 characters, and a molecular data set from 3 loci: 2 nuclear ribosomal genes (18S rRNA and 28S rRNA), and 1 mitochondrial coding gene (cytochrome c oxidase subunit I), totaling ∼3 Kb of sequence data for 76 molluscs (62 bivalves and 14 outgroup taxa). The data have been analyzed separately and in combination by using the direct optimization method of Wheeler (1996), and they have been evaluated under 12 analytical schemes. The combined analysis supports the monophyly of bivalves, paraphyly of protobranchiate bivalves, and monophyly of Autolamellibranchiata, Pteriomorphia, Heteroconchia, Palaeoheterodonta, and Heterodonta s.l., which includes the monophyletic taxon Anomalodesmata. These analyses strongly support the conclusion that Anomalodesmata should not receive a class status, and that the heterodont orders Myoida and Veneroida are not monophyletic. Among the most stable results of the analysis are the monophyly of Palaeoheterodonta, grouping the extant trigoniids with the freshwater unionids, and the sister-group relationship of the heterodont families Astartidae and Carditidae, which together constitute the sister taxon to the remaining heterodont bivalves. Internal relationships of the main bivalve groups are discussed on the basis of node support and clade stability.