Molluscan marginalia: hidden morphological diversity at the bivalve shell edge

Abstract

Molluscan shells exhibit a high and largely neglected diversity of serrations and crenulations at the growing margin. A survey of living and Cenozoic fossil bivalves indicates that serrations, in which the external ribs or interspaces between ribs extend radially beyond the general contour of the valve margin, may be symmetrical or asymmetrical. Projections whose adumbonal edge more nearly parallels the shell edge than the abumbonal edge occur on the posterior valve margins of many limids, cardiids and donacids, as well as in the arcid Anadara and a few concentrically ridged tellinids. In many cardiids, posterior serrations form as extensions of ribs, whereas ventral and anterior projections are extensions of rib interspaces. Asymmetrical serrations are almost always found on shell edges that are polished, indicating that the mantle extends slightly over the valve margin to the outside. I tentatively suggest that asymmetrical serrations enable the valves to close around siphons or other mantle extensions without injuring these soft tissues, so that the bivalve can maintain sensory contact with the environment even while the shell is shut. A preliminary comparison with brachiopods indicates that the diversity of conditions at the shell edge is much higher in bivalves. Together with the largely warm-water and marine distribution of marginal modifications in bivalves, these comparisons reflect the higher metabolic potentials of bivalves relative to brachiopods, and show that the shell edge is a rich source of evidence about function and mode of life in fossils.

Phylogenetic relationships of the family Carditidae (Bivalvia: Archiheterodonta)

“…These lineages correspond with the separation between Carditamera and Byssomera Olsson, 1961 (type species Cardita affinis), proposed by Olsson (1961). Some authors synonymized both taxa (Coan & Valentich-Scott 2012;Gonzalez & Giribet 2013) and Vermeij (2013) confirmed the distinction based on new characters. The results of the phylogenetic analyses performed here support this taxonomic separation into two taxa because C. arata is the type species of Carditamera, while C. affinis is the type species of Byssomera.…”

Clarifying phylogenetic relationships and the evolutionary history of the bivalve order Arcida (Mollusca: Bivalvia: Pteriomorphia)

“…zebra remaining Arca Linnaeus, 1759 since 645 A. noae is the type species for the genus) Oliver and Holmes (2006). andVermeij (2013) distinguished a third group of Arca species, A. tetragona and A. boucardi, and Vermeij (2013) speculated that it 648 is well removed from the two previous groups, a result supported 649 by our results as discussed above. This clade could constitute the 650 genus Tetrarca, although we did not include the type species of that 651 subgenus here, Arca tetragona Poli, 1795.…”

“…imbricata and A. noae/A. zebra, described byOliver and Holmes (2006) andVermeij (2013), the latter corresponding to 642 the subgenus Arca. Since the two groups are phylogenetically and 643 morphologically clearly distinct they could constitute separate 644 genera (with A. noae/A.…”

Revisión de la Subfamilia Crassatellinae (Bivalvia: Crassatellidae) del Paleógeno—Neógeno de Argentina

“…The resilium also helps align the two valves and resists shear forces parallel to the plane of commissure that might arise while burrowing or being predated, as do the cardinal teeth (Stanley 1970). Ventral crenulations may also assist in aligning valves; they can also impede passage of sediment and debris when the valves are slightly open (Vermeij 2013), resist compressive pressure applied by predators (Thomas 2013), and thwart attempts by predators to drill or enter the bivalve through the valve margin (Vermeij 1983).…”