Affiliation:
1. Department of Earth and Planetary Sciences University of California Davis California USA
Abstract
AbstractMolluscan shells display a high diversity of external sculpture. Sculptural elements may be symmetrical, where both edges of an element are morphologically similar, or asymmetrical, where one edge is steeper than the other. Asymmetrical sculpture can be ratcheted, with the leading edges (those in the direction of locomotion or growth) less steep than the trailing edges, or imbricated (leading edges steeper than trailing edges). While the ratcheted sculpture is better known, the diversity of imbricated sculpture has remained largely unexplored. In a survey of extant benthic shell‐bearing molluscs, we document imbricated sculpture primarily in epifaunal bivalves or on the exposed sectors of shells of semi‐infaunal bivalves. Imbricated sculpture is particularly widespread in pteriomorphian bivalves, but it is absent in the subclade Mytiloidea as well as in highly mobile Pectinidae. It also occurs in many carditid bivalves (Archiheterodonta) and in phylogenetically scattered euheterodonts. In several infaunal bivalves including species of Cardites (Carditidae), Hecuba (Donacidae), and Chione (Veneridae), comarginal elements on the posterior sector are imbricated whereas anterior comarginal ridges are ratcheted. Imbricated sculpture in bivalves tends to be concentrated on the upper (left) valves of pectinids or on the posterior sector of both valves in archiheterodonts and euheterodonts. Imbricated sculpture is uncommon in gastropods, even in epifaunal species, but does occur in the collabral ridges in some Vasidae and a few other groups. Expression of imbricated sculpture does not depend on shell mineral composition or microstructure. The ecological distribution and within‐shell pattern of expression of imbricated sculpture point to the likelihood that this type of asymmetrical sculpture is both widespread and potentially functional. Additionally, we present a potential methodology whereby shell sculpture categories (symmetrical, ratcheted, and imbricated) may be quantified by comparing the lengths of corresponding leading and trailing edges across the shell surface.
Subject
Developmental Biology,Animal Science and Zoology
Reference78 articles.
1. On the biology and functional morphology of Chama gryphoides Linné (Bivalvia; Chamidae);Allen J. A.;Vie et Milieu,1976
2. Growth and function of the micro-frills present on the Devonian brachiopod Athyris campomanesi (Verneuil & Archiac)
3. Living Chamidae of the eastern Pacific (Bivalvia: Heterodonta);Bernard F. R.;Natural History Museum of Los Angeles County Contributions in Science,1976
4. Recent Chamidae (Bivalvia) from the western Atlantic Ocean;Campbell M. R.;Malacologia,2004
5. Environmental and Biological Controls of Bivalve Shell Mineralogy and Microstructure