Showing papers by "Steven M. Stanley published in 1968"

Post-Paleozoic adaptive radiation of infaunal bivalve molluscs; a consequence of mantle fusion and siphon formation

Abstract: The Bivalvia have undergone two principal adaptive radiations since their ap- pearance in the Ordovician. The initial, Early Paleozoic radiation gave rise to epifaunal and primitive infaunal groups and was followed by a period of relative evolutionary stability in the Late Paleozoic. A second, more spectacular radiation took place during the Mesozoic and Cenozoic Eras and brought the bivalves into their present position of prominence. The well-established idea that the many new epifaunal bivalve taxa of the Mesozoic evolved to replace extinct Paleo- zoic brachiopod groups has merit, but fails to explain the rise of the 15 new infaunal bivalve superfamilies of the post-Paleozoic radiation. All new infaunal superfamilies were siphon- feeding groups, whereas siphon feeding had been virtually absent in the Paleozoic Era. Most new siphonate taxa were eulamellibranch heterodonts or their derivatives and possessed crossed-lamellar shell structure. They probably descended from a non-siphonate Paleozoic group with similar features, such as the Astartacea. Their post-Paleozoic radiation was a consequence of mantle fusion and siphon formation, which opened the way for the occupa- tion of many new infaunal ways of life that had been inaccessible to Paleozoic bivalve and brachiopod groups. radiation of infaunal bivalves in terms of life habits has revealed that the preponderance of new Mesozoic and Cenozoic groups were bur- rowers and borers that fed by means of siphons, whereas siphon feeders had been virtually ab- sent in the Paleozoic Era. Similarity of hinge dentition, gill type, and shell structure among siphon-feeding groups suggest that most shared a common ancestry, and life habit data suggest that their adaptive radiation was a consequence of mantle fusion and the evolutionary develop- ment of siphons, which enabled them to invade many new ecologic niches. Before considering the evidence for the role of mantle fusion in bivalve diversification, three in- troductory subjects will be given brief considera- tion: the criteria to be used in measuring bivalve diversity, the general evolutionary history of the Bivalvia, and the classification of bivalves according to feeding type. MEASUREMENT OF TAXONOMIC DIVERSITY