Mollusks, Hyoliths, Stenothecoids,
Artem V. Kouchinsky
Molluskan diversification was a result of the adaptation of skeletonized forms to various habitats. The ecologic radiation of Cambrian skeletonized mollusks and their possible relatives led to the appearance of all trophic groups, many of them during the Cambrian: deposit feeders (orthothecimorphs, low-spired helcionelloids, and tergomyans), scrapers and grazers (multiplated mollusks, some gastropods), suspension feeders (stenothecoids, chancelloriids, hyolithomorphs, and some macluritid gastropods, orthothecimorphs, Yochelcionella-like helcionelloids), predators, and scavengers (halkieriids and cephalopods). The distinction between suspension and deposit feeding, as well as that of semi-infaunal versus epifaunal habitats, may be meaningless for such small animals approaching interstitial sizes, as the majority of the Early Cambrian mollusks were. Size increase in Cambrian mollusks might have resulted from the invasion of shallow-water high-energy environments. Significant changes in life-cycles could have followed, one of the most important of which was possibly the appearance of the planktotrophic veliger larva.
THE CONTINUOUS Phanerozoic history of marine mollusks that bore mineralized skeletons began in the Early Cambrian. Molluskan remains constitute an important part of the earliest skeletal assemblages (Bengtson and Conway Morris 1992; Dzik 1994). In the present chapter, hyoliths, stenothecoids, and coeloscleritophorans are treated together because even now most of these are considered to be mollusks (Marek and Yochelson 1976; Bengtson 1992; Starobogatov and Ivanov 1996). Nonetheless, the systematic position among the class Mollusca of many of these Cambrian groups is still disputed (Runnegar and Pojeta 1974, 1985; Yochelson 1978; Linsley and Kier 1984; Missarzhevsky 1989; Peel 1991; Geyer 1994; Runnegar 1996). In this chapter, I follow the systematics of the principal groups of Early Paleozoic mollusks developed by Peel (1991), which is supported by morphologic-functional analyses as well as by the observed diversification pattern (Wagner 1996; Zhuravlev, this volume).