Ecologic Radiation of Cambro Ordovician Echinoderms

Echinoderms represent a modest component of the initial metazoan radiation during the Cambrian but responded to global environmental changes across the Cambro-Ordovician boundary with rapid and prolific diversification to more varied lifestyles in expanded habitats. Many attached echinoderms were preadapted to exploit carbonate hardgrounds and other stable substrates that became abundant on shallow carbonate platforms at that time, whereas other attached—and many new free-living—echinoderms evolved structures to cope with soft substrates.

Early to Middle Cambrian echinoderms are primarily known from soft substrate environments where attached suspension-feeding eocrinoids, crinoids, and edrioas-teroids clung to skeletal debris by suctorial attachment disks or were skeletally cemented by a holdfast; helicoplacoids perhaps employed other means. Vagile surface deposit-feeding echinoderms included stylophorans, homosteleans, homoiosteleans, and ctenocystoids. Echinoderms reached a diversification bottleneck in the Late Cambrian, but stemmed eocrinoids with cemented holdfasts were among the first skeletonized animals to colonize hardgrounds that became common at that time. Stylophorans, homoiosteleans, and edrioasteroids were also represented. Attached crinoids and free-living rhombiferans led the Early Ordovician radiation among suspension-feeding echinoderms and were accompanied by several other newly evolved groups with generally similar lifestyles. Vagile herbivorous echinoids and carnivorous asteroids greatly expanded echinoderm ways of life by the Middle Ordovician. This overall diversification pattern for echinoderms supports a model of two sequential evolutionary faunas in which shallow-water habitats fostered onshore origination and radiation followed by offshore expansion for many attached forms. However, the diversification pattern is not as clear among free-living echinoderm groups, and the expansion direction for several of these could have been from offshore to onshore. Bathymetry is a simplification of what must have been a complex list of controls. Most Ordovician echinoderms had regular and sturdy construction; these advanced designs were versatile and enduring by comparison with Cambrian forms, persisting through the Paleozoic and in some cases to the Recent.

DOCUMENTATION OF ECOLOGIC diversification in the fossil record provides the road map of life's temporal patterns and the context of evolutionary history. Most studies of diversification have emphasized intrinsic biotic driving factors for changes in diversification patterns and evolutionary pathways (see Sepkoski 1991 for a review), but recent field-based studies have emphasized the role of extrinsic causes. This latter approach requires extensive field observation and integration of sedimen-tologic, facies, and sequence stratigraphic information with paleobiologic observations (Guensburg and Sprinkle 1992; Rozhnov 1994; Droser et al. 1995). Broad-scale linkages are emerging as a result. For instance, we have previously correlated global environmental changes with the ecologic expansion and diversification of echino-derms and other metazoans during the Early Ordovician rise of the Paleozoic Evolutionary Fauna (Guensburg and Sprinkle 1992; Sprinkle and Guensburg 1995). Echinoderms of the Cambrian remained a modest component of the biota until favorable environmental shifts provided the catalyst for rapid ecologic expansion as part of the Ordovician radiation of metazoans (Sprinkle 1980). The purpose of this chapter is to review the ecologic radiation of Cambrian to Early Ordovician echino-derms and to analyze their diversification patterns. Direct associations of echinoderms and substrates are occasionally available when articulated specimens still adhere to attachment sites. In many other cases, however, life modes must be reconstructed from extensive field correlation of partial specimens and lithofacies, coupled with functional morphologic studies and extrapolation from better-preserved close relatives.

The ecologic radiation for Cambro-Ordovician echinoderms offered here differs from those suggested by Smith (1988: figure 12.3; Smith 1990) and Smith and Jell (1990: figure 53). Many Early Paleozoic echinoderms are interpreted by these authors to have rested unattached on, or had a distal structure inserted into, soft substrates. In contrast, evidence leads us to conclude that hard attachment surfaces were required and that this was an important limiting factor to the diversification of Cambrian echinoderms. This also implies that most Cambrian echinoderms were pre-adapted to exploit the hard substrates that became common by the Late Cambrian. These divergent functional interpretations provide an impetus for presentation of our ecologic diversification model below.

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