RICHARD B. ARONSON, Dauphin Island Sea Lab, Dauphin Island, Alabama 36528
DANIEL B. BLAKE, Department of Geology, University of Illinois, Urbana, Illinois 61801
The Phanerozoic history of marine benthic communities displays a strong environmental bias. New community types generally appeared in coastal environments and then spread to offshore habitats, replacing earlier community types along an onshore-offshore gradient (Bottjer and Jablonski 1988). During the Mesozoic, rapidly diversifying teleostean fishes and decapod crustaceans fundamentally changed community structure in shallow-water, soft-substratum habitats. The fossil record indicates that predators severely restricted or completely eliminated populations of epifaunal suspension-feeders living in those habitats (Vermeij 1987). Dense populations of ophiuroids and stalked crinoids had virtually disappeared from shallow water by the middle Cretaceous (Meyer and Macurda 1977; Oji 1985; Aronson 1992). Today, dense populations appear primarily (ophiuroids) or exclusively (stalked crinoids) in the deep sea, where predation pressure is lower than in most shallow-water environments (Gage and Tyler 1991). The predation-mediated restriction of epifaunal suspension-feeders was accompanied by a shift to infaunal, mollusk-dominated communities, which are characteristic of shallow-water, soft-substratum habitats in the Cenozoic (Sepkoski 1991).
In contrast to this general macroevolutionary trend, we discovered dense fossil populations of both ophiuroids and stalked crinoids in an upper Eocene, shallow-marine deposit at Seymour Island, 100 kilometers southeast of the tip of the Antarctic Peninsula (64°15'S 56°45'W). The La Meseta Formation on Seymour Island is a clastic deposit that probably represents a shallow submarine channel. Like most Cenozoic marine deposits that are fossiliferous, the La Meseta is rich in gastropod and bivalve shells (Stilwell and Zinsmeister 1992). Teleosts and decapods occur throughout the formation, and many of the mollusks bear antipredatory structural reinforcements suggesting intense predation pressure.
In December 1994, we located five dense assemblages of hundreds to thousands of ophiuroids ( Ophiura n. sp.) and four assemblages of tens to hundreds of isocrinid (stalked) crinoids ( Metacrinus fossilis) on Seymour Island. [Meyer and Oji (1993) and Blake and Aronson (in press-a and -b) describe the echinoderm fauna of the La Meseta Formation.] The dense ophiuroid assemblages covered a maximum area of 40 square meters and were no more than 10 centimeters thick. The crinoid assemblages were no thicker than a single layer of individuals. They were scattered over areas as large as 150 square meters, with dense concentrations covering 10-20 square meters. These autochthonous ophiuroid and crinoid assemblages appear to represent localized, short-lived populations. They were restricted to the uppermost of seven units of the La Meseta, and they were interbedded with the mollusk-rich horizons that dominate the formation.
The incidence of sublethal arm injuries can be used as a measure of predation pressure in ophiuroid and crinoid populations. For ophiuroids, sublethal damage is measured as the proportion of individuals regenerating one or more arms (Aronson 1991), and for crinoids it is measured as the proportion of regenerating brachitaxes (arm-branching series; Meyer and Oji 1993). Low incidences of sublethal damage to the ophiuroids and crinoids indicated that predation pressure was low in the Seymour Island populations (table).
The occurrence of dense ophiuroid and stalked crinoid populations in a late Eocene, shallow-water setting is anomalous. The stalked crinoids are particularly surprising, because they were supposedly driven out of shallow-water environments in the Mesozoic, tens of millions of years before La Meseta time. There was no obvious lithological or sedimentological reason for the abrupt occurrence of these ophiuroid- and crinoid-dominated horizons in the upper portion of the La Meseta Formation. Rather, it appears that changes in the physical environment in Antarctica created the ecological conditions that allowed the echinoderm populations to arise sporadically
Global cooling accelerated in the late Eocene, particularly toward the end of La Meseta time (Mackensen and Ehrmann 1992). This long-term thermal trend was accompanied by increased upwelling in the southern oceans, including around the Antarctic Peninsula (Kennett and Warnke 1992; Diester-Haass and Zahn 1996). Upwelling would have increased nutrient availability, leading to increased productivity. Increased productivity would in turn have led to increased concentrations of particulate organic material, which would have provided a food source for the dense populations of suspension-feeding echinoderms. At the same time, declining temperatures probably disrupted predator-prey relationships between the echinoderms and the fish and crustaceans that ate them. The result was localized switching from a typically Cenozoic, high-predation, mollusk-dominated ecology to a more Paleozoic-type, low-predation, echinoderm-dominated community state.
Some components of the benthic fauna of Antarctica are taxonomically, functionally, and/or ecologically archaic (e.g., Dell 1972). The retrograde character of antarctic benthic communities may well have had its origin in the disruption of predator-prey interactions at the end of the Eocene. If this hypothesis is correct, then global climate change exerted the ultimate controlling influence on the distribution of species and the nature of trophic connections in Antarctica.
This research was supported by National Science Foundation grants OPP 94-13295 to Richard B. Aronson and OPP 93-15927 to Daniel B. Blake. We thank Tatsuo Oji for his contributions to our understanding of crinoid ecology in the La Meseta Formation.
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