THE GEOLOGICAL, ECOLOGICAL AND PALEOCLIMATOLOGICAL SIGNIFICANCE OF “EXOTIC” CONTINENTAL OSTRACODES: RICK FORESTER’S LEGACY IN UNDERSTANDING OSTRACODE EVOLUTION IN LONG-LIVED LAKES

Exotic ostracodes, as recognized by Swain (1986) and defined by Forester (1991) comprise a heterogeneous assemblage of unrelated continental (mostly but not exclusively lacustrine) taxa, united by their complexly shaped and ornamented carapace morphologies. Many of these morphologies are convergent on unrelated marine taxa, but some have aberrant shapes and processes which are otherwise unique among the ostracoda. Such ostracodes are known from both modern lakes (especially the ancient Lakes Tanganyika, Baikal and Ohrid) and also from the fossil record, notably in Pliocene lake deposits of the western US, especially from Paleolake Idaho.

Forester speculated that exotic lacustrine ostracode clades might evolve in limnologically “stable” lakes. In publication and in our private conversations he argued this would be especially likely where consistently available calcium carbonate mitigated against water chemistry as a dominant adaptive constraint, allowing biological interactions (especially escalatory predator-prey interactions) to become a more dominant driver of morphological evolution. Over the past 20+ years of collection of new autecological and paleoecological data, especially from L. Tanganyika and the US Great Basin has shed new light on exotic ostracodes, supporting some of Forester’s speculations, while leaving others (particularly the escalatory hypothesis and the requirement for lacustrine “stability”) as still open to debate. Paleobiogeographical data demonstrate that individual exotic taxa are not necessarily restricted to their lake basins of origin, but can disperse (probably via riverine connections) to interconnected lakes.