Vermeij and Ruch—Miocene barnacles and their hosts
northwestern Atlantic include the epifaunal Crepidula and Placopecten, infaunal bivalves Ensis and Mercenaria, and the infaunal gastropods Euspira and Neverita. Large-bodied lineages colonizing this region from the south include the epifaunal to semi-infaunal Argopecten, Busycon, Busycotypus, Crassostrea, and perhaps Geukensia; the nonsiphonate burrow- ing bivalves Lunarca and Eontia; and the siphonate burrowers Hemimactra and perhaps Scobinopholas and Tagelus. The epifaunal mussel Mytilus and the siphonate burrowing bivalve Mya entered the region from the North Pacific via the Arctic. Among large-bodied clades, then, standing clade diversity dropped from 21 to 11 in the category of epifauna and semi- infauna, and from 10 to seven in the active infauna. These declines of 48% and 30%, respectively, mirror the greater vulnerability of nonsiphonate as compared with siphonate bivalves observed by Stanley (1986) in the late Pliocene to Pleistocene of the North Pacific. They suggest a shift from an epifauna-rich to an epifauna-impoverished community of large mollusks. Whether similar shifts have occurred during the same time interval elsewhere has not been investigated. The coast between Cape Cod and Cape Hatteras is unusual
among temperate marine regions for lacking ecological equivalents to the extinct Conradconfusus parilis. Large subtidal epifaunal predatory gastropods that do not use drilling, wedging, or chipping as means of subduing prey occur in the cold-temperate northern hemisphere (Neptunea, Buccinum), the North Pacific(Fusitriton, Kelletia), and Australasia (Argobuc- cinum, Fenion), as well as on continental shores in the tropics (Melongenidae, Triplofusus and Syrinx, among others). No large barnacle has replaced Chesaconcavus between Cape Cod and Cape Hatteras, again contrasting with the presence of large subtidal barnacles on most other coasts. Whether these absences are related to the relative dearth of subtidal hard bottoms, to the lack of suitable ancestors in the surviving species pool, or to the unusually high seasonal fluctuations in seawater temperature in this part of the northwestern Atlantic is not known. The important point that emerges from these ecological
comparisons is that the faunal composition of the modern shallow subtidal ecosystem of the temperate northwestern Atlantic is atypical. Although some of this uniqueness is attributable to current conditions, much of the peculiar character of the living fauna makes sense only in the context of history, in which extinction and subsequent colonizations from the warmer northwestern Atlantic, the cold North Pacific, and the temperate northeastern Atlantic were the most important agencies of ecological change (Vermeij, 2005; Vermeij et al., 2008). It must be noted that, despite these far-reaching changes, the temperate northwestern Atlantic ecosystem has continued to function. Although not the same as the ecosystems of the past, it remains productive, with all trophic levels represented by a mix of native survivors and newcomers. This part of the marine world offers cogent testimony to the resilience of the ecosystems in the face of significant disruptions. Let us hope that this resilience continues under human hegemony.
Conclusions
The history of life is more than simply a changing of the guard as expressed by a blizzard of names and distributions over time.
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It is instead the story of organisms interacting and evolving with each other in conditions that change over geological time. In this paper, we have set a particular example—barnacles occurring on the shells of living gastropod and bivalve hosts in the St. Mary’s Formation of the Miocene of Maryland—in the broader context of ecological changes in the temperate north- western Atlantic.We show that several guilds of large predatory gastropods and that of large-bodied barnacles, together with evidence on the life habits of the gastropod Conradconfusus parilis (one of the hosts), disappeared without replacement during the late Neogene. The lack of replacement by ecologi- cally equivalent taxa in the post-Pliocene northwestern Atlantic is surprising in view of the geographic expansion of taxa into this area from both the subtropical western Atlantic and the cool-temperate North Pacific. The Recent biota in this region is unusually impoverished compared to biotas in climatically similar assemblages elsewhere. Although extinction would seem to be largely responsible for this impoverishment, the factors preventing guild replacement in the northwestern Atlantic remain obscure. Future research should take a similar approach to that taken here to place the history of the marine northwestern Atlantic in a larger comparative framework in order to assess the roles of extinction and other agencies in determining the taxonomic and functional composition of the modern fauna in different parts of the world.
Acknowledgments
We thank G. P. Dietl and P. Kelley for carefully reviewing this manuscript, and A. Henry and J. Fong for technical assistance.
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