Late Neogene Faunal Change in the Western Atlantic

The late Cenozoic (i.e., Pliocene-Recent, the last five million years) marine fossil record of the Western Atlantic region, especially in the Caribbean, Central America, and the Coastal Plain of the southeastern United States is a particularly appropriate record in which to study the connection between environmental and faunal change because of the dramatic paleoceanographic changes associated with the rise of the Central American Isthmus (CAI). Of special interest in much work on the Western Atlantic Plio-Pleistocene has been the relationship between different environmental variables (particularly the relative roles of temperature and nutrient levels) in the marine realm and patterns of origination and extinction (Vermeij 1978,1987b, 1989,1990; Stanley 1986a; Allmon 1992b, in press; Allmon et al. 1993, 1996a,b; Jackson et al. 1993,1997; Jackson, Jung, and Fortunato 1996; Johnson, Budd, and Stemann 1995; Budd, Johnson, and Stemann 1996; Roopnarine 1996), but these studies have been without much resolution.

In this section, we examine patterns of origination and extinction of three of the better known marine groups in this region [mollusks, corals, and vertebrates (birds and mammals)], followed by an analysis of changes in nutrient supply and their effects on stages in the speciation process.

Late Cenozoic Change in the Western Atlantic: Faunas mollusks. It was long believed that mollusks suffered a late Cenozoic extinction that was more severe in the Western Atlantic than in the Eastern Pacific, especially among so-called paciphile taxa (formerly Atlantic taxa now surviving only in the Pacific; Woodring 1966), producing a modern Western Atlantic molluscan fauna depauperate relative to that of the Eastern Pacific (Dall 1892; Olsson 1961; Woodring 1966; Keen 1971; Vermeij 1978, 1991a; Stanley and Campbell 1981; Stanley 1986a; Vermeij and Petuch 1986; Stanley and Ruddiman 1995). More recent studies, however, have revised this view. It now appears that, although molluscan extinction was indeed higher in the Western Atlantic than elsewhere in the tropics (Vermeij 1991b), molluscan diversity was constant or even increased in the Western Atlantic during the last five million years, with extinctions more or less balanced by originations (Allmon et al. 1993, 1996b; Jackson et al. 1993; Jackson, Jung, and Fortunato 1996), and that the modern Western Atlantic mollusk fauna is approximately as diverse as that of the Eastern Pacific (Allmon et al. 1993,1996b).

Total molluscan diversity on the Caribbean coast of Costa Rica and Panama shows no reduction since the Late Miocene; on the contrary, diversity increased from the Early to the Late Pliocene (Jackson et al. 1993). Paciphile taxa make up only 9% of the large sample analyzed by Jackson et al. (1993), but do not show a decrease in diversity until just before the Plio-Pleistocene boundary (ca. 2.0 Ma), during a major turnover event characterized by increased rates of both origination and extinction. Extinction rates for paci-philes are, however, more than two times greater during this event than for the fauna as a whole (Jackson et al. 1993).

Gastropods as a whole in the southeastern U.S. Coastal Plain show no significant change in diversity since the Pliocene (Allmon et al. 1993, 1996b). Gastropod diversity is essentially the same today in the region as it was in the mid-Pliocene, despite approximately 70% extinction since that time. These high rates of extinction are evidently balanced by high rates of origination, although the relative timing of these two processes is not known. South of Cape Hatteras (and best represented in Florida), most of this turnover takes place in an event that occurred between 2.0 and 2.5 Ma. North of Cape Hat-teras, mollusk diversity declines during a turnover event that occurred between 3.0 and 3.5 Ma. Despite all the extinction that has occurred in the Western Atlantic, the species richness of the Recent gastropod fauna of the low- to mid-latitude Western Atlantic is not demonstrably different from that of that of the low- to mid-latitude Eastern Pacific (Allmon et al. 1993,1996b).

Studies of individual gastropod clades confirm these overall patterns. Gastropods of the Strombina group (family Columbellidae) are prominent paciphiles (Woodring 1966). Extinction in both Eastern Pacific and Western Atlantic was concentrated in a brief interval near the Plio-Pleistocene boundary, but significant origination occurred at this time only in the Pacific (Jackson et al. 1993, 1996). In the gastropod family Turritellidae, Western Atlantic species show a sharp decline in diversity in the late Neogene, but Eastern Pacific turritellids do not (Allmon 1992b).

Fewer data have been published for bivalves. Those data that are available also point to substantial origination accompanying extinction in the Late Pliocene, although at a lower rate than in gastropods. In Florida, for example, between the Late Pliocene Pinecrest Formation and the Late Pliocene-Early Pleistocene Caloosahatchee Formation, data presented by Stanley (1986a) indicate extinction in bivalves of 47.9% (versus 62.4% for gastropods) and origination of 26.7% (versus 55.2% for gastropods) (Allmon et al. 1996b). Among chionine bivalves (Roopnarine 1996), extinction of Western Atlantic species (82.6%) exceeds origination during the entire Pliocene; origination decreases in the Pacific during the same interval but is never exceeded by extinction (38.5%) (Roopnarine 1996).

corals. Reef corals in the Caribbean show a pattern of origination and extinction somewhat similar to that shown by mollusks throughout the Western Atlantic (Johnson, Budd, and Stemann 1995; Budd, Johnson, and Stemann 1996). The major difference is that molluscan turnover appears to begin in the Caribbean and Coastal Plain around 2.4 Ma, whereas coral turnover may begin as much as 1.0 m.y. earlier. Between 4.0 and 1.0 Ma, extinction and origination rates in reef corals increase roughly simultaneously (Budd, Johnson, and Stemann 1996), although in species with smaller colonies the peak in extinction is preceded by a high level of origination (Johnson, Budd, and Stemann 1995). Approximately 64% of the Early Pliocene coral fauna becomes extinct; smaller, free-living colonies living in sea grasses are most affected by the turnover, with extinction rates of 30-50% per million years; other ecological assemblages average less than 30% extinction per million years. Species richness is relatively low (30-50%) throughout much of the Early to Middle Miocene (22-9 Ma), high (80-100%) from the Late Miocene to Early Pleistocene (9-1 Ma), and intermediate since the Early Pleistocene (1-0 Ma). Both extinction and origination are accelerated between 4.0 and 3.0 Ma. Extinction rates are also high between 2.0 and 1.0 Ma. (In the Eastern Pacific, reef corals suffered nearly complete extinction during the Pliocene. The present depauperate fauna includes species of Indo-Pacific as well as Caribbean affinities [Johnson, Budd, and Stemann 1995; Budd, Johnson, and Stemann 1996; Jackson and Budd 1996].) In southern Florida, reef corals show some decline after the Pliocene (Allmon et al. 1996a; Budd, Johnson, and Stemann 1996); exten sive reefs evidently occurred farther north during the Pliocene than they do in Florida today.

marine vertebrates. The late Cenozoic marine vertebrate record is particularly well known in Florida (see Allmon et al. 1996a, and references therein) and forms a basis for comparison with the invertebrate record.

In Florida, Lower Pliocene seabirds are well known (Emslie and Morgan 1994). Particularly noteworthy in this avifauna is the diversity of taxa with modern counterparts that are associated with cold water and upwelling marine systems, such as alcids, gannets, boobies, and cormorants (Allmon et al. 1996a). Breeding (or formerly breeding) seabirds in Florida and the Dry Tortugas are limited today to only eight species, or 47% fewer than in the Early Pliocene.

Among marine mammals, compared with Late Miocene, Pleistocene, and Recent faunas in Florida, abundance and diversity in the Early Pliocene are notably high. Lower Pliocene marine mammals include 10 species of cetaceans (whales and dolphins), dominated by baleen whales, and 4 species of pinnipeds (seals and walruses) (Allmon et al. 1996a). This is in striking contrast to the present marine mammal fauna of Florida. Although over 25 species of cetaceans have been recorded from Recent Florida waters, many of these species are very rare (Layne 1965). Baleen whales in particular are now virtually unknown in Florida (see discussion in Allmon et al. 1996a).

Late Cenozoic Change in the Western Atlantic: Environments the central american isthmus. The emergence of the CAI may have begun to affect ocean circulation between the Atlantic and Pacific in the Middle to Late Miocene; deep water circulation was blocked no later than 3.6 Ma and shallow water circulation was blocked no later than about 3.0 Ma (Coates et al. 1992; Coates and Obando 1996; Collins et al. 1996). The emergence of the CAI led to separation and differentiation of Atlantic and Pacific water masses (Woodruff and Savin 1989; Wright, Miller, and Fairbanks 1991) and to changes in circulation in the Western Atlantic (Allmon et al. 1996a) and perhaps also in the Eastern Pacific (Weaver 1990). Present-day intermediate-depth Atlantic water is younger, more estuarine-influenced, and relatively nutrient-poor, whereas Pacific water is older, more lagoon-influenced, and relatively nutrient-rich. Present-day circulation in the Caribbean, Gulf of Mexico, and along the east coast of the United States is stronger, and oceanic upwelling may be weaker, than before formation of the CAI, whereas upwelling may be stronger on the west coast of Central America. (See Allmon et al. 1996a, for further discussion.)

temperature. There is evidence of significant Northern Hemisphere ice at 3.0-3.4 Ma, but almost all available data indicate that a relatively small ice volume at this time compared to the ice buildup began around 2.5 Ma (references in Allmon et al. 1996b). A growing body of data suggests that sea surface temperatures in the North Atlantic between 3.5 and 3.0 Ma were higher than at present (e.g. Cronin and Dowsett 1996 and references therein). Although the pulse of Northern Hemisphere glaciation that began around 2.5 Ma appears to have been much more significant than earlier Neogene events, it remains not clear whether low latitude sea surface temperatures declined at this time. Specifically, on the Atlantic coast of North America, there is good evidence for cooling at around 2.5 Ma north of Cape Hatteras, but no unequivocal evidence for significant cooling south of there (Allmon et al. 1996a,b).

nutrient supply. Several authors (e.g., Woodring 1966; Vermeij 1980, 1987a,b, 1989, 1990; Stanley 1986a) have suggested changes in the nutrient regimes (and resulting productivity) in the Western Atlantic as an explanation for the extinction of Neogene mollusks in the region. Good circumstantial evidence exists that (at least) local coastal upwelling and associated high productivity existed prior to around 3.0 Ma in the low-latitude Western Atlantic and then declined (Allmon et al. 1996a). In contrast, coastal upwelling and productivity appear to have changed little in the low-latitude Eastern Pacific. Evidence for upwelling in the Western Atlantic prior to 3.0 Ma includes (1) local areas of cooler temperatures in an otherwise warm Late Pliocene (Cronin and Dowsett 1990,1996; Cronin 1991); (2) vertebrate and invertebrate faunal indicators of cool waters or high productivity, or both, amidst otherwise subtropical temperatures in the Late Pliocene Pinecrest Beds of Florida (Allmon et al. 1996a); (3) carbon and oxygen isotopic evidence (Jones and Allmon 1995; Allmon et al. 1996a); (4) widespread phosphogenesis in Florida and the Carolinas throughout the Miocene and into the Early Pliocene (Riggs 1984; Allmon et al. 1996a); (5) much more common accumulation of biogenic silica in the Atlantic prior to 10-11 Ma than at present, indicating higher productivity (Keller and Barron 1983).

A Model for Faunal Evolution in the Late Cenozoic Western Atlantic

If we focus on changes in temperature and nutrient supply and ask whether and how these environmental variables might have affected origination and extinction in late Cenozoic marine faunas of the Western Atlantic, we can use the three-stage framework to construct a simple model connecting environmental and evolutionary change (Allmon, in press; figure 7.3).

Faunal data clearly indicate that both speciation and extinction were ongoing processes in the Late Pliocene Western Atlantic. It is therefore reasonable to search for causal factors that might be connected to both processes. The model presented in figure 7.3 suggests a framework for such a search. Changes in nutrient conditions in the region at this time may have increased rates of isolate formation, and thus speciation, in some taxa, while increasing rates of extinction in other taxa. Some mollusk and coral clades show both enhanced origination and extinction, although it may never be possible to determine which occurred first. If temperature did decrease in mid-low latitudes, this may also have contributed to faunal changes in much the same way. The lack of compelling evidence for temperature change, however, implicates change in nutrient conditions as the environmental factor most closely connected to these faunal patterns.

As more and more data on particular clades become available, they can be analyzed using this model to determine exactly what kinds of processes may have occurred and when they occurred. This is a very different kind of analysis than simply attributing a "regional mass extinction" to temperature decrease and subsequent or approximately synchronous speciation to a black box called "diversification."

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