Paleontologists typically treat major episodes of extinction as single and distinct events in which a major environmental perturbation results in a synchronous evolutionary response. Alternatively, the causes of biotic change may be multifaceted and extinction may lag behind the changes ultimately responsible because of nonlinear ecological dynamics. We examined these alternatives for the major episode of Caribbean extinction 2 million years ago (Ma). Isolation of the Caribbean from the Eastern Pacific by uplift of the Panamanian Isthmus was associated with synchronous changes in Caribbean near shore environments and community composition between 4.25 and 3.45 Ma. Seasonal fluctuations in Caribbean seawater temperature decreased 3-fold, carbonate deposition increased, and there was a striking, albeit patchy, shift in dominance of benthic ecosystems from heterotrophic mollusks to mixotrophic reef corals and calcareous algae. All of these changes correspond well with a simple model of decreased upwelling and collapse in planktonic productivity associated with the final stages of the closure of the isthmian barrier. However, extinction rates of mollusks and corals did not increase until 3-2 Ma and sharply peaked between 2 and 1 Ma, even though extinction overwhelmingly affected taxa commonly associated with high productivity. This time lag suggests that something other than environmental change per se was involved in extinction that does not occur as a single event. Understanding cause and effect will require more taxonomically refined analysis of the changing abundance and distribution patterns of different ecological guilds in the 2 million years leading up to the relatively sudden peak in extinction.Isthmus of Panama ͉ paleoenvironments ͉ time-lag ͉ macroevolution ͉ paleocommunities I ncreases in extinction rates are commonly correlated with major changes in environments (1-4). However, inference of cause and effect requires detailed stratigraphic control of the relative timing of events (5-7) as well as independent knowledge of the biological characteristics and fates of different taxa to tease apart the environmental factors responsible (8, 9). In addition, it has become apparent that the traditional paleontological approach to understanding macroevolutionary patterns by measuring temporal ranges of taxa reveals only part of the evolutionary narrative, and that addition of actual occurrence and abundance data significantly broadens our understanding of the ecological underpinnings of biological change (7, 10).These problems are confounded by evidence that ecological and evolutionary responses to both natural and anthropogenic perturbations may not closely coincide. Modern ecosystems commonly exhibit large scale, rapid shifts between alternative community states because interactions among organisms and their environments are nonlinear and governed by critical threshold effects (11-13). These ecological shifts result in dramatic decreases in the relative abundance of taxa associated with displaced communities, a...

Air trapped in bubbles in polar ice cores constitutes an archive for the reconstruction of the global carbon cycle and the relation between greenhouse gases and climate in the past. High-resolution records from Antarctic ice cores show that carbon dioxide concentrations increased by 80 to 100 parts per million by volume 600 ± 400 years after the warming of the last three deglaciations. Despite strongly decreasing temperatures, high carbon dioxide concentrations can be sustained for thousands of years during glaciations; the size of this phase lag is probably connected to the duration of the preceding warm period, which controls the change in land ice coverage and the buildup of the terrestrial biosphere.

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Abstract.-The marine faunas of tropical America underwent substantial evolutionary turnover in the past 3 to 4 million years in response to changing environmental conditions associated with the rise of the Isthmus of Panama, but the ecological signature of changes within major clades is still poorly understood. Here we analyze the paleoecology of faunal turnover within the family Pectinidae (scallops) over the past 12 Myr. The fossil record for the southwest Caribbean (SWC) is remarkably complete over this interval. Diversity increased from a low of 12 species ca. 10-9 Ma to a maximum of 38 species between 4 and3 Ma and then declined to 22 species today. In contrast, there are large gaps in the record from the tropical eastern Pacific (TEP) and diversity remained low throughout the past 10 Myr. Both origination and extinction rates in the SWC peaked between 4 and 3 Ma, and remained high until 2-1 Ma, resulting in a 95% species level turnover between 3.5 and 2 Ma. The TEP record was too incomplete for meaningful estimates of origination and extinction rates. All living species within the SWC originated within the last 4 Myr, as evidenced by a sudden jump in Lyellian percentages per faunule from nearly zero up to 100% during this same interval. However, faunules with Lyellian percentages near zero occurred until 1.8 Ma, so that geographic distributions were extraordinarily heterogeneous until final extinction occurred. There were also striking differences in comparative diversity and abundance among major ecological groups of scallops. Free-swimming scallops constituted the most diverse guild throughout most of the last 10 Myr in the SWC, and were always moderately to very abundant. Leptopecten and Argopecten were also highly diverse throughout the late Miocene and early Pliocene, but declined to very few species thereafter. In contrast, byssally attaching scallops gradually increased in both diversity and abundance since their first appearance in our samples from 8-9 Ma and are the most diverse group today. Evolutionary turnover of scallops in the SWC was correlated with strong ecological reorganization of benthic communities that occurred in response to declining productivity and increased development of corals reefs.