Biotic Transitions in Global Marine Diversity

Abstract

Long-term transitions in the composition of Earth's marine biota during the Phanerozoic have historically been explained in two different ways. One view is that they were mediated through biotic interactions among organisms played out over geologic time. The other is that mass extinctions transcended any such interactions and governed diversity over the long term by resetting the relative diversities of higher taxa. However, a growing body of evidence suggests that macroevolutionary processes effecting biotic transitions during background times were not fundamentally different from those operating during mass extinctions. Physical perturbations at many geographic scales combined to produce the long-term trajectory of Phanerozoic diversity. P hanerozoic trends in global marine faunal diversity continue to be of central concern to researchers interested in the history of life. After the establishment of metazoans in the Vendian and Cambrian, Phanerozoic patterns (1, 2) exhibited three easily observed characteristics (Fig. 1A): (i) intervals of increasing diversity focused mainly in the early Paleozoic and the postPaleozoic, (ii) intervals of decreased diversity and rebound associated with mass extinctions, and (iii) broad transitions in dominance among higher taxa. These attributes, and possible relationships among them, have led to two different theories (3, 4) to explain the major features of Phanerozoic diversification. One theory holds that mass extinctions constitute a distinct class of phenomena that transcended ongoing evolutionary patterns and processes operating at other times and governed long-term, global biotic transitions through wholesale removal of incumbents (5,6). The other theory is that mass extinctions played only minor roles in governing longterm diversification that was instead linked to biotic interactions over extended intervals of geologic time (4, 7).However, recent work on several seemingly disparate topics points to a third possibility: that diversification represents the summed accumulation of physically mediated transitions that took place rapidly in geographically limited venues around the world. Despite the abruptness of faunal changes locally, the apparent rates of these transitions were damped in global diversity transitions because their timing varied from venue to venue around the world, depending on the local or regional onset of physical conditions that induced the transitions. In this context, mass extinctions, although capable of mediating diversity over the long term, constitute the largest end members of a continuum of physical mechanisms that induced appreciable, rapid biotic turnover on geologic time scales (8). Here I provide a theoretical rationale for this third alternative and review evidence for episodic biological turnover on local and regional scales, linked to physical environmental transitions.

Long-Term Stasis in Ecological Assemblages: Evidence from the Fossil Record

“…This pattern may reflect decreasing susceptibility of ecological communities or their component taxa to turnover events caused by environmental perturbations of sufficient magnitude. Whether or not this interpretation is corroborated by future work, it makes the point that the fossil record can provide information on intermediate scales that connect ecological and biogeographic processes to paleontological patterns at Phanerozoic scales (Miller 1998).…”

Loss of speciation rate will impoverish future diversity

“…Each such event appears to have generated no more than 10 my of response in species diversity. Most of the rest of the Phanerozoic Eon is a record of long plateaus interspersed by quick episodes of extinction and recovery (28,29). Thus, I estimate that species diversity has spent at least 90% of the past 500 my near some steady state.…”

Survival without recovery after mass extinctions

“…Many aspects of diversification, extinction, and recovery seem to have a spatial component (e.g., 19). A preliminary test for geographic concentrations of DCWs among K-T molluscan genera can be performed by using the four regional Paleocene time-series studied in detail by Jablonski (20): the Gulf Coast of North America, northern Europe, northern Africa, and India͞Pakistan.…”

“…Such changes can affect even those taxa that crossed the extinction boundaries unscathed, but should take the largest toll on clades that have already suffered taxonomic and͞or spatial bottlenecks. Temporal coincidence does not equal causation, but causality can be tested by partitioning environmental perturbations and biotic changes spatially (19,44): if regionally restricted environmental changes prevent the postextinction recovery of taxa, then a significant subset of DCWs should be endemic to environmentally perturbed regions.…”