Taxonomic diversity gradients through geological time

Abstract

Abstract. There is evidence from the fossil record to suggest that latitudinal gradients in taxonomic diversity may be time‐invariant features, although almost certainly not on the same scale as that seen at the present day. It is now apparent that both latitudinal and longitudinal gradients increased dramatically in strength through the Cenozoic era (i.e. the last 65 my) to become more pronounced today than at any time in the geological past. Present‐day taxonomic diversity gradients, in both the marine and terrestrial realms, are underpinned by the tropical radiations of a comparatively small number of species‐rich clades. Quite why these particular taxa proliferated through the Cenozoic is uncertain, but it could be that at least part of the explanation involves the phenomenon of evolutionary escalation. This is, in essence, a theory of biological diversification through evolutionary feedback mechanisms between predators and prey; first one develops an adaptive advantage, and then the other. However, there may also have been some form of extrinsic control on the process of tropical diversification, and this was most likely centred on the phenomenon of global climate change. This is especially so over the last 15 my Various Late Cenozoic (Neogene) vicariant events effectively partitioned the tropics into a series of high diversity centres, or foci. It has been suggested that, in the largest of these in the marine realm (the Indo‐West Pacific or IWP centre), a critical patterns of islands acted as a template for rapid speciation during glacioeustatic sea level cycles. The same process occurred in the Atlantic, Caribbean and East Pacific (ACEP) centre, though on a lesser scale. Tropical terrestrial diversity may also have been promoted by rapid range expansions and contractions in concert with glacial cycles (a modified refugium hypothesis). We are beginning to appreciate that an integrated sequence of Neogene tectonic and climatic events greatly influenced the formation of contemporary taxonomic diversity patterns.

Why are there so many species in the tropics?

“…The DMP offers a way that the historical ‘out of the tropics’ dynamics and phylogenetic niche conservatism of lineages can be reconciled with the fact that the LDG is an ancient pattern, reflecting a steady-state relationship to climate dating back hundreds of millions of years (e.g. Stehli et al ., 1969; Crane & Lidgard, 1989; Crame, 2001). Needless to say, the DMP is consistent with the above suggestions that the LDG is due in large part to the temperature dependence of biotic interactions and Red Queen coevolution.…”

“…And the pattern is ancient, apparent in the fossil record dating back hundreds of millions of years (e.g. Stehli et al ., 1969; Crane & Lidgard, 1989; Crame, 2001). Even as the patterns have become clearer, however, the explanations have remained elusive and controversial.…”

Hemispheric Asymmetries in Biodiversity—A Serious Matter for Ecology

“…It is one of the most venerable, well-documented, and controversial large-scale patterns in macroecology (Willig et al 2003). Equatorial peaks in species richness have characterised the planet since the Devonian (408–362 million years ago) (Crame 2001) and are typical of a wide range of both terrestrial and marine plants and animals (Gaston 1996; Willig et al 2003). Despite the fact that this pattern has been documented since the late 1700s, sustained interest in both the regularity of the pattern and its likely underlying mechanisms is relatively modern.…”

Latitudinal gradient of taxonomic richness of ammonites in the Kimmeridgian-Volgian in the Northern Hemisphere

“…Often in these cases, the peak of taxonomic diversity, as in the extant fau nas, is not centered at the equator (Crame, 2001(Crame, , 2002Cecca et al, 2005). For instance, in Jurassic bivalves, the maximum TR is observed in paleolatitude 33°-40° (Crame, 2001).…”