Turnoverpulse

Other researchers have proposed similar ideas to coordinated stasis, coining new terms in the process - turnover-pulse and repeating faunas. Elisabeth Vrba (1985, 1993, 1995) developed a turnover-pulse hypothesis, in which a synchronized set of species extinctions and originations in many groups of animals occurs during a limited period owing to major shifts in global climate. She found a similar pattern in the different communities of vertebrates in Pliocene terrestrial environments of southern and eastern Africa (Vrba 1985). Rapid faunal replacement, following a long period of earlier stability, occurred in conjunction with a 10-15°C fall in global temperatures that started about 2.7 to 2.8 million years ago and lasted some 200,000 to 300,000 years. Vrba generalized this pattern as the

'turnover-pulse hypothesis'. In doing so, she emphasized the role of environmental disruption in prompting the transition. In particular, she emphasized the coordinated effects of both extinction and speciation as consequences of extinction by rapid change and removal of habitats favoured by species of the previous fauna, and of origination by fragmentation of habitats and resulting opportunities for speciation by the geographical isolation of allopatric populations.

Repeating faunas

Thomas van der Hammen, a Dutch palynologist, described repeated patterns in the succession of pollen communities from the Late Cretaceous through the late Cenozoic of Colombia, South America (van der Hammen 1957, 1961). Pollen abundances of ferns and angiosperms showed synchronous minima and maxima within three assemblage types designated communities A, B, and, C. Pollen community A was succeeded by community B, and pollen community B by pollen community C. The pattern then repeated with the evolution of a convergent pollen community A. As to the cause of this repeated pattern, van der Hammen looked to climatic changes: community A was adapted to very wet and warm conditions, community B to not quite so wet and warm conditions, and community C to a drier and cooler climate. Using the abundance of the Monocolpites medius group, he inferred temperature cycles showing each community going extinct near temperature minima. A pollen study on the east coast of South America confirmed van der Hammen's observations (Leidelmeyer 1966), and a Jurassic-Cretaceous study in The Netherlands reported a similar cyclical pattern (Burger 1966).

Repeating cycles of climate and sedimentation occur in the latest Eocene to the Pleistocene deposits in Nebraska (Stout 1978; Schultz and Stout 1980). Each cycle runs from a fluvial sequence (wet climate), to a less fluvial sequence (moderate climate), to a sequence with significant aeolian deposition (dry climate), mirroring the cycles of van der Hammen (1961). Larry Dean Martin (1985) related this A-B-C pattern to mammalian faunas of North America, particularly in the replacement pattern of certain convergent carnivore and ungulate adaptive types or ecomorphs (short for 'ecological morphotypes') in Cenozoic North America. Later work has explored the existence of these cycles by analysing the turnover pattern in other North American mammalian ecomorphs. Hippo, dog, bone-crushing dog, dominant artiodactyl herbivore, fossorial rodent, cat and shrew ecomorphs all reflect van der Hammen's cycles (Meehan and Martin 2003, 132-4; Martin and Meehan 2005) (Figure 8.4). Here are details of three of them:

1 Hippo ecomorphs are ambulatory, semi-aquatic herbivores that tend towards gravipor-tal bodies (built for slow terrestrial locomotion owing to a comparatively heavy body weight), whose pattern of dominance turnover shows an A-B-C iteration (Figure 8.4(a)). Coryphodon (Pantodonta: Coryphodontidae) is the first hippo ecomorph in North America and occupies this niche for nearly two complete A-B-C cycles (latest Palaeocene to Middle Eocene). In the succeeding late Eocene A-B-C cycle, members

Figure 8.4 A-B-C patterns in dominance turnover of mammalian ecomorphs in North America during the Cenozoic. (a) Dominant artiodactyl ecomorphs. (b) Fossorial rodent ecomorphs. (c) Cat ecomorphs. (d) Shrew ecomorphs. (e) Hippo ecomorphs. (f) All dog ecomorphs. (g) Bone-crushing dog ecomorphs. Source: Adapted from Meehan and Martin (2003) and L. D. Martin and Meehan (2005).

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