(g) Bone-crushing dog ecomorphs
of the family Amynodontidae (order Perissodactyla) dominate the hippo ecomorph. At this time, anthracotheres (order Artiodactyla) are present, but they do not dominate until after amynodonts become nearly extinct. Anthracotheres dominate the next two cycles until they nearly become extinct at the beginning of the Miocene. Members of Perissodactyla again dominate the next two cycles of the Miocene, but in these cycles by Rhinocerotidae. One genus of hippolike rhino (Teleoceras) survives to the latest Miocene, but afterward that ecomorph niche remains vacant in North America.
2 Dog ecomorphs (fox, raccoon dog, coyote, wolf, and hyena) may be ambush or pursuit predators and range from omnivorous to hyper-carnivorous. The first dog ecomorphs, which dominate the late Eocene cycle, are members of Hyaenodontidae (Creodonta). Except possibly amphicyonids (order Carnivora), no clearly dominant dog ecomorph exists in subcycle A of the latest Eocene. Since the Oligocene, the family Canidae (true dogs) has dominated these niches. The canid radiation is particularly striking in that each subfamily began with a fox ecomorph (Hesperocyon, Cormocyon, Leptocyon), radiated to a similar diversity, and filled the coyote/wolf, raccoon dog, and hyena ecomorphs (Figure 2(b)). Hesperocyonine canids (Carnivora) dominate in subcycles B and C of the early Oligocene. Hesperocyonines also dominate the next cycle and overlap with borophagines. Borophagines dominate the next two cycles of the Miocene. In the Late Miocene subcycle C, canines begin to dominate and continue through the Holocene as the only dog ecomorphs.
3 A subset of dog-like carnivores, bone-crushers (striped hyena, spotted hyena, and dire wolf ecomorphs), reflects an A-B-C pattern in dominance turnover. Members of Hyaenodontidae dominate for two cycles in the late Eocene to early Oligocene. Hesperocyonines then evolve bone-crushers, dominating the late Oligocene to earliest Miocene. Amphicyonids dominate the early to middle Miocene cycle and borophagines replace them in the middle Miocene. Borophagines dominate most of the next cycle, going extinct in the Pliocene. A canine (dire wolf) was the common bone-crusher in the Pleistocene of North America, but the diversity of this ecomorph was depauperate at this time.
The causes of coordinated stasis are debatable. Two chief causes seem credible - some kind of ecological locking mechanism and climatic change (e.g. Gould 2002, 920-1).
Vrba's explanation for turnover-pulses and Meehan and Martin's explanation for repeating faunas rest on climatic change. On the other hand, it is possible that communities resist change for long periods because they become 'locked' ecologically. Ecological locking is a 'mechanism by which ecological interactions prevent evolutionary change, resulting in long-lasting, stable systems capable of resisting some types of disturbance' (Morris et al. 1995, 11,272).
Whether or not ecological locking be correct, it is not the only possible explanation of the pattern of coordinated stasis. To be sure, the pattern of coordinated stasis need not imply that all species within a community are interdependent (DiMichele et al. 2004, 294). It is possible that intermittent environmental change could affect a host of evolutionary lineages that are independent of each other. If this were the case, a pattern of coordinated stasis would reflect the timing and severity of episodic environmental changes. Moreover, samples from a coordinated stasis unit need not all display identical species composition or abundance - there is room for environmental variability within a biofacies. No, synchro nized turnover breaking in upon long-lasting periods of community-wide morphological stasis define the pattern. Even so, although the pattern of coordinated stasis could simply result from episodic environmental change, some authorities interpret it as 'reflecting the influence of ecological interactions on the evolutionary trajectory of the lineages within a community' (DiMichele et al. 2004, 294).
Some critics have challenged coordinated stasis because it allegedly implies strong species interactions as a cause of the pattern (e.g. Buzas and Culver 1994; Patzkowsky and Holland 1999). Others doubt that faunas supposedly displaying coordinated stasis were proper ecological communities. In addition, some have argued forcefully that the putative patterns of persistence need testing by statistical means to establish just how much change is acceptable in assemblages that are meant to be unchanging through time (Bennington and Bambach 1996; Bambach and Bennington 1996). John Alroy (1996) carried out a statistical analysis of an extensive compilation of mammalian stratigraphical ranges and determined that the expectations of the coordinated stasis model, such as pulse-like turnover of species, did not hold. Similarly, a fresh look at the fossil data for central New York questions the existence of co-ordinated stasis at this site (Bonuso et al. 2002). Instead of using presence and absence data for species, Nicole Bonuso and her colleagues looked at relative abundance data (the abundance of all the species present over a 6 million year period) based on 38,000 specimens (Figure 8.5). In the area tested, highly controlled sampling techniques and rigorous statistical analysis, applied to a single and well-defined lithofacies (a medium-grey, non-calcareous shale representing an outer shelf environment), failed to support the coordinated stasis model, with a greater variability in species taxonomy and ecology through time than would be expected if coordinated stasis had occurred. The most abundant species did last the entire time-span, as coordinated stasis would demand, but the less common species showed more variability. Moreover, the abundant species represented only a few of all the species represented in the dataset. So, coordinated stasis seems to hold for the most abundant species but not for the less common ones, which seem to come and go through time independent of each other.
Over thirty years' work on the vertebrate faunas of northern Pakistan has generated a 10-million-year record of stasis and change in the Miocene mammalian community of the region (Barry et al. 2002). The intensively sampled Siwalik deposits (more than 40,000 fossil specimens) provide a temporal resolution of 100,000 years between 10.7 and 5.7 million years ago. This may be the finest practicable level of resolution for long sequences of vertebrate-bearing strata in any region, and allows a full examination of evolutionary and ecological change in relation to environmental factors (Barry et al. 2002). Analysis of a large suite of 115 mammal taxa showed moderately high and persistent levels of background turnover, running at 50-60 per cent, over a period of 5 million years. Three separate short-term turnover events, which also changed the nature of the mammalian community, ride upon the background turnover. The first event was the extinction of many long-lived taxa that lived in southern Asia before their demise 10.3 million years ago. Just 500,000 years separates the second and third events, which occurred at 7.8 and between 7.3 and 7.0 million years ago during a time of independently documented climate change toward intensified monsoons and the spread of grassland habitats (Dettman et al. 2000). John C. Barry and his colleagues (2002) concluded that their studies fail to support the notion of coordinated stasis and the idea of environmentally driven turnover events as the dominant mode of faunal change through time. However, William A. DiMichele and his colleagues (2004) disputed this conclusion. They noted turnover events in the Siwalik record run at three to 13 times the expected rate relative to the background turnover (background average of 1.5
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