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Seilacher A.,Engelfriedshalde 25 | Seilacher A.,Yale University
Historical Biology | Year: 2013

Mass extinctions of varying magnitude prune the continuous diversification predicted by Darwinian evolutionary processes. They are caused by events that are too rare to become adaptatively accommodated. Their effects depend not only on the nature and magnitude of the triggering event but also on the state of the biosphere at the particular time. This is most clearly shown by the existence of Golden Ages preceding all Phanerozoic mass extinctions. These coincide with greenhouse periods, in which doomed clades gave rise to heteromorphs, deviating in strange ways from established bauplans. When critically examined, the seemingly 'decadent' morphologies of Schindewolf's 'typolytic stages' turn out to have been highly functional. The paradoxical link between adaptive peaks and evolutionary failure can now be explained. Specialisation tends to increase vulnerability (1) by narrowing niches and (2) by the retention of clade-specific conservative features that happen to become fatal Achilles' Heels for entire clades in the face of a particular perturbation. Following extinctions, the availability of open niches favoured relatively rapid diversification of more innovative clades and their rise to ecological dominance (Schindewolf's 'typogenetic stage'). Although the long-term changes can be observed only in the fossil record, Golden Biotopes in the present biosphere show that the Darwinian process may also be promoted by ecological isolation. As a result, clade histories do resemble individual biographies, but for ecological rather than orthogenetic reasons. This insight may help us to deal with the present mass extinction caused by our own species. © 2013 Copyright Taylor and Francis Group, LLC. Source


Seilacher A.,Engelfriedshalde 25 | Mrinjek E.,University of Zagreb
Swiss Journal of Geosciences | Year: 2011

In contrast to Solnhofen-type conservation Lagerstätten, the platy limestones of the Benkovac Stone of the Eocene of Croatia contains a variety of trace fossils. They belong to the Nereites ichnofacies, characterized by a suite of highly specialized ichnogenera, turbiditic preservation, and distinctive pre-event versus post-event communities. In siliciclastic rocks, this ichnofacies stands for turbiditic deep-sea sediments. Also unusual is the lack of body fossils. The only exceptions are occasional plant debris and Bencovacinaconcentrica. n. gen., n. sp., which is affiliated with xenophyophorian Foraminifera. These giant protozoans survive on present deep-sea bottoms, but they were also common on the shallow-marine biomats in Ediacaran times. These findings contradict the current shallow-marine interpretation, which is based on the regional geologic setting and sedimentary structures typical for tempestites. Thus, the Benkovac Stone would be an ideal place to test the reliability of ichnological versus sedimentological depth criteria in an entirely calcareous sequence. © 2011 Swiss Geological Society. Source


Seilacher A.,Engelfriedshalde 25 | Thomas R.D.,Franklin And Marshall College
Lethaia | Year: 2012

Secondary soft-bottom dwellers share the problem that their ancestors, attached to hard substrates, had lost their mobility. On soft substrates, only a limited number of alternative tricks are available to maintain sessile organisms in life position or to right them following disturbance. Consequently, convergent adaptations have emerged in unrelated members of this ecological guild. Those of favositid corals are of particular interest because, on account of their small polyps, they were always colonial. A comparative analysis of forms adopted by favositids on soft substrates shows that key elements of their adaptive paradigms could have been achieved by self-organization. This arose without centralized control by means of inherited reaction norms of individual polyps to local environmental conditions. The unique spiral growth habit of Favosites turbinatus Billings and its emergent individuality at the colony-level of organization are explained in these terms. 'Suicidal' lids entombed altruistic marginal polyps, forming a secondary epitheca as the growing colony settled into the sediment. These lids also record the size and spacing of soft tentacles in this species. □Coloniality, Devonian, Favositida, Favosites turbinatus, morphogenesis, self-organization, Tabulata. © 2011 The Authors, Lethaia © 2011 The Lethaia Foundation. Source


Seilacher A.,Engelfriedshalde 25
Palaeontology | Year: 2013

Evolution is seen as a future-blind process driven by ecology. At geological time scales (macroevolution), complex ecosystems (including parasites) have caused periods of relative stasis, while evolutionary change may be fostered by two kinds of exceptional situation: (1) after an extinction event, the availability of open niches gave innovative groups the chance to gain ecologic power; (2) a climatically controlled Golden Age before the event allowed Darwinian optimization to exceed the constructional limits of established bauplans. Increasing specialization, however, automatically raised the vulnerability towards biologically unforeseeable events, because niches become narrower, and conservative (plesiomorphic) features happen to become fatal 'Achilles' heels' under unusual conditions. The interplay between Darwinian diversification and extinctional counter-evolution results in cascading clade histories that resemble individual life cycles without any teleonomic underpinnings. © The Palaeontological Association. Source

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