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Joined 2026-09-21 Message
Incoming links: Ordovician
A mass extinction is driven by changes that overwhelm organisms' physiological tolerances or destroy their food and habitat systems. Distinguish the initiating forcing from the linked environmental mechanisms: one event can generate several contributors.
- Large extraterrestrial impact. At the Cretaceous–Palaeogene boundary, the Chicxulub impact injected dust, aerosols and soot that reduced incoming sunlight and photosynthesis, disrupting food webs; rapid heating, fires and subsequent cooling added stresses. The impact is the trigger, while darkness and loss of primary production are proximate mechanisms.
- Large-scale volcanism. Eruption and intrusion of large igneous provinces, notably the Siberian Traps around the end-Permian crisis, release or mobilize gases and can produce rapid climate change. Carbon dioxide favors warming; sulfur-bearing emissions can cause shorter cooling and acid deposition. The outcome depends on rate, duration and interacting reservoirs, rather than eruption volume alone.
- Ocean anoxia and euxinia. Warming lowers oxygen solubility and can strengthen stratification; altered nutrient supply increases oxygen demand. Oxygen loss removes habitable marine volume, while sulfide in anoxic waters can be toxic. These processes contributed to the end-Permian marine crisis and can reinforce warming-driven mortality.
- Ocean ocean acidification and elevated carbon dioxide. Rapid addition of carbon dioxide changes seawater carbonate chemistry and can impair biomineralization, while high carbon dioxide also challenges physiological acid–base regulation. Susceptibility varies with physiology; this is a linked mechanism rather than proof that every calcifying group responds identically.
- Rapid cooling and eustatic sea level change. Late Ordovician glaciation and regression reduced shallow-marine habitat and changed temperatures and circulation; subsequent warming and oxygen loss introduced a different set of stresses. Habitat loss and climate change can therefore contribute in successive phases of one crisis.
These examples supply more than four drivers or contributors. The historical effects of volcanism, warming, oxygen loss and elevated carbon dioxide can act together rather than compete as mutually exclusive explanations.[ref]
Paleophysiology and end-Permian mass extinction by Andrew H. Knoll and others. Abstract and introduction: linked physiological effects of warming, anoxia, sulfide and hypercapnia
Silurian Created 2026-10-20 Updated 2026-11-05
The Silurian is a geological period between the Ordovician and the Devonian. Its rocks preserve evidence relevant to the diversification of jawed vertebrates and early terrestrial ecosystems.
A Silurian placoderm with marginal jaw bones resembling those of Osteichthyes illustrates its evolutionary importance: a fossil's combination of characters can challenge a simple sequence in which each major group acquires a wholly separate body plan.[ref]
A Silurian placoderm with osteichthyan-like marginal jaw bones by Min Zhu and others. 2013, character combination in Entelognathus