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Paleontologist whose research includes early life and physiological interpretations of mass extinction.

The large impact associated with the Cretaceous–Palaeogene boundary and its mass extinction. Atmospheric dust, aerosols and soot can connect an impact to reduced sunlight, suppressed photosynthesis and disruption of food webs; these environmental mechanisms distinguish the trigger from its biological effects.

Systematic variation in extinction probability with biological or ecological traits. Associations can change between events and taxonomic levels: geographic range, body size and sexual dimorphism have each been linked to survival differences in particular datasets, but none is a universal predictor across all mass extinctions.

A geological period preceding the Silurian. Its late interval includes a major mass extinction associated with glaciation, changes in eustatic sea level change and later marine oxygen stress.

Connects mass extinction selectivity with interacting warming, oxygen loss, sulfide and elevated carbon dioxide; it distinguishes environmental forcing from physiological mechanisms.

Plate tectonics changes the environments in which natural selection, gene flow and extinction operate. It influences both the persistence of habitable conditions and the geographical and chemical opportunities for evolution. It does not prescribe a unique sequence of biological innovations.

Separation of continents by continental rifting divides populations and reduces gene flow, promoting allopatric speciation. Continental collision or newly exposed land connections can reunite populations, permit dispersal and introduce competitors or predators. These mechanisms connect biogeography to continental configuration. Changing the amount and fragmentation of shallow continental shelf habitat can also change the opportunities for diversification of marine organisms.

Over long intervals, volcanic outgassing, silicate weathering, carbonate burial and subduction exchange carbon among the interior, atmosphere and ocean. In a suitable regime, warmer, wetter conditions increase silicate weathering, remove atmospheric carbon dioxide and weaken the greenhouse effect. This negative feedback can support long-lived habitability, though it depends on exposed fresh rock, available water and reaction rates.^[ref] Mountain building changes precipitation, erosion and delivery of nutrients to the ocean, affecting primary production and organic carbon burial.

The opening and closure of ocean gateways changes ocean circulation, transport of heat and the supply of oxygen to deep water. Together with movement of land through different latitudes and changes in sea level, this reorganizes climate and habitat. Enhanced organic carbon burial can change atmospheric oxygen over long intervals, while a poorly ventilated ocean can develop ocean anoxia. Such chemical changes alter which forms of metabolism are viable.

Hydrothermal circulation at spreading ridges and through fractured rock supplies chemical disequilibrium and mineral surfaces. For example, serpentinization can produce hydrogen usable by chemolithotrophy when suitable electron acceptors are available. This supplies a possible setting for abiogenesis and early ecosystems; it does not establish that life originated there.

There can also be abrupt disruption. Rapid emplacement of a large igneous province can change climate through gas release; sulfur-bearing aerosols can initially cool, whereas sustained carbon dioxide release can warm and contribute to ocean acidification and ocean anoxia. Such perturbations can contribute to mass extinction, followed by adaptive radiation among survivors. The Deccan Traps exemplify the scale of volcanism involved, but their association with the Cretaceous–Palaeogene boundary does not justify attributing the entire biological crisis to volcanism alone. Both gradual environmental reorganization and rare rapid perturbations belong in the explanation.

Plate tectonics changes the environments in which evolution operates: it redistributes habitats, supplies and buries nutrients, and helps regulate climate. It does not prescribe the direction of evolution, and its presence neither guarantees life nor demonstrates that life could not persist under another tectonic regime.

Long-term environmental regulation. Seafloor spreading and subduction exchange material between the mantle, crust, ocean and atmosphere. Volcanism and metamorphism release carbon dioxide, while exposure of fresh silicate minerals by tectonic uplift and erosion can promote chemical weathering. Coupled silicate weathering and carbonate burial remove carbon dioxide from the surface system. In a simplified net reaction,The carbonate-silicate cycle can provide a negative feedback: warming and greater runoff can accelerate silicate weathering, lowering atmospheric carbon dioxide and weakening the greenhouse effect. This requires available fresh rock and liquid water; a supply-limited chemical weathering regime need not respond strongly to warming. Plate tectonics therefore helps sustain some long-lived habitable environments without acting as an infallible thermostat.^[ref]

Nutrients and oxygenation. Tectonic uplift, erosion and hydrothermal circulation supply nutrients such as phosphorus and iron to surface environments. Changes in continental configuration, continental shelf area and ocean circulation alter primary production, nutrient recycling and ocean anoxia. Burial of organic carbon and reduced sulfur can leave an oxidizing surface budget, whereas reduced volcanic gases and freshly exposed reduced rock consume oxidants. Thus plate tectonics can influence atmospheric oxygen and the ecological opportunities for aerobic respiration, but the sign depends on competing fluxes rather than simply on the amount of volcanism.

Geographical isolation and environmental diversity. Continental rifting separates populations, favoring allopatric speciation; continental collision and the emergence of land bridges can reconnect populations, enabling dispersal and competition. Opening or closing ocean gateways changes ocean circulation and heat transport. Mountain building creates elevational habitats and rain shadows; changing continental shelf area changes shallow-water habitat. These processes can promote diversification, eliminate habitats or both. A proposed biological response should be tested against independently dated paleogeography and the fossil record, accounting for unequal preservation.

Abrupt disturbances. Large episodes of volcanism, including a large igneous province, can rapidly perturb the carbon cycle, ocean acidification and ocean anoxia; sulfur dioxide aerosols can instead produce shorter cooling episodes. Such disturbances may cause mass extinction, followed by ecological release and diversification of survivors. A mantle plume need not originate at a plate boundary, so association with a tectonically active planet is not proof that horizontal plate motion directly caused every eruption. The relevant reasoning links a dated geological forcing to its environmental effects and then to biological selectivity, rather than attributing all biological crises to plate tectonics.

Over billions of years, plate tectonics can repeatedly create habitable environments and new selective pressures. Nevertheless, models allow carbon cycling and habitable climate on some stagnant-lid planets. This makes plate tectonics a plausible contributor to sustained environmental diversity, not a demonstrated universal requirement for evolution.^[ref]

Extinction selectivity is systematic variation in the probability or rate of extinction among lineages with different traits, rather than trait-independent loss. Traits may be biological, ecological or geographical: for example, survival may differ between narrow-ranging and widespread taxa within the same extinction interval.

A test needs an explicit denominator of taxa at risk, a defined taxonomic level and time window, and a specified trait measurement. Regional disappearance is not automatically global extinction. Nor is an association necessarily a causal mechanism: traits covary with habitat, abundance and preservation, and their effects can change between background intervals and mass extinctions.

One fossil-record study is Selectivity of end-Cretaceous marine bivalve extinctions by David Jablonski and others, which compared survival of marine bivalve genera across the end-Cretaceous mass extinction against their pre-event attributes. Geographically widespread genera had lower extinction intensity than narrowly distributed ones. In the same analysis, body size, burrowing versus exposed life position, and shelf bathymetric position were not significant selectors; the apparent feeding-mode contrast was concentrated in particular groups rather than establishing a universal feeding rule.^[ref]

The logic of the test is to define the pre-event genera at risk, classify their geographic ranges using pre-event occurrences, then compare the fractions becoming extinct. Using pre-event rather than surviving ranges avoids measuring the predictor after the outcome. A two-by-two contingency table gives an elementary broad/narrow-range comparison; a logistic regression can instead retain a continuous range measure and include correlated covariates. These are explanations of the inferential design, not a claim that this paper used that exact regression specification.

A persuasive interpretation must address unequal rock exposure, collecting intensity, taxonomic revision, incomplete occurrence records and the difference between local loss and global extinction. The important result is selective survival in a real fossil dataset and the failure of several plausible alternative predictors in that setting. It is observational evidence: geographic range can correlate with unmeasured ecological properties, so the association alone does not prove a unique kill mechanism.

A mass extinction is a geologically brief, geographically widespread loss of a large fraction of species across multiple major groups, substantially exceeding background extinction rates. Severity, global extent and rapidity distinguish it from the disappearance of a local population or the ordinary loss of individual lineages. A threshold near $75\%$ of species is often used operationally, but the qualitative definition does not depend on one universal numerical cutoff. The duration is short on geological timescales, and need not mean that every species disappeared simultaneously in a single day.

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.

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]

Extinction selectivity means that the probability of extinction depends systematically on biological or ecological traits, rather than being equal across all species. An empirical association needs a specified event, group and taxonomic level; a trait protective in ordinary background turnover may not protect during every mass extinction.

Geographic range: at the end-Cretaceous crisis, geographically widespread marine bivalve genera had lower extinction intensity than narrowly distributed genera. This makes range breadth a demonstrated selective trait for that dataset. The same analysis found no clear selectivity by body size, so that result should not be generalized to every trait or event.^[ref]

Body size: large-bodied mammals suffered disproportionately during late-Pleistocene and later Quaternary losses, including the loss of many giant herbivores and carnivores. A global analysis over the last $125000$ years found strong size-selective losses across continents and trophic modes. Large size can correlate with slow reproduction, high resource demand and exposure to human hunting, but the observed size pattern and its possible mechanisms should be distinguished.^[ref]

Sexual dimorphism and reproductive investment: a study of fossil cytheroid ostracods found much higher estimated extinction rates in species with stronger male–female carapace dimorphism, interpreted as greater male investment in reproduction. This supplies a third empirical trait association; the observed relationship is not a universal claim that sexual selection is always harmful.^[ref]

The examples concern different time intervals and levels, including genera in the first example and species in the third. They demonstrate extinction selectivity without assuming one set of trait effects is universal across all crises.

A large volcanic and intrusive province associated in time with the end-Permian crisis. Gas release and interactions with surrounding sediments provide mechanisms for linked warming, ocean chemical change and physiological stress during mass extinction.