It has been suggested variously that extinction events occurred periodically, every 26 to 30 million years,[54][55] or that diversity fluctuates episodically every ~62 million years.[56]Various ideas attempt to explain the supposed pattern, including the presence of a hypothetical companion star to the sun,[57][58]oscillations in the galactic plane, or passage through the Milky Way's spiral arms.[59] However, other authors have concluded the data on marine mass extinctions do not fit with the idea that mass extinctions are periodic, or that ecosystems gradually build up to a point at which a mass extinction is inevitable.[5] Many of the proposed correlations have been argued to be spurious.[60][61]Others have argued that there is strong evidence supporting periodicity in a variety of records,[62]and additional evidence in the form of coincident periodic variation in nonbiological geochemical variables.[63]Templat:Phanerozoic biodiversityMass extinctions are thought to result when a long-term stress is compounded by a short term shock.[64] Over the course of the Phanerozoic, individual taxa appear to be less likely to become extinct at any time,[65] which may reflect more robust food webs as well as less extinction-prone species and other factors such as continental distribution.[65]However, even after accounting for sampling bias, there does appear to be a gradual decrease in extinction and origination rates during the Phanerozoic.[5] This may represent the fact that groups with higher turnover rates are more likely to become extinct by chance; or it may be an artefact of taxonomy: families tend to become more speciose, therefore less prone to extinction, over time;[5] and larger taxonomic groups (by definition) appear earlier in geological time.[66]

It has also been suggested that the oceans have gradually become more hospitable to life over the last 500 million years, and thus less vulnerable to mass extinctions,[note 1][67][68] but susceptibility to extinction at a taxonomic level does not appear to make mass extinctions more or less probable.[65]