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Why were shallow warm-water marine invertebrates especially affected by Permian extinctions?

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Permian paleogeography — Permian Period Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins near the end of the Permian Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.

Shallow warm-water marine invertebrates were particularly vulnerable during the Permian extinctions due to significant environmental stressors, primarily driven by dramatic temperature increases in ocean surface waters.12

Research suggests that water temperatures reaching 24–32 °C (approximately 75–90 °F) may have exceeded the physiological tolerance of many species, leaving only those specialized for high-temperature environments, such as shallow lagoons, to survive. Additionally, one hypothesis posits that photosynthetic symbionts living within the tissues of certain invertebrates could not withstand these rising temperatures and abandoned their hosts, leading to the decline of those organisms.12

Beyond thermal stress, other factors contributed to the widespread loss of these faunas:

Oceanic Anoxia: A lack of dissolved oxygen, potentially caused by weakened oceanic circulation, likely devastated marine life, especially bottom-dwelling species.3
Sea-Level Fluctuations: The global withdrawal of seas from continental shelves and platforms caused significant habitat loss, leading to extensive evolutionary turnover and extinction among shallow marine faunas.4

These disruptive ecological changes occurred over a protracted period, resulting in the most severe mass extinction in Earth's history.53