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What evidence does not support an asteroid impact as the main cause of the Permian extinction?

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Line graph showing the number of marine animal families from the Precambrian era to the present, with illustrations of key groups such as trilobites, ammonoids, marine reptiles, and diatoms. The graph highlights major extinction events and a rise to 1,900 living families today. — Permian extinction The diversity of marine animal families since late Precambrian time. The data for the curve comprise only those families that are reliably preserved in the fossil record; the 1,900 value for living families also includes those families rarely preserved as fossils. The several pronounced dips in the curve correspond to major mass-extinction events. The most catastrophic extinction took place at the end of the Permian Period.

The hypothesis that an asteroid or bolide impact caused the Permian extinction remains a subject of significant scientific debate, primarily because it lacks the consistent physical evidence found in other extinction events, such as the end-Cretaceous impact.12

Unlike the evidence associated with the end-Cretaceous event, there is no consistent geological record at the Permian-Triassic boundary to support an impact hypothesis. Specifically, researchers have not found an anomalous presence of iridium or associated shocked quartz—quartz grains that have been subjected to the extreme temperatures and pressures characteristic of an impact shock. Furthermore, while some scientists have proposed that a large icy meteorite could have triggered sudden cooling, this theory also lacks supporting physical evidence.23

In contrast to the impact hypothesis, many researchers point to more gradual or sustained environmental shifts, such as volcanic activity from the Siberian flood basalts, which likely contributed to oceanic anoxia and significant disruptions to the carbon cycle.42