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Why is an asteroid impact not considered the main cause of the Ordovician-Silurian extinction?
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Ordovician Period
The distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the Ordovician Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.
An asteroid impact is not considered the primary cause of the Ordovician-Silurian extinction because there is no geological evidence to support such a theory. Unlike the K–T extinction event, which is associated with a global layer of iridium—a signature of an extraterrestrial impact—no such concentration of iridium has been identified in the rock layers corresponding to the Ordovician-Silurian boundary.12
Instead, paleontologists generally attribute this mass extinction to major climatic and environmental shifts. The primary drivers are believed to be:
- Global Cooling and Glaciation: The onset of significant glaciation over the African and South American portions of Gondwana triggered a global cooling phase.12
- Sea-Level Fluctuations: The growth of glaciers caused a major drop in sea level, which drained large epicontinental seas and drastically reduced available habitats for marine organisms.12
- Environmental Instability: The subsequent rise in sea levels during the Rhuddanian Age as glaciers retreated further disrupted ecosystems, contributing to the multi-phased nature of the extinction.12