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What evidence suggests that cooling seawater contributed to Cambrian biomere extinctions?
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Cambrian Period
Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the late Cambrian Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.
Geochemical evidence indicates that Cambrian biomere extinctions were likely caused by abrupt drops in seawater temperature. Specifically, researchers have analyzed oxygen isotopes found in the skeletons of bottom-dwelling trilobites located at a biomere boundary in Texas. These isotopic signatures reveal a decrease in water temperature of approximately 5 °C (9 °F) at the boundary.1
Such a significant cooling event would have been lethal to the larvae of many marine invertebrates that were adapted to warmer ocean conditions. Following these extinction events, the shelf environments were typically repopulated by new, low-diversity trilobite faunas that migrated from deeper, cooler off-shelf environments.1