engleski

End of gigantism in tropical seas by cooling: End-Guadalupian (Permian) extinction of the photosymbiotic tropical trio

The unique Permian bivalve family Alatoconchidae has aberrant shell forms and extraordinary size up to 1 m, representing the largest bivalve group in the Paleozoic, Their occurrence is reported sporadically from Lower-Middle Perman shallow marine carbonates in 9 areas in the world (Tunisia, Craotia, Oman, Iran, Afghanistan, Thailand, Malaysia, The Philippines and Japan) that cover low-latitudes of both the Tethyan and Panthalassan domains. Alatoconchids almost always occurred in a close association with large-tested fusulines (Verbeekinidae) and/or rugose corals Waagenophyllidae of the typical Tethyan assemblage, suggesting their preferential adaption to shallow warm-water (tropical) environments. This "tropical trio" (Alatoconchidae, Verbeekinidae and Waagenophyllidae) became extinct either during the late Guadalupian or around the Guadalupian-Lopingian boundary (G-LB). Their intimate association and occurrence range suggests that these 3 taxonomically distinct clades may have shared not only a common habit but also a common cause of extinction. The shell structure of alatoconchids suggests their symbiosis with photosynthetic organisms (algae + cyanobacteria) in order to maintain their large body size that required high energy-consuming metabolism in contrast to smaller forms. The Alatoconchidae attained their largest size in the Wordian (Middle Guadalupian), probably maximizing the benefits of photosymbiosis. The subsequent extinction of the warm-water-adapted "tropical trio" both in Tethys and Panthalassa positively supports the concept that a critical cooling took place on a global scale, including low-latitude oceans. The end of gigantism in fusulines and bivalves in the Capitanian (Late Guadalupian) was likely caused by the collapse of photosymbiotic systems during a temporary temperature drop of seawater (Kamura cooling event) coupled with eutrophication that was detrimental to the tropical fauna adapted particularly to oligotrophic condition. Gigantism of bivalves occurred several times in the Phanerozoic ; e.g., Siluro-Devonian, Permian, Triassic-Jurassic and Jurassic-Craataceous, mostly in warm periods. The sea-level change in the Phanerozoic apparently synchronized with the intermittent rise and decline of bivalve gigantism, suggesting that the gigantism in low-latitudes may serve as a potential monitor of global warming/cooling in the past.

End-Guadalupian extinction; gigantism; photosymbiotic trio

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