End-Guadalupian extinction of the Permian gigantic bivalve Alatoconchidae: End of gigantism in tropical seas by cooling

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 Permian shallow-marine carbonates in 9 areas in the world (Tunisia, Croatia, 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 close association with large-tested fusulines (Verbeekinidae) and/or rugose corals (Waagenophyllidae) of the typical Tethyan assemblage, suggesting their preferential adaptation 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 suggest 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 explanation that a critical cooling took place on a global scale, including low-latitude oceans. The end of the 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 conditions. Gigantism of bivalves occurred several times in the Phanerozoic; e.g., Siluro-Devonian, Permian, Triassic–Early Jurassic, and Jurassic–Creataceous, 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 photosymbiosis-related gigantism in low-latitudes may serve as a potential monitor of global warming/cooling in the past.