End of Gigantism In Tropical Seas by Cooling: End-Guadalupian Extinction of the Gigantic Bivalve

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 was reported sporadically from the 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 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 adaptation to shallow warm-water (tropical) environments. This “tropical trio” (Alatoconchidae, Verbeekinidae, and Waagenophyllidae) became extinct immediately before the Guadalupian-Lopingian boundary (G-LB). This occurrence pattern 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 symbiosis with photosynthetic organisms (algae + cyanobacteria) for maintaining their large body size that requires high energy-consuming metabolism in comparison to smaller forms. In the Wordian (Middle Guadalupian), Alatoconchidae attained the maximum size, probably maximizing the benefits of photosymbiosis. The subsequent extinction of the warm-water-adapted “tropical trio” both in Tethys and Panthalassa positively indicates that a critical environmental change may have appeared on a global scale, and preferentially in low-latitude oceans. The end of the gigantism in fusulines and bivalves in the Capitanian (Late Guadalupian) may have been caused by the collapse of photosymbiotic systems during a temporary temperature drop (Kamura cooling event) coupled with eutrophication that was detrimental to the tropical fauna adapted particularly to oligotrophic conditions. The intermittent rise and decline of bivalve gigantism in the Phanerozoic may have been tuned by the long-term history of global warming/cooling.