THE HIRNANTIAN (LATE ORDOVICIAN) AND END-GUADALUPIAN (MIDDLE PERMIAN) MASS EXTINCTION EVENTS COMPARED

The so-called Big-5 mass extinctions of the Phanerozoic include two prominent Paleozoic episodes; i.e. the end-Ordovician and end-Permian event with large biodiversity loss. We consider that the end-Ordovician (Hirnantian) extinction could be best compared to the Middle Permian end-Guadalupian (=Capitanian) extinction, rather than to the end-Permian (Permo-Triassic boundary; PTB) extinction. The end-Guadalupian Extinction, ca. 8 m.y. before the PTB extinction, occurred as an independent episode under extremely unique global setting with the lowest sea level and lowest Sr isotopic ratios in sea water of the Phanerozoic. Multiple similarities exist between the end-Ordovician (Hirnantian) and the end-Guadalupian (Capitanian) events, such as the preferential elimination of sessile biota in the tropics, end of gigantism, a global sea-level drop, and secular changes in sea-water C and Sr isotope ratios, occurring under global cooling. The limited development of land vegetation suggests that the Ordovician extinction was restricted solely to the marine realm, with no prominent damages on land. The comparison indicates that the two extinctions of the Hirnantian and of the Capitanian have been essentially triggered by similar causes/processes; nonetheless, biotic responses were different, owing to the more oxygenated status of surface environments in the Permian after the mid-Paleozoic terrestrialization.