SLUGGISH RATES OF EVOLUTION FOR LAND ANIMALS DURING THE LATE PALEOZOIC ICE AGE: A GEOBIOLOGICAL INTERPRETATION

Accelerated burial of organic carbon caused by the luxuriant growth of coal swamps during the Carboniferous Period reduced levels of carbon dioxide in Earth’s atmosphere. This weakening of greenhouse warming triggered the expansion of massive continental glaciers situated in the Southern Hemisphere and initiated the great late Paleozoic ice age, which endured for fifty million years and occurred simultaneously with the single largest excursion of atmospheric oxygen in Phanerozoic history. By promoting the persistence of broadly adapted and widely distributed taxa, such protracted glaciation depressed rates of evolutionary turnover in the marine realm. Little is known, however, about the corresponding effects of these climatic changes on life on land. To remedy this situation, rates of origination and extinction were derived from the stratigraphic ranges of over 600 terrestrial and freshwater animal families spanning a geologic interval from the middle Paleozoic biotic invasion of the land to the terminal Permian crisis. In general agreement with evidence gathered from marine invertebrate genera and vascular land plant species, rates of origination for families of land animals abruptly plummeted roughly 320 Mya, essentially coincident with the mid-Carboniferous (Serpukhovian) onset of the ice age, and remained relatively low until extensive deglaciation in early Permian time. In contrast, rates of extinction fluctuated. A Late Pennsylvanian pulse of extinction may reflect drying climates and the concomitant transition of lycopod to tree fern-dominated land floras. Other subintervals display decreased rates of extinction. A geographic explanation for this apparent disparity between land and sea stems from the recognition that the fossil record of insects (which comprise more than 50% of sampled families) draws heavily upon tropical Euramerica, which straddled the equatorial latitudinal belt and was thus better insulated from the cold glaciated regions of the immense Pangaean supercontinent.