SUBDUED ECOLOGICAL CONSEQUENCES OF THE LATE PALEOZOIC ICE AGE

The Late Paleozoic Ice Age (LPIA) represented a major change in global climate characterized by repeated episodes of colder global average temperatures, glaciers that extended to within 30-40° of the equator, high-amplitude glacioeustatic fluctuations, and possible changes in ocean circulation. Not unexpectedly, this shift altered the taxonomic composition and dynamics of the biota by triggering a severe mass depletion of marine biodiversity, concentrated in the tropics, which was then followed by persistently low rates of speciation and extinction until the LPIA ended. Terrestrial vegetation also experienced episodes of turnover at that time. Comparatively little, however, is known about the ecological consequences of the LPIA—in what ways were the organisms that flourished in icehouse conditions different from those of a warmer world? In an effort to progress in that area, we have analyzed the diversity and distribution of ecological (functional) groups using a database of genus occurrences coded by tiering, motility, and feeding type. We find that the ecological effects of the LPIA were surprisingly muted: the marine biota was dominated by the same five ecological groups before, during, and after the LPIA, despite substantial turnover of taxa. Extinction rates did not vary significantly among ecological groups during the Viséan, when the LPIA began, and origination rates did not vary significantly among ecological groups during the Artinskian, after the LPIA ended. A slight change in rank order of ecological groups by diversity occurred during the LPIA: mobile predators (primarily goniatid ammonoids) became more common during the LPIA, whereas surficial, attached suspension-feeders (primarily brachiopods) became less common. These groups returned to pre-LPIA levels once glaciation ended, suggesting that this transition was related to climate. Although cooler temperatures would seem to favor sedentary organisms with lower metabolic requirements, it is possible that active predators like the ammonoids benefited from the corresponding increase in dissolved oxygen (a drop in seawater temperature from 25°C to 20°C, a realistic magnitude for the LPIA tropics, would result in a 9% increase in dissolved oxygen).