EXTENSION OF THE ISLAND RULE TO PERMIAN MICROGASTROPOD FAUNAS OF THE WESTERN NORTH AMERICAN ACCRETED TERRANES

The controversial Island Rule of biogeography holds that larger species undergo body size evolution to become dwarfed on isolated island environments. The generality of the rule has recently (McClain et al., 2006; Welch, 2010) been extended for gastropods in modern deep-sea environments – namely, genera with larger shallow-water species have significantly smaller species in the deep sea. The accreted terranes of the western North American Cordillera contain numerous examples of isolated Permian arc systems, many of which preserve deeper water environments, allowing for testing of the rule with paleoceanographic examples.

The Pine Forest Range of northwestern Nevada contains one such case - an unnamed limestone formation of the Black Rock Terrane that is verified by tectonostratigraphic analysis to be deeper-water, island arc deposits of Early to Middle Permian age. Macroinvertebrate fauna occur at numerous horizons in a 62 meter section with dominance (over 90% of individuals) by microgastropods (height < 10mm). A single species of the genus Ananias that comprises over 63% of the 13760 gastropod specimens collected is smaller than every other described (shallow-water) species of the genus. Similar size relations are shown by species of the genera: Naticopsis, Microdoma, Yunnania and Platyzona, among others. The Island Rule is thus further extended into the past to include these Permian gastropod examples.

Purported causes of Island Rule dwarfing for modern gastropods of the deep sea include food resource limitation and fluctuating anoxic conditions. Gamma Ray Spectroscopy of the Pine Forest Range limestones have Th/U ratios that vary from 0.59 to 18.5, with over half of the horizons less than 3.00, and thus anoxia was a likely factor in this example. The Early Triassic microgastropod dominance (e.g. Fraiser et al., 2005), and its Late Permian extension into deep shelf environments (Clapham & Bottjer, 2007) may have had an Early Permian precursor in the Panthalassic island arc faunas, shaped by Island Rule body size constraints and preserved today in the deep facies of the Pine Forest Range and other accreted terranes of western North America.