EXTREME MICROGASTROPOD SIZE-LIMITATION IN EARLY TRIASSIC SHALLOW MARINE SETTINGS,HIDDEN PASTURE, MONTANA: IMPLICATIONS FOR ENVIRONMENTAL CONDITIONS FOLLOWING THE END PERMIAN MASS EXTINCTION

The proliferation of microgastropods (smaller than 10mm) following the End Permian mass extinction has been well-documented (e.g., Batten, 1973; Fraiser and Bottjer, 2004). Whether or not Early Triassic microgastropods are evidence for a post-extinction Lilliput Effect (Twitchett, 2007) has been the subject of some debate, particularly given the report of larger gastropods from a distal locality of the Thaynes Formation in the Confusion Range of Utah (Brayard et al., 2010). However, the overwhelming numerical dominance of microgastropods in shallower settings suggests that there may have been differing environmental constraints on gastropod size in deep versus shallow settings.

Here we report the occurrence of extreme abundances of smaller-than-1mm gastropods from 27 samples throughout the Lower Triassic Dinwoody and Thaynes Formations (Induan through Spathian) at Hidden Pasture, Montana. Some samples yielded more than tens of thousands of smaller-than-1mm gastropods per kg of rock. The smaller-than-1mm gastropods were preserved as internal molds filled with phosphatic or siliceous mud. Although size sorting in two samples suggests some amount of transport, the rest of the samples exhibit no evidence for significant transport. In fact, some samples contain heterostrophically-coiled gastropods that are very fragile and likely did not experience any transport. Gastropods greater than a few mm were extremely rare in comparison with only a few fragmentary individuals found throughout the entire section.

In contrast, Thaynes Formation exposures in the Confusion Range contain at least one bed with gastropods in the 10mm to 20mm size range (Brayard et al., 2010). These larger gastropods occur in demonstrably deeper depositional settings than the nearshore marine environments of Hidden Pasture, suggesting that the gastropods were subject to different environmental constraints on body size than those at Hidden Pasture. Although the gastropods themselves are not direct evidence for specific environmental conditions, work on modern invertebrates suggests that the synergistic effects of hypoxia, lower seawater pH, and higher temperatures can limit growth and other biological parameters (e.g., Pörtner et al., 2005; Clapham and Payne, 2011).