EPIBIOSIS AS A PALEOECOLOGICAL TOOL: A PUTATIVE EXAMPLE FROM THE VIRGIN LIMESTONE (LOWER TRIASSIC, UTAH)

Epibiosis is one of the few well-preserved biotic interactions in the fossil record. In studying the spatial relationships among organisms that literally lived with each other, we may gain insight not only into the lifestyles of these organisms, but also into their common paleoenvironment and paleocommunity. In addition to all this, epibiosis and encrustation provide an essential perspective on the physical complexity of communities through geologic time. Since physical complexity and ecological complexity are linked at some scale, we can learn much about the history of ecological complexity by documenting the degree and complexity of biotic encrustation in various environments through the fossil record.

The single event of greatest global devastation to physical and ecological complexity was the end-Permian extinction event. In this event, perhaps 95% of marine species were lost to extinction, and they took many of their niches with them. Biotic encrusters were apparently absent from the Early Triassic world, at least in North America. All that remained for the duration of the Early Triassic were communities of generalist organisms arranged in simple trophic structures, with little vertical tiering or other physical structure. However, the recent discovery of putative encrusters in Lower Triassic marine rocks of southwest Utah prompts a reevaluation of the timing of reinvasion of these evacuated niches.

Although the preservation of these putative encrustations makes the identification of the encruster difficult, several spatial patterns suggest a rheotaxic or geotaxic biotic encrusting organism. The question of ecological complexity vis à vis epibiosis is a more complex one, and is even more difficult to evaluate. All such ecological hypotheses can be tested against each other within a comprehensive taphonomic framework. Taphonomy and epibiosis are always valuable tools for studying small-scale interactions, but they gain a greater significance when they allow us to document global ecological novelties. Such is the case for the reinvasion of abandoned ecological niches, namely encrusting, during the Early Triassic recovery.