DESCRIPTION OF EARLIEST ENCRUSTING ORGANISMS(?) FROM THE MESOZOIC OF NORTH AMERICA

Ecological recovery from the end-Permian extinction events was slow in marine systems worldwide. Low-diversity "disaster biotas," representing eurytopic-organism communities of simple physical and ecological structure, dominate the Early Triassic record. One useful and broadly applicable measure of community-ecological simplicity is the degree of vertical tiering manifested by organisms in a community. Tiering in early-to-middle Early Triassic (Griesbachian-Smithian) marine systems of North America was minimal, and no borers or encrusters are known. In contrast, encrusting annelids are common from the earliest Triassic of western Tethys (Italy). Here I describe the earliest biotic(?) encrustations from the North American Triassic record, from the upper Lower Triassic (Spathian) Virgin Limestone Member of the Moenkopi Formation in southwest Utah. Although the mode of preservation makes identification of the encruster difficult, sedimentological and ecological evidence suggests a biotic origin for the encrustations. Autecological evidence includes the spatial distribution of encrustations on the host shell as well as over the community of potential hosts. Documentation of the global-scale paleobiogeographic distribution of the successional reinvasion of abandoned ecospace in Early Triassic communities grants important insights into the spatiotemporal pathway taken toward biotic recovery, both at the community level and globally.