PALEOECOLOGY AND SUBSTRATE INTERACTIONS OF EPIFAUNAL SUSPENSION FEEDERS IN THE EARLY CAMBRIAN MAOTIANSHAN SHALE BIOTA, SOUTHWEST CHINA

The advent of intense vertical bioturbation and the accompanied development of the mixed layer during the Cambrian radiation caused a fundamental shift in dominant normal marine unlithified seafloor substrates from the relatively firm substrates typical of the Proterozoic to the soupier substrates typical of the Phanerozoic. Previous research has shown that this substrate transition had a demonstrable impact on the evolutionary paleoecology of early benthic metazoans. This impact is known as the Cambrian substrate revolution (CSR). The goal of this research is to examine the paleoecology and substrate interactions of epifaunal suspension feeding genera in the early Cambrian Maotianshan Shale biota of southwest China in order to gauge their response to the CSR. Relative abundance data was collected at the genus level from the Shankou biota, a distinct assemblage within the Maotianshan Shale. A total of 22,072 specimens containing 61 genera from this biota were examined and tallied. Epifaunal sessile suspension feeders (ESSU) comprise 4,239 of these specimens and 15 genera. These genera were categorized into one the following groups based on their substrate adaptations: 1) snowshoe strategists, 2) iceberg strategists, 3) holdfast strategists, 4) hard substrate attachers, 5) sediment resters, 6) shallow sediment stickers, and 7) sediment attachers (Thayer 1975, 1983; Dornbos et al., 2005). Genera in the first four paleoecological categories are interpreted as adapted to soupy Phanerozoic-style substrates whereas those in the last three categories are interpreted as adapted to firm Proterozoic-style substrates. Results reveal that a large majority (88.5%) of ESSU specimens comprising 14 genera were adapted to firm Proterozoic-style substrates. Prominent among these genera are the sediment resting brachiopod Heliomedusa (34.7%), the shallow sediment sticking chancelloriid Allonnia (14.8%), and the sediment resting sponge Saetaspongia (10.8%). These results are consistent with previous estimates based on generic diversity and indicate that the CSR was not a geologically instantaneous event. Rather, the substrate transition's impact on benthic metazoans was heterogeneous, with some normal marine environments harboring firm Proterozoic-style substrates well into the Cambrian.