Article: The community structure of the Middle Cambrian Phyllopod Bed (Burgess Shale)

The palaeoecology and taphonomy of the Middle Cambrian Phyllopod Bed fauna (Burgess Shale, British Columbia) is described. Examination of over 30 000 slabs of shale and more than 65 000 specimens, many of them showing soft-bodied preservation, provides estimates of numbers of individuals and biovolumes of approximately 100 genera belonging to twelve major groups. Life habits are diverse with a sessile and vagrant infauna and epifauna, together with a nektobenthos, being recognized; components of a separate pelagic community are also present. Trophic analysis documents deposit feeders, suspension feeders, predators, and scavengers, and reconstructs the trophic nucleus and a feeding web. Possible niche structure of various ecological categories is discussed in the context of dominance diversity curves. Most distributions are log-normal, but for epifaunal vagrant deposit (collector) feeders a geometrical distribution may support the hypothesis of niche preemption. Comparisons are drawn between the community structure of the Phyllopod Bed biota and typical Cambrian shelly faunas. In isolation the shelly component of the Phyllopod Bed has a typical Cambrian aspect, but it accounts for only c. 14% of genera and perhaps as little as 2% of individuals alive at the time of burial. Synecological pronouncements based on normal Cambrian assemblages are suspect, and the likely importance of predation is emphasized.The wider implications of this study include comparisons with younger Palaeozoic deeper-water communities in an attempt to trace the evolution of ecological analogues through time. This exercise is conducted in terms of broad categories of carnivores, suspension and deposit feeders. Some groups such as sponges, ostracodes, and trilobites persist, but probably changed in relative importance. In other cases there is evidence for ecological replacement; one example could be the rise of nautiloids and eunicid polychaetes as carnivores and scavengers.