INTRODUCTION Late Neoproterozoic and early Phanerozoic body and trace fossils commonly exhibit strange morphological adaptations and paleoenvironmental distributions (e.g., Fig. 1). At this time, the basic body plans of large metazoans were first evolving, and much research has been expended toward understanding the evo- lutionary relationships of these ancient animals. Of particular importance is that while this evolutionary play of metazoan body plan evolution was taking place, the ecological stage was shifting. Two changes in the biological dimensions of the marine ecologi- cal stage were especially important. First was the advent and development of predation, which, together with additional bio- logical and geochemical factors, fostered the evolution of miner- alized skeletons (e.g., Vermeij, 1989; Bengtson, 1994). The second change in the biological dimensions of the eco- logical stage occurred in seafloor sediments, which act as the sub- strate on and in which benthic organisms live. This change was caused by increasing disturbance of sediments by bioturbation (e.g., Droser, 1987; Droser and Bottjer, 1989) (Fig. 2). Through analogy with the development of agriculture and its resulting effects upon soils, Seilacher and Pflüger (1994) have termed this change the agronomic revolution. Late Neoproterozoic seafloors were typically characterized by well-developed microbial mats (e.g., Gehling, 1986, 1996, 1999; Schieber, 1986; Hagadorn and Bottjer, 1997, 1999) and poor development of sediment mixing by vertically oriented burrowing (e.g., Droser et al., 1999; McIlroy and Logan, 1999) (Fig. 2). Sediment layers on the seafloor thus had relatively low water content and were characterized by a sharp water-sediment interface. Work on carbonates (e.g., Awramik, 1991) and more recently on siliciclastics (e.g., Hagadorn and Bottjer, 1997, 1999) has shown that in the Cambrian shallow marine environments characterized by seafloors covered with microbial mats became increasingly scarce, largely due to increas- ing vertically oriented bioturbation (Fig. 2). This change to a more Phanerozoic-style seafloor resulted in relatively greater water content of seafloor sediment and a blurry water-sediment interface, which led to the first appearance of a mixed layer. Mixed layers constitute the soupy upper few centimeters of the substrate that are homogenized by bioturbation and are charac- teristic of later Phanerozoic fine-grained substrates (e.g., Ekdale Vol. 10, No. 9 September 2000 GSA TODAY A Publication of the Geological Society of America INSIDE • Research Grants, p. 12 • Section Meetings Northeastern, p. 16 Southeastern, p. 18 • Happy Birthday, NSF, p. 22 Cambrian Substrate continued on p. 2 Figure 1. Looping and meandering trace fossil Taphrhelminthopsis, made by a large Early Cambrian bioturbator, on a bedding plane from Lower Cambrian Poleta Formation, White-Inyo Mountains, California. Such traces, consisting of a central trough between lateral ridges, occur in sandstones deposited in shallow-marine environments. Evidence indicating original presence of microbial mats is found in associated strata, and morphological features of these traces suggest they were produced on seafloor by active ingestion, or perhaps grazing, of underlying sediments (Hagadorn et al., 2000). The Cambrian Substrate Revolution David J. Bottjer, Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, dbottjer@usc.edu James W. Hagadorn, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, hagadorn@caltech.edu Stephen Q. Dornbos, Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, sdornbos@usc.edu ABSTRACT The broad marine ecological settings prevalent during the late Neo- proterozoic–early Phanerozoic (600–500 Ma) interval of early metazoan body plan origination strongly impacted the subsequent evolution and development of benthic metazoans. Recent work demonstrates that late Neoproterozoic seafloor sediment had well-developed microbial mats and poorly developed, vertically oriented bioturbation, thus producing fairly stable, relatively low water content substrates and a sharp water-sediment interface. Later in the Cambrian, seafloors with microbial mats became increasingly scarce in shallow-marine environments, largely due to the evolution of burrowing organisms with an increasing vertically oriented component to their bioturba- tion. The evolutionary and ecological effects of these substrate changes on benthic metazoans, referred to as the Cambrian substrate revolution, are presented here for two major animal phyla, the Echinodermata and the Mollusca.