The Neoproterozoic—Cambrian biological and geological events between 630 to 530 or so million years ago, are among the most fascinating topics in life history today because the first animals appeared and radiated amidst environmental changes of major magnitude. Not only are paleontologists and geologists interested in these events, but so are the general public, biologists, and theologians, among others. Indeed, this interest was manifested in a cover story in Time Magazine (Nash, 1995), another in the National Geographic (Monastersky, 1998), and in newspapers and other outlets. It is an intriguing time period, for most people are interested in our origins-of life itself, of animals, and of humans. The topics of this book then are nearly unique in this respect. The Paleontological Society is therefore pleased to present a Short Course and book summarizing the evidence, interpretations and hypotheses relating to this interval of time, with a focus on the origin and early evolution of the first known animals through a period of about 100 million years. Like all of the Society's Short Course books, this one is designed to provide a current review an important topic in paleontology for college instructors, interested professionals, and students. Such a book cannot cover the topics in great detail, but the authors have provided up-to-date references to additional sources.

Microbes and the sedimentary record of their activity, microbialites, have existed since the Archaean. During the Neoproterozoic-Cambrian interval, there is a marked shift in the fossil record of microbialites with the widespread proliferation of thrombolites (clotted microbialites) and the appearance and proliferation of dendrolites (microbialites composed of dendritic clusters) alongside stromatolites (laminated microbialites). Calcimicrobes also diversified during this interval. The end of this resurgence coincided with the increase in invertebrate taxa at the end of the Early Ordovician. The ìMicrobialite Resurgenceî has long been recognized but the importance of the Neoporoterozoic microbialite record for deciphering trends of this interval has not been so widely realized. It is critical to note that some of the oldest skeletal invertebrates and calcimicrobes are found in Neoproterozoic microbial reefs. Also, microbe-invertebrate synecology and invertebrate abundance trends during this interval suggests that a simple model that keys the resurgence tied in to a lack of competition from reef-building sponges and corals may not be valid. Current re-evaluation of the environmental conditions, principally the fluctuations in the geochemical conditions of the global oceans and atmosphere, will likely shed new light on understanding the Neoproterozoic-Cambrian microbialite record.

Carbonate units deposited during the Precambrian-Cambrian transition records a unique δ13C profile that is useful for chemostratigraphic correlation. However, the Precambrian-Cambrian boundary is currently defined within siliciclastic units where δ13C data are not available. The mixed siliciclastic-carbonate succession from the southern Great Basin, USA, records the appropriate fossils in the siliciclastic strata interbedded with carbonate strata that record the appropriate shifts in δ13C to facilitate correlation between the lithologic end-members. Ultimately, the integrated dataset demonstrates that vertical burrowing and the onset of widespread biomineralization was essentially synchronous.

The Cambrian explosion can be thought of as the culmination of a diversification of eukaryotes that had begun several hundred million years before. Eukaryotes - one of the three domains of life — originated by late Archean time, and probably underwent a long period of stem group evolution during the Paleoproterozoic Era. A suite of taxonomically resolved body fossils and biomarkers, together with estimates of acritarch and compression fossil diversity, suggest that while divergences among major eukaryotic clades or 'super-groups' may have occurred as early as latest Paleoproterozoic through Mesoproterozoic time, the main phase of eukaryotic diversification took place several hundred million years later, during the middle Neoproterozoic Era. Hypotheses for Neoproterozoic diversification must therefore explain why eukaryotic diversification is delayed several hundred million years after the origin of the eukaryotic crown group, and why diversification appears to have occurred independently within several eukaryotic super-groups at the same time. Evolutionary explanations for eukaryotic diversification (the evolution of sex; the acquisition of plastids) fail to account for these patterns, but ecological explanations (the advent of microbial predators) and environmental explanations (changes in ocean chemistry) are both consistent with them. Both ecology and environment may have played a role in triggering or at least fueling Neoproterozoic eukaryotic diversification.

The Late Neoproterozoic or Ediacaran biota contains a variety of enigmatic fossils of uncertain, but likely metazoan, affinities. The protistan group Choanoflagellata and Metazoa share a common ancestor predating the first fossils by perhaps 100's of millions of years. Sponge choanocytes closely resemble choanoflagellates, establishing a morphologic similarity as well. Fossils in the late Neoproterozoic may represent stem or early groups of cnidarians, while others resemble eumetazoans and bilaterians. These organisms occurred on all continents except Antarctica, and occupied four major habitats from prodeltaic to deep slope environments in each area. Their paleoecology was complex but similar to modern soft-bodied slope organisms. Ediacaran trophic structures were complex as well and included a wide variety of feeding types from detritovores, herbivores on microbial mats, filter-feeders, and predators. Ediacaran assemblages thus constitute the evolutionary and ecological precursors of later Phanerozoic and modern biotas.

The Precambrian-Cambrian transition saw the burgeoning of diverse skeletal organisms (“small shelly fossils”), represented in the fossil record by spicules, tubes, tests, conchs, shells, and a variety of sclerites and ossicles. Whereas calcareous biomineralization as such may have been facilitated by changes in ocean chemistry at this time, the utilization of biominerals in mineralized skeletons is a different process. The massive appearance of skeletons is most likely an epiphenomenon of the general radiation of body plans and tissues. The “choice” of biominerals (mainly calcium carbonates, calcium phosphates, and silica) may reflect the environmental conditions under which the particular skeleton first evolved.

Biogeographic patterns from early Cambrian trilobites are used to evaluate the nature and timing of the Cambrian radiation. Results from a phylogenetic biogeographic analysis reveal that patterns of vicariance are compatible with a vicariant distribution of trilobites across what were originally joined elements of the supercontinent Pannotia; further, there is limited evidence for coordinated range expansion or geo-dispersal by these trilobites. As Pannotia had split apart sometime between 550-600 Ma this suggests that trilobites, and by extension several other metazoan taxa, had begun to diversify by this interval. This result suggests that there may have been some period of cryptic diversification by metazoans prior to the Cambrian radiation, though the inferred length of this interval is not as long as that invoked by some molecular studies. Perhaps trilobites existed at low population densities in marginal environments before they became paleontologically emergent. Even though the results suggest some apparent gap in the fossil record, the evolutionary signature of this gap is still preserved in the paleobiological patterns from the fossil record, indicating that the fossil record is still the one best source of data on the nature of key episodes in the history of life, like the Cambrian radiation.

The Cambrian Radiation was expressed as major changes in marine communities. High-diversity skeletalized metazoan faunas appeared and persisted with little change in the most proximal onshore habitats and show little expansion into the offshore through much of the long (ca. 24 m.y.) pretrilobitic Placentian Epoch on the Avalon microcontinent. These small shelly fossil communities and a large number of coelomate burrowers that appeared earlier in the Placentian comprise the Placentian Ecologic Evolutionary Unit (new)—the initial stage of the Cambrian Evolutionary Fauna. Early members of a number of high-level metazoan groups were concentrated in onshore habitats vacated by the demise of the Ediacaran fauna, where they diversified and, possibly, originated no earlier than latter part of the Placentian Epoch (543–ca. 519 (m.y.a.). Development of unstable substrates and predation in open shelf habitats occupied coelomate trace producers may have been a key factor in the restriction of diverse skeletalized metazoan faunas to peritidal habitats, where mineralized skeletons may have served as protection from desiccation and UV-damage. In contrast, the oldest trilobites are most diverse and abundant in offshore habitats, and their appearance in habitats dominated by large trace producers suggests a protective role of their mineralized integument.

A consortium dominated by archaeocyaths and calcified microorganisms (calcimicrobes) constructed the first metazoan reefs during an eleven-million-year interval of the Early Cambrian. However, archaeocyaths were not the first metazoan reef dwellers; the weakly calcified organism Namacalathus, and the more heavily biomineralized organism Namapoikia, occupied microbial reef environments during the late Neoproterozoic, but they were not involved in reef construction. Throughout the late nineteenth century, and during most of the twentieth century, the biological affinities of archaeocyaths were unsettled, which caused paleobiologists to avoid including them in analyses of the Cambrian fauna. However, in the late twentieth century the discovery of living, aspiculate sponges led to a consensus among archaeocyathan workers that these fossils represent an extinct class of aspiculate, calcareous sponges. The majority of archaeocyathan-calcimicrobial reefs are relatively small, lenticular mounds, typically about a meter thick and a few meters in diameter, but the archaeocyathan-calcimicrobial consortium also constructed massive, ecologically zoned, wave-resistant, framework reefs. The Great Siberian Cambrian Reef Complex is 200-300 km wide and stretches for 1500 km across northern Siberia. A consensus has not yet been found among Cambrian reef workers concerning photosymbiosis, which is such an important aspect of the ecology of modern coralgal reefs. Two extinction events hit during the Early Cambrian, the second of which is associated with a eustatic sea-level drop. The attendant marine regression eliminated reefs, and the archaeocyathan-calcimicrobial reef community disappeared. While the disappearance of reefs at this time is perfectly understandable, it is nevertheless surprising that the metazoan-calcimicrobe reef-building consortium was not able to recover within a few million years. Approximately forty million years passed æ from the end of the Early Cambrian to the beginning of the Middle Ordovician æ before metazoans finally returned to reef-building. We present seven hypotheses to explain this metazoan-reef-free window. The testing of these hypotheses will, in part, be the challenge of the next phase of research on Early Cambrian reefs.