380 PRECAMBRIAN/Vendian and Ediacaran in the heavy isotope of carbon and, as it went into circulation in marine water, caused the precipitous drop or boundary excursion in the d13C value It therefore seems reasonable to interpret the Vendian carbon isotope curve as a record of gradually increasing biotic productivity, with sequestering of much of the organic matter within and below the seafloor microbial mats, followed by a relatively sudden release of part of this organic matter deposited at the end of the Vendian as a result of increased burrowing intensity Other factors, such as destabilization of gas hydrates in seafloor sediments, may also have been involved in these isotopic excursions A number of other phenomena can perhaps be traced to what has been called the Cambrian Substrate Revolution The Vendian and Cambrian both saw an increase in the proportion of calcified filamentous microbes (such as Girvanella), which were perhaps less palatable to mat grazers than filamentous cyanobacteria and unprotected algae With all the new sediment and organic matter in suspension, filter feeding probably became more possible throughout the water column, leading to the evolution of the first tiered filter feeders in the Cambrian The only Vendian organisms that were likely to engage in suspension feeding were the cloudinids, which lived close to the sediment–water interface Finally, assuming that we are interpreting the secular carbon isotopic curve correctly, it is entirely possible that oxygen levels increased in the Vendian due to the sequestration of organic matter Whether or not increasing oxygen levels influenced metazoan evolution is not known, although it seems fair to say that early burrowing animals would not have required high levels of oxygen The earliest animal habitat appears to have been the submicrobial mat environment, where oxygen levels would probably have been rather low considering the relative abundance beneath the mats of hydrogen sulphide and other reduced compounds Glossary abiogenically A term applied to rocks formed by processes not directly influenced by living organisms anactualistic processes Processes that occurred at one time in the Earth’s past, but which are no longer operational today cloudinid A late Vendian calcareous shelly fossil consisting of closely nested, thin-walled tubes or cones Thought to represent one of the earliest examples of a shelly animal fossil Includes the genera Cloudina and Sinotubulites coelomic spaces The compartments that house the rigid, fluid-filled body cavity present in many animals The coelom serves as a hydrostatic skeleton conulariid Any member of an enigmatic group of Vendian/Cambrian to Triassic shelly organisms They formed conical, often pyramidal, tapering cones with transverse ribbing, composed of calcium phosphate Ediacaran Any member of a group of marine, megascopic fossils with a metacellular growth pattern Found primarily in strata deposited before the Cambrian period Assigned to extinct Kingdom Vendobionta frondose forms Ediacarans with a leaf, palm, or frond body form Garden of Ediacara A palaeoecological theory that holds that the marine ecosystems of the Vendian were largely free of megascopic predators and thus allowed organisms such as Ediacarans to survive unmolested using photosymbiotic, chemoautotrophic, and osmotrophic life styles holobiont A single integrated organism, as opposed to a colonial organism hydrostatic skeleton A fluid-filled internal organ or support structure within an animal’s body that can be kept rigid or made limp by control of internal water pressure metacell A single or isolated modular unit of a metacellular organism; usually consists of a single enlarged cell metacellularity Term applied to organisms that are either multicellular (such as animals and plants) or consist of clusters or metacells (such as Ediacarans and certain types of aquatic algae) Mirovia The Precambrian superocean that surrounded Rodinia molecular clock Any gene or gene sequence used by biologists in an attempt to determine the evolutionary time of divergence from a common ancestor between two or more groups of organisms belonging to different species osmotrophy A feeding strategy utilizing osmosis or direct absorption of nutrients peristaltic burrowing A burrowing strategy in metazoans that consists of rhythmic muscular contractions along the length of the body Rodinia A supercontinent consisting of all or nearly all of the continents Consolidated one billion years ago (in an event referred to in North America as the Grenville Orogeny), this supercontinent broke up into smaller continents by the process of plate tectonics and continental drift before the Cambrian Sinian The Precambrian geological period immediately preceding the Vendian period