CAMBRIAN PERIOD

oldest time division of the Paleozoic Era, extending from 540 to 505 million years ago. The rocks that originated during this interval of geologic time make up the Cambrian System; they contain the earliest record of abundant and varied life forms. The Cambrian Period is often divided into the Early Cambrian Epoch (540 to 520 million years ago), the Middle Cambrian Epoch (520 to 512 million years ago), and the Late Cambrian Epoch (512 to 505 million years ago). Cambrian rocks are widely distributed on all the continents. Most are of sedimentary origin, and many show evidence of deposition in or near shallow seas that had invaded far into the continental interiors. Some such epicontinental sea deposits have been found to be thousands of metres thick. Fossil evidence and paleomagnetic data indicate that the landmasses were scattered during Cambrian time, but none was located in the polar regions. One of the major continents, Laurentia, consisted largely of present-day North America and Greenland and lay across the Equator throughout most or all of the period. Another, smaller continent, called Baltica, was located in the middle to high latitudes of the Southern Hemisphere. Baltica was composed primarily of northern Europe, along with what is now Scandinavia. The largest of the Cambrian continents was Gondwana. Composed essentially of modern South America, Africa, southern Europe, a large portion of the Middle East, India, Australia, and much of Antarctica, Gondwana extended from the low northern latitudes to the high southern latitudes. It is thought that several crustal fragments dubbed exotic terranes lay near to or were attached to the margin of Gondwana that now constitutes North Africa. There were several other small landmasses (e.g., Siberia and Kazakhstania) in the low latitudes. In all likelihood, these consisted of separate lithospheric plates (the immense rigid slabs of crust and upper mantle that make up the Earth's surface). The Cambrian was a period of relatively little tectonic activity. It represents a transition from the continental fragmentation of the end of the Precambrian to the continental accretion of the later Paleozoic. Geologic evidence suggests no collisions between lithospheric plates or associated mountain-building during Cambrian time. There are, however, indications in the rock record of limited volcanism linked with volcanic island arcs. The landscape of the Cambrian Period was quite desolate, something akin to that of the Moon, since no plants or animals were present on the terrestrial surface. The average climate that existed during the Cambrian is thought to have been warmer and more equable than it is at present. This condition is indicated by the complete absence of glacial deposits of Cambrian age and the widespread occurrence of carbonate deposits, which today form only in the warm-temperate and tropical regions of the world. The accumulation of evaporite sequences of Early Cambrian age in what are now southern Siberia and sectors of the eastern United States further suggests warm to hot conditions. There is no Cambrian record of terrestrial or freshwater flora and fauna, and no vertebrate fossils have been found in beds of this age. The known Cambrian biota is restricted to marine environments. Fossil remains in Cambrian rocks include the oldest representatives of most animal phyla with either mineralized shells or skeletons. The dominant form of this kind is the trilobite, a major subgroup of the arthropods distantly related to the modern horseshoe crab. For this reason, the Cambrian is sometimes referred to as the Age of Trilobites. Other important groups include the graptolites, conodonts, annelids, brachiopods, chordates, ctenophores, echinoderms, mollusks, and sponges. Various algae and algalike forms also flourished during the Cambrian Period. earliest time division of the Paleozoic Era, extending from about 540 to 505 million years ago. The Cambrian Period is often divided into the Early Cambrian Epoch (540 to 520 million years ago), the Middle Cambrian Epoch (520 to 512 million years ago), and the Late Cambrian Epoch (512 to 505 million years ago). Rocks formed or deposited during this time are assigned to the Cambrian System, which was named in 1835 by Adam Sedgwick for successions of slaty rocks in southern Wales and southwestern England. They contain the earliest record of abundant and varied life forms. The corresponding period and system names are derived from Cambria, the Roman name for Wales. Additional reading C.H. Holland (ed.), Cambrian of the New World (1971), Cambrian of the British Isles, Norden, and Spitsbergen (1974), Lower Palaeozoic of the Middle East, Eastern and Southern Africa, and Antarctica (1981), and Lower Palaeozoic of North-Western and West-Central Africa (1985), are detailed surveys of Lower Paleozoic rocks. Correlation charts, explanatory notes on rocks and faunas, and extensive references are found in Reinhard Wolfart, The Cambrian System in the Near and Middle East (1983); J.H. Shergold et al., The Cambrian System in Australia, Antarctica, and New Zealand (1985); W.T. Chang, The Cambrian System in Eastern Asia (1988); Kaisa Mens, Jan Bergstrm, and Kasimiera Lendzion, The Cambrian System on the East European Platform (1990); and Vladimir A. Astashkin et al., The Cambrian System on the Siberian Platform (1991). Michael E. Taylor (ed.), Short Papers for the Second International Symposium on the Cambrian System, 1981 (1981), contains research reports from many parts of the world. Descriptions of Cambrian life-forms and those of succeeding geologic periods are found in Raymond C. Moore et al. (eds.), Treatise on Invertebrate Paleontology (1953 ), a comprehensive multivolume work; part A contains a discussion of Cambrian biostratigraphy. Stephen Jay Gould, Wonderful Life: The Burgess Shale and the Nature of History (1989), analyzes a famous Cambrian biota and its significance in the history of life. Richard A. Robison Cambrian environment Paleogeography Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the The geography of the Cambrian world differed greatly from that of the present. The geographic reconstruction in thefigure is based on integrated geologic and biological evidence. Fossils in continental-shelf deposits indicate the presence of at least three major faunal provinces during much of the Cambrian Period. The most distinct faunal province surrounded the continent of Laurentia. Paleomagnetic evidence indicates that Laurentia was located over the equator during most or all of Cambrian time. This geographic interpretation is supported by the presence of thick, warm-water, carbonate-platform deposits that accumulated in a broad belt encircling the continent. These carbonates are commonly flanked on the inner shelf by lagoonal shale and nearshore sandstone. On the outer shelf, the carbonates commonly grade into laminated mudstone and shale that accumulated in deeper water. At times, two almost mutually exclusive subfaunas were separated by temperature and salinity barriers in the shallow water on the carbonate platforms. Inner, restricted-shelf deposits are characterized by sparse, low-diversity faunas that tend to be highly endemic. Outer, open-shelf deposits are characterized by common to abundant, high-diversity faunas that are widely distributed around the continent. Fossils are usually most abundant and most diverse near the outer margin of the carbonate platform. Because Laurentia has remained nearly intact structurally, it is ideal for studying the relationships between Cambrian environments and faunas around a low-latitude Cambrian continent. Another Cambrian faunal province surrounded the small continent of Baltica, which was located in middle to high southern latitudes. Cambrian shelf deposits of Baltica are relatively thin, rarely exceeding 250 metres in thickness, and are composed of primarily sandstone and shale. Seemingly as a consequence of cool-water environments, carbonate deposits are relatively minor and very thin. The wide distribution of many species from the nearshore to deep-shelf environments of Baltica suggests no significant restriction in shelf dispersal like that caused by the shallow carbonate platforms of Laurentia. The largest Cambrian faunal province is that around the continent of Gondwana, which extended from the low northern latitudes to the high southern latitudes, just short of the South Pole. Rocks and faunas of Gondwana show major changes that correspond to its great size and wide range of climates and environments. The Antarctic and Australian sectors of Gondwana were in low latitudes and have extensive carbonate deposits, although those of Antarctica are poorly exposed through the present-day polar ice cap. Faunal differences and paleomagnetic evidence suggest that present-day North and South China were on separate tectonic plates. Extensive carbonate deposits in both regions, however, indicate that both plates were in low latitudes. South China has strong faunal similarities to both Australia and Kazakstan, but details of the Cambrian geographic relationships remain unclear. Several terranes seem to have been near or attached along the margin of Gondwana in high southern latitudes (the northern Africa sector), but many details of their Cambrian geographic relations are unknown. (Terranes are fault-bounded fragments of the Earth's crust characterized by a geologic history markedly different from that of neighbouring crustal segments.) These terranes now make up much of southern Europe and parts of eastern North America. Cambrian deposits in all the terranes are chiefly sandstone and shale and include few or no carbonates. Their faunas closely resemble those of Baltica at generic and higher taxonomic levels, but differences at the species level suggest some geographic separation. Siberia was a separate continent located in the low latitudes between Laurentia and Gondwana. Faunal affinities suggest that it lay relatively close to equatorial Gondwana. Present-day Kazakstan seems to be composed of several microcontinental blocks that were in all likelihood separated during the Cambrian. These blocks were amalgamated after the Cambrian, and the composite continent, Kazakhstania, collided with Siberia during the late Paleozoic. Few Cambrian faunas from continental-slope and deep-ocean environments are known. Limited information from these is important, however, for demonstrating affinities between deep, cool-water faunas from all latitudes and shallow, cool-water faunas from high latitudes. Close similarity between the observed distribution patterns of Cambrian and modern marine arthropods has been used as persuasive evidence for thermally stratified Cambrian oceans in lower latitudes and for a thermocline separating warm-water and cool-water layers. The inferred thermocline as well as wide oceanic separation were likely causes for the high endemism of the Laurentian faunal province. This interpretation is supported by Middle Cambrian deposits in North Greenland where, in a few tens of kilometres, normal Laurentian shelf-margin trilobite faunas grade into deepwater faunas like those in the shallow-shelf deposits of Baltica. In a similar pattern, trilobite species in deepwater faunas of Late Cambrian age in the western United States and southeastern China are the same, but shallow-water faunas of the same age in the two regions have few genera in common. As large lithospheric plates continued to move during the Phanerozoic Eon, terranes of various sizes were displaced. Endemic Cambrian fossils, in conjunction with such other geologic evidence as physical stratigraphy, have been useful in helping to identify the geographic origins of some terranes, particularly those that have undergone substantial displacement. Examples are northern Scotland with Laurentian faunas, eastern Newfoundland with Baltic faunas, southern Mexico (Oaxaca) with Gondwanan (South American) faunas, and west-central Argentina (Precordillera) with Laurentian faunas. Another important consequence of continued plate movement has been the formation of large mountain ranges by crumpling where plates have collided. Pressure and heat generated during collisions since the Cambrian have folded, faulted, and metamorphosed significant volumes of Cambrian rock, especially that from the outer margins and slopes of many continental shelves. No crustal rocks in today's oceans have been found to be older than Mesozoic in age, and apparently most pre-Mesozoic deposits that accumulated in the deep ocean basins have been destroyed by subduction into the Earth's interior. Relatively abrupt changes in sea level may have significantly influenced Cambrian environments and life. A global drop in sea level is suggested by extensive unconformities (i.e., interruptions in the continuity of depositional sequence) and changes in sedimentary rocks near the EarlyMiddle Cambrian boundary. The time represented by such unconformities in sectors of Laurentia and Baltica bounding the Iapetus Ocean has been called the Hawke Bay event. An apparent absence of a coeval unconformity in western North America seems to be an anomaly. Thick, uninterrupted shelf deposits in this sector of Laurentia, however, may have resulted from abnormal shelf subsidence caused by cooling of crustal rocks following a late Precambrian plate-rifting event. Temporal correlations with unconformities on other continents lack precision. Nevertheless, it is perhaps significant that a number of characteristic Early Cambrian animal groups were either exterminated or severely restricted in their geographic distribution at about the same time in the world's shallow-shelf environments. Among biostratigraphically important trilobites, the olenellids were exterminated around Laurentia, the holmiids were killed off around Baltica, and the redlichiids vanished around Gondwana. Also, diverse and abundant reef-dwelling archaeocyathans disappeared from most low-latitude, warm-water continental shelves. A significant rise in sea level is suggested by rather abrupt and extensive displacements in sedimentary environments and biotas in the middle Middle Cambrian (the Ptychagnostus gibbus zone). Lowland areas were flooded, as in parts of Baltica. In warm-water shelf sections of the world, it is common for coarse-grained, shallow-water, carbonate rocks to be abruptly overlain by fine-grained, deeper-water, laminated limestone or shale. Adaptive radiation of the pelagic agnostoid trilobites was greatly accelerated in open-oceanic environments following this event, perhaps in response to expanded habitats. Missing faunas and an unconformity, which define the boundary between the Dresbachian and Franconian stages in peripheral areas of North America, suggest another significant drop in sea level during the early Late Cambrian. Evidence for two other lesser changes in Cambrian sea level has been identified near the CambrianOrdovician boundary. Associated minor unconformities have provided a problem for selection of a boundary stratotype, which ideally should be located in an uninterrupted stratigraphic section. Several regions of Cambrian volcanism have been identified. Australia was especially active, with large areas in the northern and central regions covered by flood basalts during the Early Cambrian and with residual activity into the Middle Cambrian. Basalts and mafic intrusives in southeastern Australia formed in a volcanic island-arc setting during the Early and Middle Cambrian. Volcanic suites of similar age also are present in New Zealand and in parts of Antarctica (northern Victoria Land, Ellsworth Mountains, and Pensacola Mountains). Other significant Lower and Middle Cambrian volcanic deposits are present in southern Siberia and western Mongolia (Altai and Sayan mountains), eastern Kazakstan and northwestern China (Tien Shan), and northeastern China. Cambrian volcanics are scattered along the easternmost margin of the United States, but most are probably island-arc deposits that were accreted to Laurentia after the Cambrian. In the southern United States ( Oklahoma), granitic intrusives and basaltic and rhyolitic extrusives are associated with a large tectonic trough that was formed by Early and Middle Cambrian crustal extension. Minor volcanic deposits, mainly ash beds and thin flows, are widely known. In general, these have received little study, but some are suitable for determination of isotopic ages. Zircons from a lower Lower Cambrian (pre-Tommotian) volcanic ash bed in New Brunswick, Can., have a uranium-lead age of 531 million years. Volcanic tuffs near inferred TommotianAtdabanian boundaries in both Morocco and southwestern China have yielded similar dates of 521 million years. The tectonic history of the Paleozoic is much better known than that of the Precambrian. In general, however, late Precambrian history seems to have been characterized by continental fragmentation, whereas Paleozoic history was characterized by continental accretion. The Cambrian was a period of transition between those tectonic modes, and continents were scattered, apparently by fragmentation of a late Precambrian supercontinent. Major Cambrian and early Ordovician tectonism affected large areas of Gondwana in what are now Australia, Antarctica, and Argentina. Multiphase tectonism in Antarctica is called the Ross Orogeny, and in Australia it is known as the Delamerian Orogeny. At least some of the volcanic activity noted above, particularly that of volcanic island arcs, is evidence that seafloor spreading and crustal subduction were active geologic processes. Paleoclimate Global climate during Cambrian time was probably warmer and more equable than it is today. An absence of Cambrian glacial deposits and an abundance of widespread, warm-water, carbonate deposits both suggest higher average temperatures than at present. The absence of glacial deposits of Cambrian age is more notable because such deposits are common and widespread in the upper Precambrian, and they accumulated again during the Ordovician in northern Africa as Gondwana began to move over the South Pole. An apparent absence of either land or landlocked seas at the Cambrian poles may have prevented the accumulation of polar ice caps. Cambrian life The long history of life on Earth has been punctuated by relatively abrupt changes. Some have argued that the greatest change of all occurred in marine environments near the PrecambrianCambrian boundary. Fossils from Cambrian rocks include the oldest representatives of most animal phyla having mineralized shells or skeletons. A lack of observed connecting links suggests that processes of biomineralization evolved independently in several phyla. Whether or not soft-bodied representatives of some of these phyla originated during the Precambrian but have no preserved record is a debated question. Nevertheless, the hard parts of Cambrian animals had a much greater potential for preservation than soft parts, and they mark the beginning of a diverse fossil record. Fossil record of the Precambrian-Cambrian transition Preservation of the record of the PrecambrianCambrian transition was significantly affected by global changes in sea level. During latest Precambrian time, the sea level was relatively low, resulting in areally restricted oceans and expanded continents. Throughout much of the Cambrian, rising seas gradually flooded vast land areas. Sediment was eroded from the continents and deposited in adjacent seas. Because of low sea level, the sedimentary and fossil records of the PrecambrianCambrian transition are generally most complete toward the outer margins of continental shelves. As a corollary, the time gap represented by the boundary surface generally increases in landward directions. Absence or serious incompleteness of a transitional record in most areas, particularly in those of classical Cambrian studies, contributed significantly to the long-held notion of an abrupt or sudden appearance of Cambrian fossils. This was compounded by a general deficiency in knowledge of Precambrian biotas before the mid-1900s. Considering the biological importance of the PrecambrianCambrian transition, it is somewhat surprising that the primary impetus for its detailed study came from a project undertaken to establish international agreement on a suitable boundary stratotype. Before the project was initiated in 1972, reasonably complete stratigraphic sections across the transition were either largely unrecognized or ignored. Since 1972, information about the transition has accumulated at an accelerating rate. Although many details remain to be learned, the general history of this momentous interval is becoming clear. The PrecambrianCambrian biotic transition, once thought to be sudden or abrupt, has been found to include a succession of events spread over many millions of years. It commences with the appearance of the animal kingdom (i.e., multicelled organisms that ingest food), but the date and details of that event remain obscure. At least three informal phases in the transition can be identified by progressively more diverse and complex biotas. The earliest phase, of late Precambrian age, is characterized by fossils of soft-bodied animals known from many localities around the world. These organisms may have appeared as early as 650 million years ago and are commonly called the Ediacaran fauna. The fossils are predominantly the imprints of soft-bodied animals. Their extraordinary preservation, usually in sandstone or shale, was probably the result of rapid burial and protection by smothering sediment. Most of the fossils are relatively simple, and many resemble worms, sea pens, and jellyfish. Dwelling traces like those of modern sea anemones are also common. Higher taxonomic assignments are controversial, however, because critical diagnostic features are not evident. Some paleontologists have assigned Ediacaran body fossils to the extant phyla Annelida, Coelenterata, and Arthropoda, whereas others have regarded them as members of extinct taxonomic groups of high rank. Some adherents of the latter viewpoint have suggested that the Ediacaran fauna was terminated by a major extinction event, but direct evidence of an abrupt faunal replacement has not been detected in any stratigraphic section. Other kinds of fossils also provide valuable clues about life during Ediacaran time. Photosynthetic organisms include unicellular blue-green algae (cyanobacteria) and acritarchs (probable algae), both of low diversity. Individuals of some species were probably abundant, however, and may have been an important source of food for Ediacaran animals. Hard parts of animals, primarily known from Africa and China, are mainly dwelling tubes composed of calcium carbonate and other compounds. Most were probably secreted by sessile, filter-feeding, wormlike animals. Although rare and of low diversity, these forms are significant because they signal the advent of biomineralization. The oldest unequivocal trace fossils, mainly crawling trails, are also of Ediacaran age. The trails suggest that locomotion of the trace makers was accomplished by waves of muscular contraction, like that in annelids and sea slugs, and not by legs. All but the latest Ediacaran trace fossils are relatively simple, suggesting limited and primitive behaviour patterns. Their low diversity further suggests that few kinds of mobile animals lived on the Ediacaran seafloor. The second phase of the PrecambrianCambrian biotic transition is characterized by a marked increase in the diversity of its shelly fauna and a lack of trilobites. It is near the lowest stratigraphic occurrence of this fauna that the PrecambrianCambrian boundary stratotype has been placed. The fauna includes that of the Tommotian Stage, as applied in Russia, and it has often been referred to as the Tommotian fauna. It is known from many localities around the world, but time correlations lack precision. A general acceleration in biotic diversity during this second phase is the beginning of the so-called Cambrian explosion. Fossils of the second phase, which may be locally abundant, represent several new animal groups of Paleozoic aspect. Calcified archaeocyathan sponges diversified rapidly and were the first skeletal metazoans to develop a modular growth habit. They also evolved a complex symbiotic relationship with reef-building blue-green algae. Mollusks, preserved in both shale and limestone, include at least four classes (Monoplacophora, Gastropoda, Hyolitha, and Rostroconchia). Brachiopods made their appearance but are low in diversity. Several problematic groups are represented by an astonishing array of small mineralized tubes, scales, and spicules. The presence of arthropods, the first animals to develop legs, is indicated by characteristic trace fossils. The skeletal remains of arthropods are not preserved in the fauna, however, presumably because they were not mineralized. Other trace fossils show a marked increase in abundance and diversity as well as an expansion of behaviour patterns that reflect improvements in locomotion, greater ability to penetrate sediment, and new foraging strategies. The third phase of the PrecambrianCambrian biotic transition commenced with the appearance of mineralized trilobite skeletons, which approximately correlates with the base of the Atdabanian Stage of the Lower Cambrian, as conceived in Russia. Subsequent adaptive radiation of the trilobites was exceptional, and their remains dominate most later Cambrian deposits. For this reason, the Cambrian Period has sometimes been called the Age of Trilobites. The known Cambrian biota was restricted to marine environments. At least 11 extant animal phyla (Annelida, Arthropoda, Brachiopoda, Chordata, Ctenophora, Echinodermata, Hemichordata, Mollusca, Onychophora, Porifera, and Priapulida), including most of those with a fossil record, first appear in Cambrian rocks. Most of these rapidly diversified as they seemingly adapted to numerous unfilled ecological niches. Another five phyla (Nemertea, Phoronida, Platyhelminthes, Pogonophora, and Sipuncula) are questionably known from Cambrian fossils. The only extant animal phylum with a good fossil record that is not known from Cambrian rocks is the Bryozoa, which first appears in rocks of Early Ordovician age. A summary of the principal biotic groups of the Cambrian is given below. Cambrian rocks Types and distribution Rocks of Cambrian age occur on all of the continents, and individual sections may range up to thousands of metres thick. The most fossiliferous and best-studied deposits are principally from marine continental-shelf environments. Among the thicker and better-documented sections are those in the Cordilleran region of western North America, the Siberian Platform of eastern Russia, and areas of central and southern China. Other well-documented, fossiliferous, but thinner sections are in Australia (especially western Queensland), the Appalachian Mountains of eastern North America, Kazakstan, and the Baltic region (notably Sweden). Lateral changes in the composition of Cambrian rocks resulted from regional differences in environments of deposition. Nearshore deposits are commonly composed of siliceous sandstone. This usually grades seaward into siltstone and shale, which formed by accumulation of finer-grained sediment in deeper water where the seafloor was less affected by wave action. Extensive carbonate platforms, analogous to the modern Bahama Banks, developed along some continental shelves that were in low latitudes during Cambrian time. Rapid production of carbonate sediment in this warm, shallow-water environment resulted in massive deposits of Cambrian limestone and dolomite. Examples are exposed in the Cordilleran region of North America, in north-central Australia, along the Yangtze River in central China, and along the Lena River emanating in Siberia. Few Cambrian rocks from land environments have been documented, and most of those are of limited areal extent. They mainly represent deposits of floodplains and windblown sand. Without plants or animals, the desolation of Cambrian landscapes must have rivaled that of any present-day desert. In the absence of plants with roots to hold soil in place, Cambrian lands in general probably eroded more rapidly than they do now. Relative sea level rose significantly during the Cambrian, but with fluctuations. This is indicated by both the geographic distribution and the stratigraphic succession of sedimentary deposits. In North America, for example, early Cambrian marine deposits covered only marginal areas, but late Cambrian marine deposits covered much of the continent. A similar distribution of marine rocks is present on other continents. In stratigraphic sections from continental shelves that were in low latitudes it is common for a basal, nearshore sandstone to be overlain by a transgressive succession of more seaward shale and carbonate rocks. Shelf sections from high latitudes may be mostly or entirely sandstone, or a basal sandstone may grade upward into shale, but most of these sections contain evidence of marine transgression. Exceptions to the general Cambrian sea-level pattern are commonly attributable either to local tectonism or to different rates of sediment accumulation. The most likely explanation for the general rise in Cambrian sea level seems to be increased thermal activity and related swelling of spreading ridges between lithospheric plates, which would displace vast quantities of seawater. It has been suggested that the general Cambrian transgression exerted an influence on adaptive radiation by greatly increasing the area of shallow seas where life was most abundant. Correlation Time correlation of Cambrian rocks has been based almost entirely on fossils. The most common fossils in Cambrian rocks are trilobites, which evolved rapidly and are the principal guide fossils for biostratigraphic zonation in all but rocks below the Atdabanian Stage or those of equivalent age (Table). Until the mid-1900s almost all trilobite zones were based on members of the order Polymerida. Such trilobites usually have more than five segments in the thorax, and the order includes about 95 percent of all trilobite species. Most polymeroids, however, lived on the seafloor, and genera and species were mostly endemic to the shelves of individual Cambrian continents. Therefore, polymeroid trilobites are useful for regional correlation but have limited value for intercontinental correlation, which has been difficult and subject to significant differences in interpretation. From the 1960s, investigators began to recognize that many species of the trilobite order Agnostida have intercontinental distributions in open-marine strata. These trilobites are small, rarely exceeding a few millimetres in length, and they have only two thoracic segments. Specialized appendages, which were probably useful for swimming but unsuitable for walking on the seafloor, suggest that they were pelagic. Agnostoids make up less than 5 percent of all trilobite species, but individuals of some agnostoid species are abundant. This fact, together with their wide geographic distribution and rapid evolution, makes them valuable for refined intercontinental correlation. Agnostoids first appear in upper Lower Cambrian rocks but did not become common or diversify significantly until the middle of the Cambrian. Therefore, agnostoids have their greatest biostratigraphic value in the upper half of the Cambrian System. A comprehensive trilobite zonation in Sweden has frequently been cited as a standard for correlation. Other kinds of fossils have had more limited use in Cambrian biostratigraphy and correlation. Among them are the archaeocyathan sponges in the Lower Cambrian and brachiopods throughout the Cambrian, but use of both groups has been hampered by problems of endemism. Small mollusks and other small shelly fossils, mostly of problematic affinities, have been employed for biostratigraphy in the Tommotian Stage, but their utility is also limited by endemism. Conodonts appear in the uppermost Precambrian but are rare in most Cambrian rocks except those of latest Cambrian age, when adaptive radiation of conodont animals accelerated. Wide species distributions, rapid evolution, and abundance make conodonts excellent indexes for global biostratigraphy in uppermost Cambrian to uppermost Triassic rocks. Since roughly the 1980s, trace fossils have been used with limited precision to correlate uppermost Precambrian and basal Cambrian strata. Although the biostratigraphic use of such fossils has many problems, they nevertheless demonstrate progressively more complex and diverse patterns of locomotion and feeding by benthic (bottom-dwelling) marine animals. For example, Phycodes pedum, which initially appears in basal Cambrian deposits, is the first regularly branching burrow pattern.