Occurrence

Trilobite taxa differ in their temporal and geographic distributions. The study of trilo-bite distributions provides information on the ecologic evolution of the group, even if this information cannot be directly related to specific niches. Occurrence, along with analogy and functional considerations, can constrain hypotheses about trilobite life habits (Fortey 1985). For example, the widespread geographic occurrence of agnos-tid trilobite species, upon which much of intercontinental Cambrian biocorrelation rests, supports morphology-based arguments that these animals were free-swimming and possibly pelagic (see the section "Specialized Morphologies and 'Morphotypes' " above). Pioneering studies of the global distribution of Cambrian trilobites (e.g., Richter and Richter 1941; Repina 1968, 1985; Cowie 1971; Jell 1974; Taylor 1977; Shergold 1988) indicate broad faunal provinces during Cambrian time. Faunal data are broadly consistent with other indicators of Cambrian global paleogeography. Lau-rentian shelf faunas are apparently the most distinctive, a characteristic consistent with the notion that Laurentia was geographically isolated during Cambrian times. A widespread shelf fauna occurs about the peri-Gondwanan margin, although the restriction of many elements to specific regions suggests some paleolatitudinal control of faunal distribution. Faunas adapted to cooler waters had more widespread occur-

Figure 17.6 Contrasting biogeographic distributions of two Late Cambrian trilobite genera. The distribution of Erixanium, which shows a worldwide equatorial distribution (Stitt et al. 1994), was apparently constrained by factors related to latitude. In contrast, Maladioidella shows a widespread peri-Gondwanan distribu tion (Rushton and Hughes 1996). The greatest constraint on its distribution was crossing oceanic basins. The Maladioidella distribution tract is superimposed over the Erixanium tract for graphic clarity only. Source: Base map provided by Chris Scotese.

Figure 17.6 Contrasting biogeographic distributions of two Late Cambrian trilobite genera. The distribution of Erixanium, which shows a worldwide equatorial distribution (Stitt et al. 1994), was apparently constrained by factors related to latitude. In contrast, Maladioidella shows a widespread peri-Gondwanan distribu tion (Rushton and Hughes 1996). The greatest constraint on its distribution was crossing oceanic basins. The Maladioidella distribution tract is superimposed over the Erixanium tract for graphic clarity only. Source: Base map provided by Chris Scotese.

rence than did faunas restricted to equatorial shelf environments (e.g., Wilson 1957; Taylor 1977). Cambrian trilobite faunas have now been described from most parts of the world, but a great deal of phylogenetic analysis is necessary before the potential of Cambrian trilobites for assessing paleogeography is fully realized. Studies of the global distributions of genera or species suggest that ecologic factors controlling distributions differ markedly, even among taxa with broadly similar morphologies (figure 17.6).

In spite of these problems, the distributions of distinctive morphotypes suggest consistent broad patterns of trilobite distribution among paleocontinents. For example, distinctive oryctocephalid, olenid, eodiscid, and agnostid trilobites are common in slope facies and had wide geographic distributions during life. These forms are characterized by thin cuticles, which is common also in Ordovician trilobites inhabiting deeper water (Fortey and Wilmot 1991). Offshore benthic polymerids commonly share the "olenimorphic" morphotype, consisting of multisegmented thoraces with narrow axes and wide pleurae (Fortey and Owens 1990a). A comparison of Laurentian and Siberian faunas suggests morphologic and distributional similarities among shelf faunas, even though phylogenetic relationships among these faunas remain unclear. For example, diverse assemblages of Late Cambrian trilobites are known

Figure 17.7 Cartoon cross section of northern North America during the Late Cambrian serotina interval, showing lithofacies and associated trilobite biofacies. The biofacies represent distinguishable faunal associations. Eup-Eur,

Ka-Yu, and Bie represent level-bottom biofacies; Pl-Ca and Log are outer platform bank-edge biofacies. Source: Modified from Ludvig-sen and Westrop (1983: figure 17).

Figure 17.7 Cartoon cross section of northern North America during the Late Cambrian serotina interval, showing lithofacies and associated trilobite biofacies. The biofacies represent distinguishable faunal associations. Eup-Eur,

Ka-Yu, and Bie represent level-bottom biofacies; Pl-Ca and Log are outer platform bank-edge biofacies. Source: Modified from Ludvig-sen and Westrop (1983: figure 17).

from outer shelf facies in both paleocontinents (Pegel' 1982; Ludvigsen and Westrop 1983). Similarly, Late Cambrian nearshore assemblages show relatively reduced diversity (Hughes 1993), although unusual morphotypes can be common in these facies (Hughes et al. 1997).

At a smaller geographic scale, repeated associations of taxa have been recognized as biofacies (e.g., Lochman-Balk and Wilson 1958). In many cases, biofacies can be related to specific paleoenvironments, and a variety of statistical methods have been used in their definition (e.g., Ludvigsen and Westrop 1983; Pratt 1992; Babcock 1994; Westrop 1995). Detailed biofacies analyses have been undertaken in North America for portions of Cambrian time. In many cases these biofacies possess distinctive suites of trilobites and can be related to specific lithofacies. For example, the Sunwaptan Euptychaspis-Eurekia biofacies consistently shows a similar array of taxa in approximately constant proportions and is always associated with light-colored shelf packstones and grainstones (Ludvigsen and Westrop 1983; Westrop 1986) (figure 17.7). However, other biofacies, such as the Kathleenella-Yukonaspis biofacies, which occurs in light-colored wackestones from ramp settings, show considerable variation in constituent taxa and their relative abundances (e.g., Ludvigsen and Westrop 1983; Westrop 1995). Different biofacies vary markedly in the numbers of constituent taxa, their taxonomic integrity, and the range of lithofacies that they occupy. Hence, tracking the temporal and geographic establishment and demise of biofacies, and their constituent taxa, can provide insight into the ecologic evolution of Cambrian communities.

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