Morphospace studies abstract and simplify information on morphologic variation, using a variety of mathematic algorithms. The principal advantage of this approach is that, provided that the same information is abstracted for each specimen analyzed, relationships among taxa can be evaluated within a uniform reference frame. Hence the user can be confident that like is being compared with like, largely independently of taxonomy. Here the term morphospace rather than morphologic is used here, because these studies consider the relative placement of individuals within a space that is defined by the same set of individuals. Because morphospaces are based on a sample of the overall morphology, the extent to which they summarize the group's morphologic variation depends on the degree to which the sample is representative of total morphology. Morphospace approaches have been used to address a variety of questions within Cambrian trilobites (e.g., Ashton and Rowell 1975; Schwimmer 1975), but the most relevant application to the ecologic evolution of the Trilobita has been attempts to assess the morphologic diversification of trilobites throughout their evolutionary history (Foote 1990, 1991, 1992, 1993).
Using an analysis of the outline of the cranidium in trilobites that have a dorsal suture and the outline of the cephalon in forms that do not, Foote (1989) suggested that the morphologic diversity of polymerid trilobites increased from the earlier to later Cambrian, followed by a sharp increase in diversity in the later Ordovician (Foote 1991) (figure 17.4A). Although the earlier Cambrian shows the lowest overall diversity, the transition to the later Cambrian is not marked by a significant jump in area of occupied morphospace, despite the large increase in numbers of species sampled. Furthermore, the variance of earlier Cambrian trilobites apparently exceeds that of later Cambrian forms (Foote 1993) (figure 17.4B). The transition from Cambrian to later Ordovician was marked by the appearance of several distinct trilobite morphotypes, which went on to dominate the remainder of trilobite evolutionary history. Given the roughly similar volume of morphologic space occupied by earlier and later Cambrian trilobites, the greater variance of earlier forms, and the profound difference in numbers of species in each interval, estimates of diversity must be corrected to assess the effects of differing sample sizes. These analyses showed that earlier Cambrian morphologic diversity might actually have been higher than that of the later Cambrian (Foote 1992). Alternatively, if the increased sample size of later Cambrian trilo-bites reflects an absolute increase in taxic diversity during that time, it may suggest that the later Cambrian diversification of trilobites was morphologically constrained.
Foote's work permits an improved understanding of the morphologic diversification of trilobites, but interpretation of his data is complicated by the fact that the cephalic structures studied were not homologous among all the trilobites surveyed (Foote 1991). Early Cambrian olenelloids lacked a dorsal facial suture, and so the outline of the cephalon was used as a proxy for cranidial form. The cephalic outline includes the genal spine, a character showing considerable variation, and the presence of this spine contributes to the high disparity among olenelloid taxa (Foote 1991: text-figure 3) relative to forms in which cranidial outline was used. The argument that inclusion of the genal spine increases intragroup variability is supported by the pattern shown in the Ordovician cheirurids, which also occupied a larger proportion of morphospace than other groups and show a greater intrataxon disparity. Cheirurids had a proparian facial suture, with the result that their genal spines were also in
Figure 17.4 A, Morphological diversification of Cambrian and Ordovician trilobites as expressed by the first two principal components of Fourier coefficients of the outlines of cephalic structures (Foote 1989, 1993). Note the relatively constant area of morphological space occupied from the earlier Cambrian through
Figure 17.4 A, Morphological diversification of Cambrian and Ordovician trilobites as expressed by the first two principal components of Fourier coefficients of the outlines of cephalic structures (Foote 1989, 1993). Note the relatively constant area of morphological space occupied from the earlier Cambrian through earlier Ordovician. B, Morphological variance of trilobites. Note that the variance of earlier Cambrian trilobites is slightly higher and shows greater error estimates than that of the later Cambrian. Source: Figures computed by Mike Foote.
cluded in the data set. Whether this anomaly explains the relatively high variance in Early Cambrian trilobites as revealed by rarefaction analysis (Foote 1992) is unclear. Nevertheless, high variation in the Early Cambrian is consistent with the appearance of five trilobite orders during that time, each with a distinctive morphology.
Estimates of the range of maximum sizes within a group provide a measure of ecologic diversity, because maximal body size is directly related to ecologic activity (McKinney 1990). Analysis of the size ranges of Cambrian trilobites was attempted using data on
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Late Cambrian -
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