Paleozoic record of morphologica diversity in blastozoan echinoderms.

Abstract

There has been extensive debate about the magnitude and implications of morphological diversity in early Paleozoic animals, with some workers using apparently rapid initial diversification to infer unusual evolutionary processes. Analysis of discrete morphological characters shows that initial morphological diversification in the echinoderm subphylum Blastozoa was so pronounced that morphological diversity relative to taxonomic diversity was greatest in the Cambrian, whereas morphological diversity itself was greatest in the Middle and Upper Ordovician. Thus, a small number of Cambrian taxa sparsely occupied a large range in morphological space, whereas subsequent diversification involved expansion and filling of morphospace. A measure of clade-shape asymmetry and a method for statistical testing of lade shape are used to show that morphological diversity is significantly concentrated early in the history of the Blastozoa. The subphylum represents the highest biologic level at which temporal patterns of morphological diversity have been analyzed. Because this study is based on explicit morphological analysis, not taxonomic proxies for morphological diversity, the results are not artifacts of taxonomic practice.The early Paleozoic represents an extraordinary phase of life's history, involving extensive radiation of basic body plans, macroevolutionary response to vast ecological opportunities, and, arguably, substantial genomic evolution (1-14). Patterns of morphological diversity are central to our understanding of such large-scale evolutionary processes in early metazoans, but very little of our knowledge is based on direct analysis of morphology. Taxonomic diversity is often used to study the radiation of form but provides only an imperfect proxy for morphological diversity (2,4,(15)(16)(17). In fact, it is discrepancies between morphological and taxonomic diversity that may most clearly illuminate the early metazoan radiations (2-5, 15, 16).Echinoderms are a major component of the early Paleozoic radiation. Previous studies have emphasized the pace of morphological evolution in echinoderms based on the temporal occurrence of novelties (8, 18). Such an approach is inadequate for assessing diversity. Each organism consists of primitive and derived characters; therefore, exclusive focus on novelties would mean that different characters were studied for each species, largely defeating the purpose of considering all species simultaneously. Moreover, taxonomic turnover plays an important role in patterns of diversity. For example, a clade that contains lineages with the primitive and derived states of a character represents greater morphological diversity than a similar clade in which lineages with the primitive state have become extinct, leaving only lineages with the derived state. Study of novelties alone would overlook this important distinction. For these reasons, analyses presented here are based on quantification of primitive and derived aspects of morphology.Because it is difficult to quantify organi...

Extinction as the loss of evolutionary history

“…The impact on morphological disparity is apparent from the large number of clades lost (trilobites, blastoids, and the tabulate and rugose corals) or severely affected (articulate brachiopods, echinoids, ammonoids, radiolarians, bryozoans, and foraminifera). The loss of disparity is confirmed by quantitative studies of disparity among brachiopods (45), ammonoids (46), and crinoids and blastoids (48,98). Carbon isotopes indicate a significant loss of primary productivity (19) that persisted for Ϸ2 million years (99,100).…”

Trait-based diversification shifts reflect differential extinction among fossil taxa

“…We measure morphological disparity as the average pairwise dissimilarity among species [i.e., the differing characters between two taxa=characters coded for both taxa (64)]. We use cumulative disparity rather than standing disparity (i.e., the average pairwise dissimilarity among all S taxa in a dataset and the average pairwise dissimilarity among the oldest S/2 taxa in that dataset).…”

What limits the morphological disparity of clades?

“…When multiple disparity indices are used to capture variation in the same aspects of form, these often show a high degree of congruence. However, conflicting patterns are possible, depending upon the variance structure of the data [28][29][30][31][32].…”

“…Several indices of disparity assess the distribution of taxa in such morphospaces: for example, by adding the ranges or variances on successive axes (a boxing approach), using convex hulls or determining the mean distance between all pairs of taxa. Indices can then be used to compare the disparity of constituent subclades, or to track the morphospace occupation of one or more groups through time, thereby building up a disparity profile [6,[28][29][30][31][32]. When multiple disparity indices are used to capture variation in the same aspects of form, these often show a high degree of congruence.…”