Decoupling of taxonomic diversity and morphological disparity during decline of the Cambrian trilobite family Pterocephaliidae

Abstract

Although discordance between taxonomic diversity and morphological disparity is common, little is known about the underlying dynamics that drive this decoupling. Early in the history of the Cambrian trilobite family Pterocephaliidae, there was an increase in taxonomic diversity and morphological diversity. As taxonomic diversity declined in the later history of the clade, range of variation stayed high and disparity continued to increase. However, per-branch rates of morphological evolution estimated from a recent phylogeny decreased with time. Neither within-trait nor within-species variation increased or decreased, suggesting that the declining rates of morphological evolution were more likely related to ecological opportunity or niche partitioning, rather than increasing intrinsic constraints. This is further supported by evidence for increased biofacies associations throughout the time period. Thus, the high disparity seen at low taxonomic diversity late in the history of this clade was due to extinction -either random or targeting mean forms -rather than increased rates of morphological evolution. This pattern also provides a scenario that could account for instances of low taxonomic diversity but high morphological disparity in modern groups.

Extinction Space-a Method for the Quantification and Classification of Changes in Morphospace Across Extinction Boundaries

“…Outside of restricting taxa in the postextinction sample to species belonging to families occurring in the preextinction interval, we did not explicitly consider phylogenetic relationships between species of ammonoids in this analysis. The inclusion of ghost ranges frequently increases the range of morphology, particularly earlier in the evolutionary history of a clade (e.g., Wills ; Hopkins ), because of the backward skew in diversity counts (e.g., Wagner ; Lane et al. ).…”

Novelty and Innovation in the History of Life

“…Numerous studies of individual groups have shown that diversity and disparity are often decoupled, particularly early in the history of a clade. This pattern has been found not only for major clades [28,[109][110][111], but also within carnivores [31], salamanders [112], trilobites [113] and anomodont therapsids [114]. A recent analysis of disparity in 98 metazoan clades through the Phanerozoic found a preponderance of clades with maximal disparity early in their history [29].…”

Magnitude versus direction of change and the contribution of macroevolutionary trends to morphological disparity

“…Morphological evolution of Cambrian pterocephaliid trilobites. Phylomorphospace for trilobites based on principal components (PC) analysis of landmark data described in Hopkins (, b), realized during the beginning of the Steptoean Stage (top), the middle of the Steptoean Stage (middle), and the end of the Steptoean Stage (bottom). White circles, aphelaspidid trilobites (outgroup); black squares, pterocephaliine trilobites (with the genus Pterocephalia circled in the middle panel); grey triangles, housiine trilobites (with the genus Housia circled in the bottom panel).…”