Abstract
To better understand the patterns and processes shaping large-scale phenotypic diversification, I integrate palaeobiological and phylogenetic perspectives to investigate a ~200-million-year radiation using a global sample of Palaeozoic crinoid echinoderms. Results indicate the early history of crinoid diversification is characterized by early burst dynamics with decelerating morphologic rates. However, in contrast with expectation for a single “early burst” model, morphospace continued to expand following a slowdown in rates. In addition, I find evidence for an isolated peak in morphologic rates occurring late in the clade’s history. This episode of elevated rates is not associated with increased disparity, morphologic novelty, or the radiation of a single subclade. Instead, this episode of elevated rates involved multiple subclade radiations driven by environmental change toward a pre-existing adaptive optimum. The decoupling of morphologic disparity with rates of change suggests phenotypic rates are primarily shaped by ecologic factors rather than the origination of morphologic novelty alone. These results suggest phenotypic diversification is far more complex than models commonly assumed in comparative biology. Furthermore, palaeontological disparity patterns are not a reliable proxy for rates after an initial diversifying phase. These issues highlight the need for continued synthesis between fossil and phylogenetic approaches to macroevolution.
Highlights
Major fluctuations in the rate of morphologic evolution are evident across the Palaeozoic radiation of eucladid crinoids (Fig. 1)
Rates significantly drop during the Devonian, reaching their Palaeozoic minimum during the late Devonian Frasnian-Famennian stages
Rates were elevated throughout much of the late Carboniferous, with a singular burst of morphologic evolution occurring during the Moscovian stage that resulted in the only significant post-Silurian peak in morphologic rates (Fig. 1, Methods and Materials)
Summary
To better understand the patterns and processes shaping large-scale phenotypic diversification, I integrate palaeobiological and phylogenetic perspectives to investigate a ~200-million-year radiation using a global sample of Palaeozoic crinoid echinoderms. The lack of support for adaptive radiation in comparative data may reflect statistical difficulties in testing for early bursts[14], the discordance between fossil and phylogenetic approaches highlights the need to consider alternative models of morphologic diversification and the importance of considering taxonomic and temporal scales when relating pattern to process[9,15]. To test whether large-scale patterns of morphologic evolution within a long-lived clade are primarily shaped by early burst-like dynamics or increased phenotypic constraint, I apply recently developed methods integrating both phylogenetic and palaeobiologic perspectives to empirically document ~200 million years of morphologic diversification using a major clade of marine invertebrates, the eucladid Crinoidea To account for uncertainty in tree topology, divergence times, and branch lengths, I estimated mean and median rates of character change over a random sample of time-calibrated trees obtained from a Bayesian posterior distribution of fossil tip-dated phylogenies (Materials and Methods)
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