Trilobite evolutionary rates constrain the duration of the Cambrian explosion
John R. Paterson, Gregory D. Edgecombe, and Michael S. Y. Lee
PNAS published ahead of print February 19, 2019
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Edited by Andrew H. Knoll, Harvard University, Cambridge, MA, and approved January 9, 2019 (received for review November 12, 2018)
Significance
The Cambrian explosion was arguably the most important biological event after the origin of life. Extensive research has been devoted to understanding when it began but far less on when this burst of evolution ended. We present a quantitative study that addresses these issues, using a large new dataset of Cambrian trilobites, the most abundant and diverse organisms during this time. Using probabilistic clock methods, we calculate rates of evolution in the earliest trilobites virtually identical to those throughout their Cambrian fossil history. We conclude that the Cambrian explosion was over by the time the typical Cambrian fossil record commences and reject an unfossilized Precambrian history for trilobites, solving a problem that had long troubled biologists since Darwin.
Abstract
Trilobites are often considered exemplary for understanding the Cambrian explosion of animal life, due to their unsurpassed diversity and abundance. These biomineralized arthropods appear abruptly in the fossil record with an established diversity, phylogenetic disparity, and provincialism at the beginning of Cambrian Series 2 (∼521 Ma), suggesting a protracted but cryptic earlier history that possibly extends into the Precambrian. However, recent analyses indicate elevated rates of phenotypic and genomic evolution for arthropods during the early Cambrian, thereby shortening the phylogenetic fuse. Furthermore, comparatively little research has been devoted to understanding the duration of the Cambrian explosion, after which normal Phanerozoic evolutionary rates were established. We test these hypotheses by applying Bayesian tip-dating methods to a comprehensive dataset of Cambrian trilobites. We show that trilobites have a Cambrian origin, as supported by the trace fossil record and molecular clocks. Surprisingly, they exhibit constant evolutionary rates across the entire Cambrian, for all aspects of the preserved phenotype: discrete, meristic, and continuous morphological traits. Our data therefore provide robust, quantitative evidence that by the time the typical Cambrian fossil record begins (∼521 Ma), the Cambrian explosion had already largely concluded. This suggests that a modern-style marine biosphere had rapidly emerged during the latest Ediacaran and earliest Cambrian (∼20 million years), followed by broad-scale evolutionary stasis throughout the remainder of the Cambrian.
Cambrian explosion evolutionary rates trilobites Bayesian tip-dating morphological clock
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