Evolutionary explosions and the phylogenetic fuse

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Abstract

A literal reading of the fossil record indicates that the early Cambrian (c. 545 million years ago) and early Tertiary (c. 65 million years ago) were characterized by enormously accelerated periods of morphological evolution marking the appearance of the animal phyla, and modern bird and placental mammal orders, respectively. Recently, the evidence for these evolutionary `explosions' has been questioned by cladistic and biogeographic studies which reveal that periods of diversification before these events are missing from the fossil record. Furthermore, molecular evidence indicates that prolonged periods of evolutionary innovation and cladogenesis lit the fuse long before the `explosions' apparent in the fossil record.

Section snippets

New evidence from molecular data

Thanks to the increased availability of DNA sequence information, molecular tests of distant divergence events are becoming common9, 10, 11, 12, 13, 14, 15, 16(Appendix A). Divergence dates from molecular data are generally expected to be earlier than those from palaeontological data because they simply estimate the point at which genetic intermixing ceases while fossils record when a species has developed diagnostic characters, as well as attained fossilization. However, the extent of this

The Cambrian evolutionary explosion

The base of the Cambrian marks not only the appearance in the historical record of animal phyla, but also of many of their component major classes. There is no question that this time marked the nearly simultaneous appearance of hard shells (and hence commonly-preserved fossils) in many animal groups, but this may be decoupled from the appearance of the phyla and classes themselves. Among the arthropods, for example, there are Cambrian records of crustaceans, and stem group arachnoids, as well

The origin of modern bird and placental mammal orders

The history of birds and placental mammals presents many interesting similarities. Modern forms are abundantly represented in the favourable depositional environments of the late Palaeocene/early Eocene, but Mesozoic bird and mammal fossils are relatively rare and mostly represent `archaic' forms (e.g. multituberculates and enantiornithines) not closely related to their modern counterparts5, 7, 33. However, recent fossil discoveries have started to reveal tantalizing evidence of a Cretaceous

The phylogenetic fuse

Evolution has proceeded by innovation on the one hand and radiation on the other; and the two are not necessarily coupled. The fastest radiations from the recent past—the insect and bird endemics which evolved on Hawaii over the past five million years or so would be an example—have produced striking opportunistic adaptations but not, apparently, fundamentally new designs. Conversely, there are examples of major Baupläne such as represented by phyla today— chaetognaths or echiuroids are two

Acknowledgements

We thank many colleagues for stimulating discussions and access to unpublished data, especially David Wagner, David Penny and Tom Kemp. Chris Simon, Michael Benton, David Archibald and an anonymous reviewer provided helpful suggestions that improved the manuscript. Matt Wills generously provided a diagram. Financial support was provided in part by the UK NERC, and New Zealand Marsden Fund.

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