The calcareous reef alga Halimeda (Chlorophyta, Byropsidales): a cretaceous genus that diversified in the cenozoic

Abstract

The genus Halimeda (Order Bryopsidales) is an important contributor of calcareous sediments to present day reefs. By ∼80 million years ago the genus appeared to have achieved limited distribution in both Old and New Worlds; today its 33 extant species are widely distributed throughout the tropics. Halimediform genera of the Ordovician exhibit considerable morphological similarity to the modern genus. However, the first records of Halimeda fossils appear to be Permian, with the greatest pre-Cenozoic species diversity being achieved during the latter part of the Cretaceous. The long paleohistory is capped by an apparent burst of speciation associated with the Holocene. At least three time periods during the ca. 260Mya of Halimeda history are likely to have had major impacts on evolution of the genus: (1) Cretaceous-Tertiary boundary events; (2) closing of the circumtropical Tethyan seaway with associated Messinian crisis; and (3) final closure of the Panama seaway. However, little concerning the relationship of these events to Halimeda phylogeny can be gleaned from its recorded paleohistory. The first strong phylogenetic data for the genus, provided by cladistic analyses of 18sDNA sequences, identify principal lineages corresponding to the three major taxonomic sections (Rhipsalis, Opuntia and Halimeda). These lineages provide the major axes of a proto phylogeny, and represent an apparent differentiation of the genus into at least 3 basic thallus designs. The most complex of the phylogenetic trees presented also indicates a geographic separation of rhipsalian species into Atlantic and Pacific clades. It is likely that these two major events occurred during the Cenozoic, with the second associated with a vicariant event such as the cessation of the circumglobal Tethys current. Research by paleontologists is especially needed for developing time references for major evolutionary events which will lead to setting an evolutionary clock for Halimeda independent of molecular data. Paleontologists also have an important role to play in testing the hypothesis that the burst of speciation in the Holocene is an artifact of differential collection, differential preservation or both, and that some of this speciation occurred in preceeding epochs. Investigation of these hypotheses, coupled with better samples, will reveal much more of the history of the genus during the period of its greatest success, the Cenozoic.