Integrating phylogeny, molecular clocks, and the fossil record in the evolution of coralline algae (Corallinales and Sporolithales, Rhodophyta)

When assessing the timing of branching events in a phylogeny, the most important tools currently recognized are a reliable molecular phylogeny and a continuous, relatively complete fossil record. Coralline algae (Rhodophyta, Corallinales, and Sporolithales) constitute an ideal group for this endeavor because of their excellent fossil record and their consistent phylogenetic reconstructions. We present the evolutionary history of the corallines following a novel, combined approach using their fossil record, molecular phylogeny (based on the 18S rDNA gene sequences of 39 coralline species), and molecular clocks. The order of appearance of the major monophyletic taxa of corallines in the fossil record perfectly matches the sequence of branching events in the phylogeny. We were able to demonstrate the robustness of the node ages in the phylogeny based on molecular clocks by performing an analysis of confidence intervals and maximum temporal ranges of three monophyletic groups of corallines (the families Sporolithaceae and Hapalidiaceae, as well as the subfamily Lithophylloideae). The results demonstrate that their first occurrences are close to their observed appearances, a clear indicator of a very complete stratigraphic record. These chronological data are used to confidently constrain the ages of the remaining branching events in the phylogeny using molecular clocks.