This study represents the first in a series tracing the early evolution of a dominant Caribbean reef coral, the Montastraea “annularis” species complex, using a combination of morphometric and phylogenetic approaches. It focuses on Costa Rica and Panama; additional geographic locations and reef environments will be treated in subsequent work. To distinguish species, new landmark methods are developed by comparisons with genetically characterized modern colonies from Panama. The fossil material consists of transverse thin-sections of 94 well-preserved specimens of M. annularis-like corals collected in Plio-Pleistocene reef sequences in the Limón basin of Costa Rica and the Bocas del Toro basin of Panama. The landmarks comprise 27 spatially homologous points, which define the thickness and structure of the corallite wall and associated costosepta. Bookstein size and shape coordinates are analyzed using cluster analysis and canonical discriminant analysis, and a total of 10 morphotypes are distinguished. Cladistic analyses are performed using characters derived partially from morphometric data. A matrix consisting of 16 taxa and eight characters is analyzed using global parsimony and a sister group of two species as the outgroup. The results reveal two distinct evolutionary groups, which are distinguished by the new corallite wall characters. One group contains one modern species, and the other contains a second; the relationships of the third are poorly resolved.

Despite the low number of equally parsimonious trees, high numbers of plesiomorphic taxa, long range extensions, and lack of agreement with genetic data indicate that the new characters alone are inadequate for completely interpreting evolutionary relationships, and more samples and characters are needed. Nevertheless, these preliminary results do show that three modern species of the M. “annularis” complex arose prior to accelerated extinction at the end of late Pliocene to early Pleistocene faunal turnover of Caribbean reef corals, and two may have originated younger than 4 Ma. Six or more new species may be represented in upper Pliocene to lower Pleistocene sequences in Costa Rica and Panama. Coexistence of predominantly pre- and post-turnover clades may have been responsible for the high diversity observed within the species complex in these two sequences.