MORPHOLOGIC DISPARITY DURING THE EARLY EVOLUTION OF A COMMON CARIBBEAN REEF CORAL

Morphometric analyses show that more than nine species existed within the Montastraea “annularis” complex during its early evolution in the Mio-Pliocene of the Dominican Republic, and many had durations of up to 2-5 million years. In addition to higher diversity, comparisons with modern members of the complex show that the Dominican Republic species were more variable both within and among colonies, suggesting less colony-wide coordination and fewer growth-related morphologic constraints during the early evolution of the species complex. Speciation within the complex has resulted in continued narrowing of this initial morphologic variability, both within and among species. Phylogenetic analyses further indicate that the three modern members of the complex are not sister species but belong to different clades, two of which diverged during the early to middle Pliocene prior to Plio-Pleistocene reef coral turnover. The three modern members of the M. “annularis” complex are thus survivors of previously more diverse, distinct evolutionary groups.

Samples consist of 208 colonies of M. “annularis”-like corals (less than four septal cycles), which were collected along four river sections in the Cibao Valley of the northern Dominican Republic. Five stratigraphic intervals are represented, ranging from approximately 17.3-13.1 Ma to 3.7-3.4 Ma (with a gap at ~13.1-8.3 Ma). Data for morphometric and phylogenetic analyses are based on coordinates of 27 spatially homologous points or landmarks, which define the thickness and structure of the corallite wall and associated costosepta. Numbers of septa, relative development of septal cycles, and measures of colony growth form are also recorded. Species are recognized using Bookstein size and shape coordinates as variables in cluster and canonical discriminant analyses. Variation within and among colonies is estimated using principal component analysis. Cladistic analyses are based in part on characters derived from morphometric data.