Nonprogressive Evolution in a Clade of Cretaceous Montastraea-Like Corals

A phylogeny of Cretaceous Montastraea-like corals was constructed and used to evaluate the importance of differential speciation, selective extinction, and developmental constraints in the evolutionary history of the clade. Colonies assembled from localities across the central and western Tethyan region were subdivided into four stratigraphic levels: (1) Neocomian to lower Albian, (2) upper Albian to Cenomanian, (3) Turonian to Campanian, and (4) Maastrichtian. Ten corallite characters were measured on transverse thin sections of each colony, and analyzed following a three-step procedure: (1) species were recognized using all-inclusive average linkage cluster analyses (UPGMA) and a series of iterative canonical discriminant analyses; (2) species were separated by stratigraphic level and linked between levels by comparative analysis of the resulting discriminant scores; and (3) ancestor-descendant relationships were interpreted within and among adjacent levels using phenetic and cladistic approaches. A composite tree was then examined with respect to biogeography. The results suggest that a total of 16 species existed during the Cretaceous, only 2 of which extended between stratigraphic levels. Speciation events between levels 1 and 2 were associated with the radiation of "small-corallite" forms from a "large-corallite" form, as its distribution expanded southward into the Tethyan realm. Limited speciation and stasis predominated among species within the clade between levels 2 and 3. Speciation events between levels 3 and 4 were associated with the radiation of predominantly "large-corallite" forms from a "small-corallite" form, as the clade became restricted to the Caribbean. Extinction initially focussed on "small-corallite" forms, but later shifted to "large-corallite" forms. Morphologic change was constrained between two extremes, the lower limit of which involved a minimum corallite diameter possibly set by developmental or ecological constraints, and the upper by limitations to increases in number of septa. The overall net result was that of nonprogressive evolution within the clade.