DOES ONTOGENY RECAPITULATE PHYLOGENY? AN EXAMPLE FROM EXTANT BRACHIOPOD CRURAL TYPES

Heterochrony has been hypothesized to play a role in the evolutionary transitions among crural types in brachiopods (Rhynchonellida), but these hypotheses have yet to be tested rigorously. Crura position the lophophore in the mantle cavity and are thus important functionally; they have also been significant in structuring the classification of these brachiopods, and suggesting patterns of evolutionary change within rhynchonellide lineages. Three types of information are required to test hypotheses of heterochrony: shape change relative to size change; ontogenetic age; and phylogenetic relationship. We quantified morphological change among juvenile individuals of different sizes (and ages) from several species of Recent Rhynchonellida using three-dimensional geometric morphometric techniques and multivariate statistical analyses to establish the relationship between crural size and shape over ontogeny. This, along with separate morphological and molecular phylogenetic hypotheses of Recent rhynchonellides previously generated, provides a foundation for investigating hypotheses of heterochronic change among the six Recent rhynchonellide crural types, from among the 17 named types. Not unexpectedly, statistically significant allometric shape change in each of the first three principal components is associated with ontogenetic size increase in raduliform crura. As size increases, crural length increases, the distal tip rotates medially, crural width decreases relative to length, and crural curvature increases ventrally. More interestingly, a cluster analysis of the loadings on the first three principal components suggests that successively older juvenile raduliform crura in Notosaria nigricans are more similar in shape to adult crural morphs (spinuliform and falciform) present in several different genera than they are to adult raduliform morphs of N. nigricans. Documenting in greater quantitative detail the intriguing and complex relationship between crural size and shape over ontogeny and phylogeny will allow us to test the extent to which our classification of Rhynchonellida represents a defensible proxy for evolutionary patterns in this group. It will also provide a more detailed picture of phylogenetic patterns as they might be revealed in ontogenetic transformations.