Developmental modularity in living and fossil fishes

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Research on the evolution of living and fossil forms has benefited from the linkage between paleontology and evolutionary developmental biology (evo-devo). Both, neontologists and paleontologists are now viewing adult organism as the result of complex interactions among developmental stability mechanisms and developmental instability. Among the stability processes, phenotypic modularity has been given much attention because of its importance in the evolution of complex anatomical systems. Among the various definitions of modularity, a module could be defined as a network of interacting elements behaving as relatively independent units of development. Highly integrated changes in size and shape during ontogeny have been identified as modular units (e.g., human skull and limbs). Another type of ontogenetic change corresponds to sequences of chondrification (cartilage formation) and ossification (bone formation) that can be described in terms of (1) the time of first appearance of an element, (2) the relative timing within morphological units and (3) the direction of formation among serial elements within morphological units. Direction of ossification has been used to identify phenotypic developmental modules. Only a few modules have been identified in the skeleton of fishes (e.g., the similarity between the dorsal and anal fins). Phenotypic modularity of postcranial elements (vertebral column and fin supports) in living and fossil fishes will be investigated externally (size and shape) and internally (chondrification and ossification sequences) at three levels: (1) integration during ontogeny, (2) integration between ontogeny and environment, and (3) integration between ontogeny and phylogeny. In other words, we will be addressing three fundamental questions about developmental modules: (1) How do they form during the early life stages of a fish?; (2) Are they influenced by environmental factors or stresses?; and (3) Can we track down their evolution? This research program will help to understand developmental mechanisms that have been and are still responsible for the evolution of fishes, the most diversified group of living backboned animals.