Modularity and developmental patterning in the evolution of early jawed vertebrates

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Approximately 540 million years ago, the ancestor of all craniates, which gave rise to today's hagfishes, lampreys, cartilaginous (ratfishes, skates and sharks) and bony fishes (ray-fined fishes, coelacanths, lungfishes and tetrapods), acquired distinctive embryological features - the neural crest and the epidermal placodes (specialized groups of embryonic cells forming early during organogenesis). These two developmental novelties are accountable for numerous anatomical characteristics of craniates, such as the brain and craniofacial cartilages and bones. No other developmental novelties have been associated to the early evolution of jawed vertebrates, or gnathostomes (acanthodians, placoderms, cartilaginous and bony fishes). As a result, basal gnathostome interrelationships are poorly resolved. Most likely, the recognition of new developmental features such as developmental modules (a network of interacting elements behaving as relatively independent units of development) and patterning (the order of formation of these elements) can help solving gnathostome phylogeny. All anatomical modules that have been previously proposed will be compiled to reconstruct a theoretical model of gnathostome modularity. The presence of these fundamental modules will be tested in living gnathostomes by looking at (1) their internal pattern of variation and (2) their pattern of formation. These modules and patterning will be evaluated in fossil taxa and subsequently used to reconstruct phylogeny. In other words, we will be addressing three questions about the evolution of modularity and patterning in jawed vertebrate history: (1) What was the modular organization of gnathostomes at their origin? (2) Can we still recognize these modules and their associated patterns of formation in living and fossil gnathostomes? (3) Can we reconstruct the gnathostomes' tree of life based on new modular and patterning characters? This research program will help understanding developmental mechanisms that have been responsible for the evolution of jawed vertebrates, the taxonomic group we belong to and from which we inherited fundamental anatomical and developmental characteristics.