Functional aspects of the armour in the early vertebrates

The possible factors stimulating the development of carapaces in the early vertebrates are discussed; biological factors are considered as primary. The specific vertebrate assemblages, which most probably formed in the preSilurian times and included higher developed representatives offishes, e.g. chondrichthyans and osteichthyids, caused the changes in the locomotion of agnathans, and consequently in their body shape. The absence of the paired and some unpaired fins in these vertebrates was to some extent compensated by rigidly fixed exoskeletal elements (keels, spines). Though in rectilinear motion the armoured agnathans could compete with fishes in swimming speed they never reached manoeuvrability of fishes with a complete set of fins. Having exhausted the potential of the development of their exoskeleton, the agnathans became extinct.