Meeting Abstract
Axial regionalization is one of many skeletal modifications that allowed tetrapods to make the transition from water to land. The presence of five morphologically distinct regions along the vertebral column (i.e., cervical, thoracic, lumbar, sacral and caudal) has frequently been linked to the demand of weight-bearing and walking on land. This condition has long been thought to be restricted to tetrapods. While osteichthyans have long been characterized by a column divided solely into two regions (i.e., abdominal and caudal), recently, five regions were described in a Carboniferous actinopterygian fish, Tarrasius. Furthermore, a cervical region has recently been identified in some extant actinopterygians. Although rarely investigated, chondrichthyan axial regionalization has been suggested in the anterior part of the column (e.g., synarcuum of chimaeras and rays). Considering these facts, the regionalization of the chondrichthyan vertebral column has been investigated using an ontogenetic series of cleared-and-stained specimens of black dogfish (Centroscyllium fabricii). At least three axial regions have been recognized based on the chondrification and mineralization of vertebral components (e.g., centra, ribs, arches, spines) as well as on specific conditions (e.g., size and shape, fusion). Description of the axial regionalization in sharks provides insights on the apparition of axial patterning in early gnathostomes.
