| The question of whether a hagfish is a true vertebrate or not has profound implications about the ancestry of the clade. New anatomical evidence allows a test of their systematic position. With dissections and serial sections of original specimens, and with a literature review, a comparative analysis revealed homologues in the chondrocranium and musculature of hagfish shared with lampreys, gnathostomes, and extinct jawless vertebrates. The analysis also identified intermediate characters that foreshadow gnathostomes, including a possible precursor of the synovial joint and tendon-like pseudocartilages. However, the hierarchical organization of the homologues presented an enormous challenge to recovering a phylogenetic signal. The traditional morphological view of the vertebrate head was overturned to formulate new evolutionary models for the ancestry of vertebrates and the origin of the gnathostome jaw. A phylogenetic analysis placed hagfish more basal than other living vertebrates, and a new definition was proposed for the clade Vertebrata. |
