Vertebral development and amphibian evolution

Amphibians provide an unparalleled opportunity to integrate studies of deve lopment and evolution through the investigation of the fossil record of lar val stages. The pattern of vertebral development in modern frogs strongly r esembles that of Paleozoic labyrinthodonts in the great delay in the ossifi cation of the vertebrae, with the centra forming much later than the neural arches. Slow ossification of the trunk vertebrae in frogs and the absence of ossification in the tail facilitate the rapid loss of the tail during me tamorphosis, and may reflect retention of the pattern in their specific Pal eozoic ancestors. Salamanders and caecilians ossify their centra at a much earlier stage than frogs, which resembles the condition in Paleozoic leposp ondyls. The clearly distinct patterns and rates of vertebral development ma y indicate phylogenetic separation between the ultimate ancestors of frogs and those of salamanders and caecilians within the early radiation of ances tral tetrapods. This divergence may date from the Lower Carboniferous. Comp arison with the molecular regulation of vertebral development described in modern mammals and birds suggests that the rapid chondrification of the cen tra in salamanders relative to that of frogs may result from the earlier mi gration of sclerotomal cells expressing Pax1 to the area surrounding the no tochord.