 <unk> like <unk> are known from both large adult specimens and small larvae , showing an extreme change in body shape . In these species , the shape and proportions of skull bones change in the early stages of development . The ornamentation on the surface of the skull roof also develops at this time . Small , regularly spaced pits are the first to form , followed by larger ridges . As development continues , the external gills disappear . Small teeth that once covered the palate are lost . The postcranial skeleton does not develop at the same rate as the skull , with ossification ( the replacement of cartilage by bone ) happening more slowly . Vertebrae and limb bones are poorly developed , ribs and fingers are absent in the early stages , and the <unk> and ischium are entirely absent through most of development . Once maturity is reached , most bones have fully formed and growth rate slows . The bones of some <unk> like <unk> show growth marks , possibly an indication that growth rate varied with the change in seasons . Fossils of <unk> like <unk> and <unk> show that individuals grew larger past maturity . The oldest individuals usually have more pitting on their skulls with deeper sulci . 
