Figure 1

Four age classes of cervical Ambystoma tigrinum vertebrae. Scale is in millimeters. Left to right: Young Larval show an open (unfused) neural arch and open centrum with little or no ossification; Young Terrestrial are characterized by an open neural arch and constricted, or partially fused, centrum with little ossification; the Paedomorphs are typified by a fused neural arch and an open centrum with some ossification present; the Old Terrestrial are described by a fused neural arch and a closed, or fused, centrum with visible ossification.