Developmental timing and life history events of plethodontids. The solid black line represents morphogenic changes for three alternative life history strategies (A-C): direct development, biphasic, and paedomorphic. Dashed horizontal and vertical grey lines indicate metamorphosis and maturation, respectively. (A) Direct developing plethodontids metamorphose inside of the egg, usually followed by a terrestrial juvenile stage and terrestrial maturation. (B) Biphasic plethodontids typically have aquatic eggs, aquatic larval stages (of varying durations), and metamorphose into a more terrestrial morphology prior to, or around the time of, maturation. (C) Paedomorphs mature while still in the larval form prior to metamorphosis, which may, or may not, subsequently occur. Accelerations and decelerations in somatic morphogenesis and maturation can alter the timing of life history events (D-F). The solid grey lines represent an example of ancestral morphogenic changes, and in these examples the ancestral life history is biphasic (the same as B). (D) Dotted green line shows a trajectory with an accelerated timing of metamorphosis compared to its ancestor. In this case, the acceleration still results in a biphasic descendent, but with an abbreviated larval period. Further acceleration of morphogenic changes (completed within the egg) could lead to direct development. (E) Dotted red line shows a trajectory with decelerated timing (or permanent postdisplacement) of metamorphosis compared to its biphasic ancestor. This example of deceleration shows neoteny, where the deceleration results in maintenance of the larval form beyond maturation (paedomorphosis). (F) Dashed green vertical line represents an acceleration in the timing (or predisplacement) of maturation to an age prior to metamorphosis, resulting in a progenic descendant exhibiting larval form paedomorphosis. It is important to note that accelerations (and decelerations) in maturation time also occur within life history categories. Terminology is from  and the figure is a synthesis from [7, 23, 26, 27] with modifications and additions.