throughout this period. Neoteny has been implicated as a developmental cause for a number of behavior changes, as a result of increased brain plasticity and extended childhood. Progenesis (or paedogenesis) can be observed in the axolotl (Ambystoma mexicanum). Axolotls reach full sexual maturity while retaining their fins and gills (in other words, still in the juvenile form of their ancestors). They will remain in aquatic environments in this truncated developmental form, rather than moving onto land as other sexually mature salamander species. This is thought to be a form of hypomorphosis (earlier ending of development) that is both hormonally and genetically driven. The entire metamorphosis that would allow the salamander to transition into the adult form is essentially blocked by both of these drivers. Paedomorphosis by progenesis may play a critical role in avian cranial evolution. The skulls and beaks of living, adult birds retain the anatomy of the juvenile theropod dinosaurs from which they evolved. Extant birds have large eyes and brains relative to the rest of the skull; a condition seen in adult birds that represents (broadly speaking) the juvenile stage of a dinosaur. A juvenile avian ancestor (as typified by Coelophysis) would have a short face, large eyes, a thin palate, narrow jugal bone, tall and thin postorbitals, restricted adductors, and a short and bulbous braincase. As an organism such as this aged, they would change greatly in their cranial morphology to develop a robust skull with larger, overlapping bones. Birds, however, retain this juvenile morphology. Evidence from molecular experiments suggests both fibroblast growth factor 8 (FGF8) and members of the WNT signalling pathway have facilitated paedomorphosis in birds. These signalling pathways are known to play roles in facial patterning in other vertebrate species. This retention of the juvenile ancestral state has driven other changes in the anatomy that result in a light, highly kinetic (moveable) skull composed of many small, non-overlapping bones. This is believed to have facilitated the evolution of cranial kinesis in birds which has played a critical role in their ecological success. Peramorphosis Peramorphosis is delayed maturation with extended periods of growth. An example is the extinct Irish elk. From the fossil record, its antlers spanned up to wide, which is about a third larger than the antlers of its close relative, the moose. The Irish elk had larger antlers due to extended development during their period of growth. Another example of peramorphosis