The axolotl (top middle), also known as a Mexican salamander, is a neotenic salamander, closely related to the tiger salamander. Many species within the axolotl’s genus are either entirely neotenic or have neotenic populations. In the axolotl, metamorphic failure is caused by a lack of thyroid stimulating hormone, which is used to induce the thyroid to produce thyroxine in transforming salamanders. Continue reading for more.