are mexican salamanders the only kind of neotenic salamanders

No, Mexican salamanders like the Axolotl are not the only kind of neotenic salamanders. Neoteny is a trait found in various species across the animal kingdom, including amphibians such as salamanders. Apart from the Axolotl, there are several other salamander species that exhibit neoteny. Here are a few examples:

1. Ambystoma opacum (M大理石纹蝾螈): Also known as the Marbled Salamander, this species retains some larval characteristics as an adult, although it undergoes metamorphosis to some extent.

2. Taricha granulosa (北方粗皮螈): The Rough-skinned Newt is another example of a neotenic salamander. It retains aquatic traits and lays eggs in water, even though it spends part of its life on land.

3. Necturus (洞螈属): The members of this genus, often referred to as Mudpuppies or Water Dogs, are also neotenic, retaining many juvenile features into adulthood and living in water.

4. Cryptobranchus alleganiensis (大鲵): The Hellbender, North America's largest salamander, is sometimes considered neotenic because it retains certain larval characteristics such as its gills, although it is adapted for life in fast-moving streams.

5. Dicamptodon (肺螈属): This genus includes the Pacific Giant Salamander and the Eastern Giant Salamander. While they undergo metamorphosis, they retain some neotenic features, like their large size relative to other adults of their family.

It's important to note that the degree of neoteny varies among species; some may retain more juvenile characteristics than others. The Axolotl is one of the most well-known and extreme examples due to its complete lack of metamorphosis.