John - Rhyacotriton are not cousins of Ambystoma and Dicamptodon. All current data place them closer to Amphiumidae and Plethodontidae.
The four named and possibly 4-5 unnamed species are restricted to icy clean mountain streams of the Pacific Northwest. Their speciation has been impacted by the carving and shifting of a millions-of-years-old river, the rise and eruptions of SEVERAL volcanos, AND repeated floods of lava hundreds of meters deep which covered state-sized areas before flowing into the Pacific. The type locality for R.cascadae is actually IN that basalt, where it has been carved through by that very river, more or less at the foot of one of those volcanos. [Wahkeena Falls, Columbia River Gorge, Larch Mountain/Mt. Hood]
