Comments on the Osteology and Phylogeny of Ambystomatid Salamanders

Abstract

The family Ambystomatidae is a strictly Nearctic family, probably derived from the Asiatic Hynobiidae in the late Mesozoic or very early Tertiary. Osteological evidence indicates that the Pacific Coast forms Dicamptodon and Rhyacotriton were derived very early, and independently, from the proto-ambystomatid stock. These forms are so distinct from each other and from the remainder of the family that recognition of three separate subfamilies is proposed. These are the Dicamptodontinae, including Dicamptodon and, presumably, the fossil genus Ambystomichnus; the Rhyacotritoninae, including only the genus Rhyacotriton; and the Ambystomatinae, including all other forms. Two well-defined genera are recognized in the subfamily Ambystomatinae. One of these, Rhyacosiredon, contains only four species. The other, Ambystoma, is in turn divided into three subgenera, Ambystoma, Bathysiredon, and Linguaelapsus. Four imperfectly delineated species groups are recognizable in the sub-genus Ambystoma - the mexicanum group (which includes the fossil species kansense), the tigrinum group, the opacum group, and the maculatum group. It is postulated that Dicamptodon and Rhyacotriton developed from a branch of the original proto-ambystomatid stock that established itself in the western part of the continent very early in the history of the family, while the primary center of dispersal for the remainder of the family was in the northeast. Establishment of this eastern group probably preceded the Miocene, with the various species groups being developed not later than the early Pliocene, and many of the extant species by or in the early Pleistocene.