Description

Ambystoma ordinarium measures between 70 and 75 mm SVL at sexual maturity, reaching a maximum size of 86 mm SVL for terrestrial adults. This salamander has a narrow head, and generally bears 16-24 tooth-rakers on the 3rd arch. Pattern and color may vary. Adult individuals generally sport a uniformly dark to black dorsum, but mottling may also be present. Some adults retain the larval coloration, consisting of slight ventral, lateral, and dorsal rows of light silver-yellow specks from axilla to groin (Anderson and Worthington 1971; Shaffer 1984).

Larvae of Ambystoma ordinarium resemble those of Ambystoma opacum, and have sparse, but evenly distributed melanophores in addition to the rows of light silver-yellow specks. The larval fins are well-developed, and darken and mottle at maturity. Larval gills are reduced in size but bushy. Ambystoma ordinarium larvae reach a maximum size of 100 mm SVL and 191 mm in total length. Ambystoma ordinarium possesses features of both pond-dwelling and mountain brook-dwelling species. The dorsal and ventral fins of hatchlings are fully developed, as is typical of pond-type larvae, while gills are both diminutive and bushy, as is typical of mountain brook-type larvae. (Anderson and Worthington 1971; Anderson 1975).

Ambystoma ordinarium is a member of the Ambystoma tigrinum complex, a group of salamanders which has undergone rapid lineage diversification and radiated throughout North America, from southern Canada to central Mexico. Ambystoma ordinarium is thought to have split from Ambystoma dumerilii 7-10 million years ago. Recent work has shown that Ambystoma ordinarium is genetically distinct but that mitochondrial introgression has occurred from Ambystoma dumerilii, which is a paedomorphic species found at nearby Lake Patzcuaro (Weisrock et al. 2006).