DIAGNOSIS A single salamander vertebra (DMNH 44765) was recovered from the cave. The specimen is amphicoelous, with the centrum and most of the neural arch preserved. The diapophyses and parapophyses are broken distally, but there is some indication that they supported bicipital ribs. On the ventral side of the vertebra a single spinal nerve foramen pierces each side of the centrum immediately posterior to the transverse processes where they meet the centrum. Single intravertebral spinal nerve foramina appear in this position in the trunk vertebrae of Ambystomatidae, Plethodontidae, Salamandridae, and Sirenidae (Edwards, 1976). The larger size, reduced transverse processes, and distinct vertebral morphology of sirenids exclude them from further consideration. The amphicoelous condition of the Porcupine Cave fossil argues against assignment to the Salamandridae, a group in which the vertebrae are usually opisthocoelous (Estes, 1981). Discrete characters permitting separation of ambystomatids from the diverse plethodontids are often lacking. In general, the vertebrae of North American plethodontids are relatively slender, with long, narrow centra and a very low neural arch supporting a weak neural spine (Tihen and Wake, 1981). In Ambystoma and the Porcupine Cave fossil the vertebrae are more robust, and there is an extremely reduced neural spine (= neural ridge) with a posteriorly vaulted neural arch terminating at its posterodorsal end in a small dimple or depression (Hilton, 1948). This feature and the overall morphology of the fossil permit identification to the Ambystomatidae.
REMARKS The specimen is likely from a species of Ambystoma, the only currently recognized extant genus of the family and the only one to which Pleistocene fossils have been allocated. The Ambystoma lineage has a fossil record purportedly extending back to the Oligocene (Holman, 1968). Ambystoma tigrinum is present in Park County today (Hammer-son, 1999) and is known to occur within a short distance of
Porcupine Cave (see the section "Comparison with Local Modern Fauna").
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