diagnosis AND REMARKS A total of 67 vertebrae are assigned to Natricinae and have the following characteristics

FIGURE 11.5. DMNH 27888; precloacal vertebra referred to Natricinae shown in anterior (left) and left lateral (right) views. Scale bar = 2 mm.

Precloacal Vertebra

FIGURE 11.5. DMNH 27888; precloacal vertebra referred to Natricinae shown in anterior (left) and left lateral (right) views. Scale bar = 2 mm.

(figure 11.5): a well-developed hypapophysis that is predominantly posteriorly angled (not distinctly directed ventrally); prezygapophysial accessory processes that are elongate spines; and very well-developed osseous paralymphatic channels that are present in all specimens. Recognition of Natricinae solely on the basis of vertebral morphology is difficult because vertebral morphology in natricines appears to reflect a more generalized morphology shared with other colubroid lineages, notably the Elapidae. Qualitative vertebral characteristics are widely used in an effort to distinguish these lineages (e.g., Hoffstetter, 1939; Holman, 1979; Rage, 1984), but such characters are not demonstrated to be uniform with respect to individual, ontogenetic, intracolumnar, and intraspecific variation for a wide range of snake taxa (see also comments by Cadle, 1987, 1988; Kluge, 1993; Czaplewski et al., 1999a). Studies on the variability of similar characters applied to differentiate other snake taxa on the basis of vertebrae demonstrated considerable individual and intracolumnar variation (Kluge, 1988; Bell and Mead, 1996).

The issue here is really one of higher-order ambiguity; thus there do not appear to be any discrete vertebral characters that can be used to separate all natricines from all elapids, and we are not confident in the use of characteristics previously applied by others to recognize natricines, especially with respect to elapids. As a result, we employed continental-scale geographic data in our analysis and compared the Porcupine Cave sample to the only North American elapids, Micrurus and Micruroides. Many extant species of Micrurus are known from throughout the Americas (Campbell and Lamar, 1989), but we were unable to examine vertebral morphology for any species other than M. tener (species taxonomy follows that adopted by Crother et al., 2001), the only species now inhabiting the United States (where it is found along the Gulf coast from Texas to Florida, and north along the east coast into North Carolina). Micruroides is monotypic; the single species M. euryxanthus inhabits the Sonoran Desert region of the southern United States and northwestern Mexico (Campbell and Lamar, 1989). The highest elevation recorded for M. tener is close to 2000 m (Roze and Tilger, 1983; Campbell and Lamar, 1989), and that for M. euryxanthus is 1800 m (Roze, 1974).

Both M. tener and M. euryxanthus have a relatively short neural spine, a distinctive vertebral morphology that is not found in the Porcupine Cave specimens we refer to Natricinae.

Although this character is not universally diagnostic within either Elapidae or Natricinae, its absence in the Porcupine Cave sample, combined with the geographic and elevational range in North American natricines and elapids, indicates that the most parsimonious assignment of the Porcupine Cave specimens is to Natricinae.

There are currently six species of natricine snakes inhabiting Colorado (Nerodia sipedon, Thamnophis cyrtopsis, T. elegans, T. proximus, T. radix, and Tropidoclonion lineatum). Thamnophis elegans inhabits the immediate vicinity of the cave today and is known from elevations as high as 3992 m in Colorado (Hammerson, 1999). The past distribution, thermal tolerance, and elevational extent of these taxa remain unknown and cannot be clarified by study of the material from Porcupine Cave.

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