Systematic Status of the Colubrid Snake, Leptodeira discolor Günther

by William Edward Duellman

Fig. 2. Lateral view of the left maxillary of Tantalophis discolor (Günther). (KU No. 40143). × 17.

A thin and otherwise small parotid gland or "venom sac" extends posteriorly from beneath the eye to about the angle of the jaw; a minute duct connects with the anteromedian surface and extends to the fleshy part of the mouth at the posterior end of the maxillary (Figure 3).

Fig. 3. Lateral view of the head of Tantalophis discolor (Günther), showing the position and relative size of the parotid gland. (KU No. 40143). × 17.

Hemipenis.—In situ the hemipenis extends to the posterior edge of the thirteenth caudal. The unforked part of the organ is bedecked with numerous heavy longitudinal folds alternating with thinner folds. The basal parts of the two heads are covered with moderate sized spines, those closest to the base and the sulcus being the smallest. The distal parts of the heads are covered with calyces. The sulcus bifurcates on the unforked part of the organ at a point about two-thirds of the distance from the base to the division of the organ. The sulcus is a deep groove between heavy folds proximally and is a shallower furrow distally (Figure 4).

Fig. 4. Hemipenis of Tantalophis discolor (Günther). The organ was cut on the ventral surface and opened. (KU No. 40143). × 4.

Relationships.—Using Dunn's (1928) arrangement of the American colubrid snakes as a guide permits the taxonomist to group Tantalophis with several genera, some of which occur in South America and others in the West Indies. Although the significance of such generic characters as scale pits and nature of the hemipenis is not clear, these characters must, of necessity, be utilized in attempting to ascertain the relationships of Tantalophis to other colubrid snakes. Assuming that the primary divisions of the American colubrids into subfamilies based on the nature of the sulcus spermaticus and the presence or absence of hypapophyses on the posterior vertebrae have some reality, Tantalophis must be placed in the subfamily Xenodontinae comprising genera chiefly South American in their distribution, but with several genera in Middle America and a few in North America and the West Indies. In order to limit the number of genera to be compared with Tantalophis, only those xenodontines having apical pits and bifurcate hemipenis are considered. These include Cyclagras, Drepanoides, Hypsirhynchus, Ialtris, Leimadophis, Pseudablabes, Siphlophis, Tachymenis, Tomodon, and Trypanurgos. Aside from differences in scutellation, Leimadophis, Siphlophis, and Trypanurgos have the heads of the hemipenes terminating in a disc, and Ialtris has a plicate hemipenis. Tomodon has basal spines on the hemipenis. The hemipenes of the other genera have proximal folds, distal spines, and distal calyces, not greatly unlike the condition found in Tantalophis. Of these, Cyclagras, Hypsirhynchus, and Pseudablabes have round pupils and certain differences in scutellation. Drepanoides and Tachymenis have elliptical pupils like those of Tantalophis, but Tachymenis has only one apical pit, and Drepanoides has one apical pit or none. In the above characters no especially close relationship between Tantalophis and any one of these genera is apparent.

If the characteristics usually employed in distinguishing and relating genera are ignored and other more subjective criteria are used, the relationships of Tantalophis still remain obscure. Of the xenodontine genera Tantalophis approaches Leimadophis in general physiognomy; perhaps it represents an early divergent stock of Leimadophis that has undergone radical changes in the hemipenis and other characters. On the other hand, if the nature of the hemipenis is of no importance in defining supergeneric groups of colubrid snakes, Tantalophis may have its relationships with Leptodeira and Hypsiglena. Although herpetologists have been working intensively on American colubrids for many decades, the relationships of the majority of the groups are not well understood. Until the hemipenes and skulls of all of the forms have been studied and compared, and the evolutionary significance has been determined for the characters of the hemipenes, dentition, and apical pits, our knowledge of the relationships of these snakes will be incomplete.

Remarks.—The individual on which this paper is based is the only specimen of the species with definite locality data. It is from a locality six miles southeast of Tamazulápam in northwestern Oaxaca. This town lies at an elevation of about 6500 feet in the upper reaches of the Balsas Basin, an arid interior valley that expands in its upper end to form a broad basin of rolling and dissected terrain ranging from about 4000 to 6800 feet in elevation. The countryside around Tamazulápam is arid and supports plants of the genera Prosopis, Acacia, Ipomoea, and Cassia, and also columnar cacti. Oaks and pines grow on the limestone hills rising above the rolling valley. Tantalophis may be endemic to the Balsas Basin, as are many other species of reptiles. However, if the snake has its relatives to the south in lower Central America and South America, such a distribution seems unlikely, even for an apparent relict.