Rhadinella donaji, Campbell, Jonathan A., 2015

Campbell, Jonathan A., 2015, A new species of Rhadinella (Serpentes: Colubridae) from the Pacific versant of Oaxaca, Mexico, Zootaxa 3918 (3), pp. 397-405 : 398-403

Rhadinella donaji sp. nov.

Figure 1

Holotype. Adult male, UTA R- 4233 (original field no. JAC 277) from 13.6 km SW Villa Sola de Vega, 16.454873 N, - 97.002701 W, 2195 m above sea level (hereafter asl) [sometimes called San Miguel Sola de Vega], Oaxaca, Mexico, collected 8 June 1974 by Jonathan A. Campbell. The individual was found dead on the road during the early morning hours of an overcast day.

Diagnosis. Small, gracile snake of the genus Rhadinella in which the head is only slightly distinct from neck. The head pattern alone is diagnostic from all other species of Rhadinella , consisting of a mostly pale dorsum with irregular head blotching followed by an immaculate pale collar ( Fig. 1 ). Rhadinella donaji can easily be distinguished from most species of Rhadinella , such as R. hannsteini and R. kinkelini , which posses a distinct body pattern of longitudinal stripes. Rhadinella donaji can be most easily confused with R. schistosa , R. pilonaorum , and R. posadasi , all of which have a mostly dark dorsum, pale dashes on most dorsal scales, and often an indistinct dark vertebral line. Characteristics defining R. donaji include seven supralabials; third and fourth infralabials contacting posterior genials; 166 ventrals in single male; a mostly pale head with a relatively small amount of brown mottling; a pale collar that extends three scales posterior of the ultimate supralabial; an extremely faint dark vertebral stripe that involves only the vertebral scale row; no lateral stripe, but pale dashes that are less developed or absent on the third scale row; no dark pigment on lateral edges of ventrals; and only a very small amount of faint brown pigment on lateral edges of subcaudals, without darker pigment along subcaudal midline. Although the head proportions do not seem to vary significantly among species of Rhadinella , in R. donaji the frontal is slightly wider than long (2.6 w x 2.3 l), occupying 81.3% of the distance across the top of the head as measured from lateral edges of supraoculars, and the suproculars are relatively narrow (0.4 w x 1.3 l), whereas in R. pilonaorum , R. posadasi , and R. schistosa the frontal is longer than wide and the supraoculars are relatively broad.

Rhadinella donaji most closely resembles R. pilonaorum , but the latter taxon differs in having an auburn head cap heavily mottled in black ( Fig. 2 ), followed by an pale orange collar; the fourth and fifth infralabials contact the posterior genials; there are eight supralabials (rarely seven), and males have 151–154 ventrals. The pale collar in R. pilonaorum extends no more than two scales posterior to the ultimate supralabial (vs. three); the posterior edges of supralabials are boldly edged with brown forming vertical barring; the parietals are heavily mottled with brown; the temporals are heavily edged with brown; dorsal color is dark, but a poorly defined vertebral stripe occupies the vertebral and adjacent half scale rows; an inconspicuous lateral stripe is present on adjacent halves of third and fourth scale rows; the lower edges of first scale row and lateral edges of ventrals distinctly edged with brown; and the lateral edges of ventrals are distinctly brown, with irregular pigment scattered along ventral midline of tail.

Rhadinella posadasi differs from the new species in having an almost uniformly dark head cap followed by a pale collar ( Fig. 3 ); the fourth and fifth infralabials contact the posterior genials; 136–141 ventrals are present in males; a pale collar extending two scale rows posterior to ultimate supralabial; dorsal head plates, except posterior portion of parietals, including temporals mostly dark brown with small pale flecking; anterior supralabials to behind eye are mostly pale with no vertical barring, upper portion of these scales dark; posterior one and half to two supralabials dark; no trace of a lateral stripe; the lateral edges of ventrals and subcaudals with conspicuous brown pigment; and midventral surface of tail with almost continuous brown pigment along adjacent subcaudal sutures.

Rhadinella shistosa differs from the new species in having a dark head cap followed by a U-shaped collar that is often broken along the middorsal line; there 1+1 temporals (vs. 1+2); the tail is relatively short, accounting for only about 20% of total length; fourth and fifth infralabials contacting posterior genial; eight supralabials; 145–147 ventrals in males; supralabials mostly dark brown, but have a pale central spots or line; lateral edges of the ventrals and subcaudals are brown; and 16–17 maxillary teeth.

Description of holotype. Adult male as indicated by well developed hemipenes and cloacal ridges; head length 7.9; snout-vent length 220; tail incomplete, length 48+; eye diameter 1.0; naris to anterior edge of eye 2.0; internasal suture 0.3, about 17% length of prefrontal suture; frontal 2.5, about 93% as long as its distance from tip of snout, 63% as long as parietals. Dorsal scales smooth, in 17/17/17 rows; apical pits appear to be absent; head scales arranged in typical colubrid fashion with paired internasals, paired prefrontals, broad median frontal, narrow paired supraoculars, and paired parietals; supralabials 7/7, fourth below orbit; infralabials 7/7, first to third in contact with anterior genials, third and fourth in contact with posterior genials; preoculars 1/1, subpreocular absent; postoculars 1/1; temporals 1+ 2; preventrals 1; ventrals 166; cloacal plate divided; tail incomplete, subcaudals 40, divided.

In preservative (alcohol after formalin) head mostly pale yellow, extending onto the lips; lateral surface of head mostly pale except for dark brown pigment present on rostral above lingual groove, along lower loreal suture, around periphery of eye, on primary temporal, and onto last two supralabials; nasals and internasals mostly pale, prefrontals and anterior portion of frontal mottled with dark brown, with dark pigment extending medially on frontal for most of its length; parietals almost entirely pale except for small amount of brown mottling on anterolateral and posteromedial edges along their common suture; white collar three to three and half dorsal scales long extending ventrally to include preventral and first three ventrals; dorsum and sides of body dark brown; vertebral scale row very slightly darker than adjacent rows, forming almost indistinguishable vertebral line; upper rows of lateral body scales with small pale dashes, becoming larger on lower rows and occupying the central and anteroventral portion of each scale, especially on first two scale rows; ventral surface of head and body immaculate white, including all of ventral scales; dorsal pigment barely encroaching on lateral portion of subcaudals. Immediately after death, pale orange-brown cap extending posteriorly to level just posterior to parietals, bordered posteriorly by cream-colored collar.

The left maxilla bearing 11 teeth (possibly 12 allowing for one at the anterior tip of the bone). Teeth increasing in size posteriorly, with fifth and sixth teeth noticeably enlarged, except for posteriormost tooth, which is noticeably smaller; tooth nine even with anterior edge of ectopterygoid process; teeth arranged in straight line except for slight curvature of maxilla; no tooth noticeably offset; no pronounced diastema; slight gaps between sockets 7–8 and 9–10.

The right retracted hemipenis (base partly everted) extending to approximate level of fifth subcaudal, with muscle inserting at level of subcaudal 22. This organ was removed and pinned flat for examination. It is somewhat contorted and possibly shorted owing to its partial eversion. Although it is less spinose, it is similar to the hemipenes of Rhadinella pilonaorum (UTA R-21729) from near Laguna del Pino, Guatemala (ca. 1050 m asl).

Etymology. This species is offered to the Princess Donají [pronounced softly in Zapotec as Donashi], who according to Zapotec legend was the beautiful granddaughter of King Cosijoeza, the last great king of the Zapotec civilization. She was taken hostage and subsequently decapitated by factions warring with the Zapotecs. Her head was lost, only to be discovered miraculously much later still in a fresh state by a shepherd digging up a flower. Her visage is represented on the coat of arms of the city of Oaxaca de Juárez.

Proposed Standard English and Spanish names: Donaji’s Slender Leaflitter Snake and Hojarasquera Delgada de Donaji , respectively.

Distribution and habitat. The type-locality is a heavily karstic region with many sharp rock ridges and pinnacles ( Fig. 4 ) forming a honeycombed landscape that makes field work difficult. Pine-oak forest covers the landscape and in some areas a moderate accumulation of leaf litter may be found in rock crevices. The slope of the type-locality is drained by the upper tributaries of the Río Atoyac, which is a tributary of the Río Verde.

The snakes now placed in the genus Rhadinella have had a somewhat tortuous taxonomic history. Bailey (1940) provided a summary of Mexican species of Rhadinaea (sensu lato) in which he considered only one species of what later was to become part of the Rhadinaea godmani group, namely R. lachrymans . However, he pointed out that R. lachrymans was distinct from all other species he reported from Mexico, but that it possessed similarities with R. godmani from western Guatemala. Stuart & Bailey (1941) described several new species of Rhadinaea from the highlands of Guatemala. They relegated two of these species, along with R. godmani and R. serperaster , to what they called the Rhadinaea lachrymans group, which corresponds well with what was subsequently termed the Rhadinaea godmani group by Myers (1974).

The genus Rhadinella was first proposed by Smith (1941) to accommodate a single species ( Rhadinella schistosa ) of small, dark colubrid from the Atlantic versant in Veracruz, Mexico. He suggested that the generic relationships of this taxon were with Rhadinaea and Diadophis . This monotypic taxonomic arrangement persisted until Myers (1974) placed the taxon in the genus Rhadinaea . However, he recognized the uniqueness of Smith’s (1941) taxon and its affinity with a number of other species allocated at the time to Rhadinaea and placed them in the Rhadinaea godmani species group, following in part Stuart & Bailey (1941). Myers (1974) provided a thorough diagnosis for the group, which included 17–21 dorsal scale rows; ultimate maxillary tooth in line with preceding teeth, not offset laterally; usually more than two posterior teeth noticeably enlarged; diastema absent or small; hemipenis slightly bilobated with a basal naked pocket; subpreocular absent; cloacal ridges usually present in adult males of most species; and certain similarities of color pattern. Myers (2011) resurrected Rhadinella for the Rhadinaea godmani group, with particular emphasis on the anatomy of the maxillary dentition.

With the addition of Rhadinella donaji , the genus Rhadinella currently comprises 17 species. These small, gracile snakes are inconspicuous inhabitants of the forest floor and most species are represented by meager material in museum collections. Although a few species of this group became known as early as the latter half of the 1800s (Günther 1895; Cope 1870, 1871; Boettger 1898), the majority have been described after the turn of the last century, including a number of species described only in the last several decades ( Holm & Cruz 1994; Smith & Campbell 1994; Mendelson & Kizirian 1995; Köhler & McCranie 1999; McCranie 2006). Most species of Rhadinella are restricted to Nuclear Central America with 14 species distributed between the Isthmus of Tehuantepec and the Nicaraguan Depression. One species, Rhadinella serperaster , occurs in Isthmian Central America, and two species, Rhadinella schistosa and Rhadinella donaji occur to the west of the Isthmus of Tehuantepec.

Figures

FIGURE 1. (A) Dorsal view of the holotype of Rhadinella donaji (UTA R- 4233, male), showing details of head pattern, head length 7.9 mm. (B) Holotype of R. donaji showing entire body, snout-vent length 220 mm. (C) Lateral aspect of left maxilla of holotype having length of 3.7 mm.

FIGURE 2. Dorsal view of Rhadinella pilonaorum (UTA R- 21730, female) from near Laguna del Pino, ca. 1050 m asl, Santa Rosa, Guatemala. Snout-vent length 198, total length 318.

FIGURE 3. Dorsal view of Rhadinella posadasi (UTA R- 21728, male) from S slope Volcán Santa María, Finca El Faro, 875 m asl, ca. 4 km N El Palmar. Snout-vent length 199, total length 313. Photo taken immediately after death.

FIGURE 4. Satellite image of region around type-locality (16.454873 N, - 97.002701 W) of Rhadinella donaji, showing highly karstic nature of landscape. Circle represents collection site; inset of state of Oaxaca showing location of type-locality.