The genus OxyrhOpus ( Serpentes : Dipsadidae : Xenodontinae ) in Guyana : morphology , distributions and comments on taxonomy

Considerable confusion exists regarding taxonomy and distributions of Oxyrhopus in northern South America. Specimens and records of Oxyrhopus from Guyana were examined. Oxyrhopus melanogenys, O. occipitalis and O. petola occur in Guyana. Data on morphology and colouration of these species are presented. Species distributions and characters to distinguish among the species are discussed.

The aim of this paper is to confirm which species of Oxyrhopus occur in Guyana, to establish characters that distinguish among the species, and to provide data on morphology, colour pattern and distributions.

RESuLTS
Snakes of the genus Oxyrhopus in the Guiana Shield region exhibit the following suite of characters: scale rows 19-19-17; dorsal scales smooth with two apical pits; anal single; subcaudals divided; upper labials 8, 4 th and 5 th enter orbit; temporals normally 2+3.Meristics of specimens examined are summarised in Table 1.Characters useful for distinguishing among species of Oxyrhopus occurring in the Guiana Shield region are in Table 2. Details of each species are presented below.
Oxyrhopus occipitalis differs from O. formosus by its slender body form (more robust in O. formosus), snout yellow and top of head brown (entire head yellow or cream), adults red with very faint body bands (adults with prominent black bands)(Table 2).
Of the two species, only O. occipitalis occurs in the Guiana Shield.All of the Oxyrhopus from the Guiana region that have been identified as O. formosus are actually O. occipitalis.We include O. formosus in Table 2 only to summarise differences between it and O. occipitalis.

Colour and Pattern
Dorsal ground colour red or reddish orange; snout and supralabials yellow; top of head medium to dark brown.A white anterior body band (sometimes obscure), 3-4 dorsal scales in width, approximately 2 cm behind the head.Faint, pale orange body bands with very narrow dark borders; width of pale orange bands equal to the length of 2-4 dorsal scales.Bands are usually symmetrical, but in one specimen there is a half-band, on one side only, between otherwise symmetrical bands.Bands on body 15-17, on tail 5-6.Underside of head yellowish orange.Venter white; dorsal pigment barely touches the edges of some ventrals.Underside of tail banded orange and white, corresponding to dorsal pale orange bands and red ground respectively (Fig. 1A-C).
There is considerable ontogenetic change in colour pattern.In most cases bands are prominent in juveniles but become inconspicuous in adults (Martins & Oliveira, 1999).However, a juvenile with the typical adult pattern of inconspicuous bands has been reported from southern Venezuela (C.L. Barrio-Amorós, pers.comm.).Bands are often more visible in preservative, less conspicuous on living specimens.Photographs are in Starace (1998 [as O. formosus]), Martins & Oliveira (1999 [as O. formosus]) and Campbell & Lamar (2004).
This paper reports the first records of O. occipitalis from Guyana.Specimens were collected in forest; habitat description is in Ernst et al. (2005) and Kok & Kalamandeen (2008).Distribution in Guyana is mapped in Fig. 2.

Taxonomic history
Oxyrhopus melanogenys was originally described from the southwestern Amazon region.This species has been confused with the similar-appearing O. trigeminus.Oxyrhopus melanogenys can be distinguished from O. trigeminus by its entirely black head (snout white with black spots in O. trigeminus), by the outer black bands of the triads narrower than the central black band (all three black bands equal) and by the white bands of triads narrower than the black bands (equal to the black bands)(Table 2).
Historical reports of O. trigeminus in Guyana are based on two specimens (BMNH 1990.19 and1990.20)from "Demerara" (= Georgetown, Guyana) donated by Captain E. Sabine between 1818 and 1824.However, the "Demerara" locality must be regarded as highly dubious, because numerous specimens donated by Sabine to BMNH, purportedly from Demerara, are really from other localities (Underwood, 1993;C. Mc-Carthy, pers. comm.).Donnelly et al. (2005) reported a specimen of Oxyrhopus trigeminus from Iwokrama in central Guyana, but the specimen was re-identified as O. melanogenys by RDM and AL.Lancini (1979) and Kornacker (1999) mentioned O. trigeminus from Venezuela (photograph in Kornacker, 1999), but Zaher & Caramaschi (1992) demonstrated that these are really O. melanogenys.All other known specimens of O. trigeminus are from south of the Amazon River (Zaher & Caramaschi, 1992).
Consequently, we exclude O. trigeminus from the fauna of Guyana and the Guiana Shield.Any reports of O. trigeminus from the region are actually of O. melanogenys.We include O. trigeminus in Table 2 only to point out differences between it and O. melanogenys.
A taxon identified as Oxyrhopus aff.melanogenys has been reported from the Guiana Shield (Zaher & Caramaschi, 1992;Starace, 1998).There are no significant morphological differences between Oxyrhopus melanogenys and O. aff.melanogenys.The two taxa differ solely in the degree of fusion of anterior triads, a character that we feel insufficient to distinguish between species.We therefore consider O. aff.melanogenys to be O. melanogenys.
Material examined -Six males, five females and one juvenile (Appendix).

Morphology
Maximum total length in our sample is 810 mm; maximum reported length is 930 mm (Starace, 1998).Lower labials 10; six in contact with shields.In AMNH 152279 and 152280 the uppermost posterior temporal on the left side is fused with the adjacent posterior scale to produce an elongate scale, equal in length to the parietal; temporals on the right side are normal.IRSNB 17940 has a similar scale fusion on both sides of the head.Measurements of adults are in Table 1.The single juvenile measures 183 mm SVL, tail 49 mm, with 190 ventrals and 82 subcaudal pairs.The tail of the juvenile is 21% of its total length, vs. 23-27% in adult males and 20-22% in adult females.

Colour and Pattern
Oxyrhopus melanogenys has a triad colour pattern (a triad is a repeating series of bands, black-whiteblack-white-black, separated by red or orange interspaces) (Figs.1D, 3).In all but one O. melanogenys examined (n = 10) the body bands anterior to the triads are symmetrical (nuchal red/orange, black, white, black, red/orange, then triads).In CSBD HR700 some of the body bands anterior to the triads are asymmetrical.In all other specimens, symmetry breaks down in the first triad; the anterior white band of this triad is symmetrical, but the posterior is not.In many individuals all subsequent triads, anterior to the vent region, are asymmetrical.The vent is in the central black band of a triad; this is the first symmetrical triad in all but two snakes; in these two individuals (AMNH 140898, MAD 1368) the triad anterior to the triad that straddles the vent is also symmetrical.In three specimens, AMNH 8094, AMNH 98211 and MAD 1368, triads become symmetrical just posterior to the midpoint between snout and vent, then become asymmetrical again, then once again symmetrical at the vent triad or the triad anterior to it.Triads anterior to vent 15.5-20.5;triads on the tail are difficult to count because tails become very dark, with triads indicated by only a few light flecks (Fig. 3).In IRSNB 17941 and CSBD HR700, the anterior colour pattern consists of white body bands on a dark background, with triads detectable only posteriorly (2.5 anterior to vent in IRSNB 17941, 4.5 in CSBD HR700), when red interspaces become visible laterally.In AMNH 8094, AMNH 98211, MAD 1368 and ROM 39439, the nuchal band is complete and symmetrical, broader laterally than middorsally.In AMNH 140898, 152279 and 152280 the nuchal band and the next interspace are present laterally but incomplete dorsally.In AMNH 98211, AMNH 152279, AMNH 152280 and ROM 39439, the interspaces between the anteriormost triads do not extend up to the middorsal line.In IRSNB 17912 and IRSNB 17941 the nuchal band is incomplete, only barely distinguishable laterally.In IRSNB 17912 the next interspace is present, but red bands are then absent until slightly posterior to the midpoint between snout and vent.In IRSNB 17941 the next red interspace is absent, red bands are absent except at the very end of the body where two red bands are barely distinguishable laterally.The width of the interspaces between triads is variable, even within the same individual; in some cases they are equal to the outer black triad bands and in others they are wider, equal to the central black bands.In most specimens, dark pigment is present on outer edges of ventrals anteriorly, extending further onto ventrals posteriorly, often extending across ventrals that are immediately anterior to vent.In AMNH 8094 and AMNH 140898 the dark bands touch only the outer edges of ventrals.
In general, larger, presumably older, individuals have less prominent markings than do smaller, younger specimens.Partial melanism was reported by Starace (1998), who noted that the nuchal band is always present in partially melanistic individuals; similar melanism was observed in central French Guiana by PJRK (unpublished data).

Distribution in the Guiana Shield
Zaher & Caramaschi (1992) map the distribution of Oxyrhopus melanogenys.Additional locations in the Guiana Shield, other than Guyana, are in Starace (1998) and Gorzula & Señaris (1999).Oxyrhopus melanogenys is a forest dweller; habitat description is in Donnelly et al. (2005) and Kok & Kalamandeen (2008).Distribution in Guyana is mapped in Fig. 2.

Taxonomic history
Oxyrhopus petola has a very large range, occurring from Mexico to Bolivia.Several subspecies have been described, usually based on colour pattern (Bailey, 1970).Chippaux (1986) expressed doubts about the criteria used to define the subspecies, stating that O. petola might be a highly variable taxon, or a complex of species.
Material examined -four males, one female, one juvenile (Appendix).

Colour and Pattern
Terminology used to describe the colour pattern of Oxyrhopus petola has been inconsistent.Beebe (1946) and Abuys (1985) describe the pattern as red bands on a black ground; Roze (1966), Chippaux (1986) and Starace (1998) describe the pattern as alternating black and red bands; Bailey (1970) and Kornacker (1999) describe the species as having black bands with red interspaces.We follow the latter in the interest of consistency within the genus Oxyrhopus, in which the arrangement of black bands is often diagnostic.
Relative widths of bands and interspaces are variable.Bailey (1970) stated that in Oxyrhopus petola from northern South America "…[black] bands [are] approximately equal in length, separated by interspaces about one scale wide".In specimens from French Guiana and Venezuela the interspaces are approximately equal in size to the black bands (Roze, 1966;Chippaux, 1986;Starace, 1998;Kornacker, 1999).Abuys (1985) showed a photo of an individual with very narrow and asymmetrical interspaces, and stated that interspace width is variable in specimens from Surinam.The bands are wider than the interspaces in an individual from Surinam photographed by Moonen et al. (1979).In some individuals very narrow white bands are present between the black bands and the red interspaces (Beebe, 1946;Kornacker, 1999).Melanism was reported by Abuys (1985) and Chippaux (1986).Esqueda et al. (2007) reported partial albinism in a specimen from Venezuela.
In specimens from Guyana the interspaces are red or reddish brown and the black bands are approximately 2X the width of the interspaces (bands 3-6 scales in width, interspaces 1-3 scales).Bands and interspaces may be asymmetrical.Venter whitish, immaculate, with dorsal black bands contacting the outer ends of the ventrals (Fig. 4).

Remarks
Although some recent work has contributed to the understanding of the systematics and distributions of Oxyrhopus, species identifications can still be problematic.Some widespread taxa, such as O. formosus, O. melanogenys and O. petola, may contain more than one species.Further study using morphological and molecular methods will be necessary to determine this.

TAbLE 1 :
Ranges of measurements of Oxyrhopus adults from Guyana.