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| Proahaetulla antiqua
Mallik, Achyuthan, Ganesh, Pal, Vijayakumar & Shanker, 2019
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Abstract
The Western Ghats are well known as a biodiversity hotspot, but the full extent of its snake diversity is yet to be uncovered. Here, we describe a new genus and species of vine snake Proahaetulla antiqua gen. et sp. nov., from the Agasthyamalai hills in the southern Western Ghats. It was found to be a member of the Ahaetuliinae clade, which currently comprises the arboreal snake genera Ahaetulla, Dryophiops, Dendrelaphis and Chrysopelea, distributed in South and Southeast Asia. Proahaetulla shows a sister relationship with all currently known taxa belonging to the genus Ahaetulla, and shares ancestry with Dryophiops. In addition to its phylogenetic position and significant genetic divergence, this new taxon is also different in morphology from members of Ahaetuliinae in a combination of characters, having 12–13 partially serrated keels on the dorsal scale rows, 20 maxillary teeth and 3 postocular scales. Divergence dating reveals that the new genus is ancient, dating back to the Mid-Oligocene, and is one of the oldest persisting monotypic lineages of snakes in the Western Ghats. This discovery adds to the growing list of ancient lineages endemic to the Agasthyamalai hills and underscores the biogeographic significance of this isolated massif in the southern Western Ghats.
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| Fig 2. (A) Maximum likelihood tree showing the relationship of Proahaetulla antiqua gen. et sp. nov. within family Ahaetuliinae. Value on each node indicates the bootstrap support. (B) Photograph of holotype specimen in life. (C) Dorsal and lateral view of head. (D) Dentition arrangement of maxillary arch. |
Systematics
Ahaetuliinae Figueroa, McKelvy, Grismer, Bell, Lailvaux, 2016
Ahaetuliinae Pyron, Burbrink, Wiens, 2013 (invalid nomen).
Ahaetullinae Zheng & Wiens, 2015 (invalid nomen).
Proahaetulla gen. nov.
Type species, by present designation: Proahaetulla antiqua sp. nov.,
by monotypy.
Etymology: The generic epithet Proahaetulla gen. nov. stems from the generic nomen Ahaetulla indicating the early divergence of the lineage from the rest of the Ahaetuliinae members. Gender feminine. The specific epithet antiqua is Latin for ‘antique’ or old, a term alluding to the evolutionary age or antiquity of this new taxon.
Diagnosis:
Lineage diagnosis.
- Proahaetulla gen. nov. is a member of the subfamily Ahaetuliinae, and shares ancestry and a sister relationship with Aheatulla clade.
- It shows high genetic divergence and differs from members of the genus Ahaetulla (including A. nasuta, A. pulverulenta, A. prasina, A. anomala, A. fronticinta, A. fasciolata and A. mycterizans) with genetic distances of 14.1–17.4% on Cytb, 16.0–18.1% on ND4 and 5.2–6.5% on 16S genes.
- Morphologically, it is characterized by the presence of horizontally elliptical pupil; concave loreal region, enabling a near-binocular vision; snout produced forward to a fine point; and bright-green dorsum. Proahaetulla gen. nov. differs from Ahaetulla (A. anomala, A. nasuta, A. dispar, A. pulverulenta, A. mycterizans, A. fasciolata, A. fronticincta & A. prasina) in showing the following combination of morphological characters: 12–13 rows of mildly serrated, keeled dorsal scales starting from the nape till the dorsal scales that are situated in the row adjacent to the cloacal plate; strongest keels on the mid vertebral row (7th or 8th row) of scales; keels consecutively weaker in the paravertebral row of scales (dorsal scales smooth in most Ahaetulla, partly keeled near sacral region in A. perroteti); a set of 20 maxillary teeth (12–16 in Ahaetulla) and post-ocular scales 2–3 (2 in Ahaetulla); greenish yellow tongue with black speckles (vs. reddish, purplish or brownish tongue in Ahaetulla spp.).
- Proahaetulla gen. nov. differs from all the known genera in Ahaetuliinae as follows: pupil horizontally elliptical (vs. rounded in Dendrelaphis, Chrysopelea); snout tip pointed (vs. rounded in Dendrelaphis, Chrysopelea); dorsum verdant green (vs. never totally green in Dendrelaphis, Chrysopelea, Dryophiops); dorsal scales keeled (vs. smooth in Dendrelaphis, Dryophiops, Ahaetulla); vertebral scales not enlarged (vs. enlarged in Dendrelaphis); dorsal body scales without apical pits (vs. with apical pits in Dendrelaphis, Chrysopelea, Dryophiops); maxillary teeth 20 (vs. < 16 in Ahaetulla; > 22 in Dendrelaphis, Chrysopelea); mid body scale rows 13–15 (vs. not less than 15 in Dryophiops, Ahaetulla; 17 in Chrysopelea) (Table 3).
Proahaetulla antiqua gen. et sp. nov.
Relationships: The new lineage is a sister taxon to the genus Ahaetulla, as recovered in our phylogeny. Our tree topology varies slightly in comparison to previous reconstructions, likely due to the use of the different nucleotide substitution models for the analyses (Bayesian analysis with mixed substitution models vs. ML analysis with GTRG model). In addition, we were unable to include other available RAG1 sequences from the GenBank as they could not be aligned with our generated RAG1 sequences. Regardless of the variation in the topology within the Ahaetulla clade, there is very strong support for the node of interest (the relationship between Proahaetulla gen. nov. and Ahaetulla) in both the analyses. The generic status of Proahaetulla gen. nov. is further supported by the date of divergence (26.57 Ma) from its MRCA, indicating that it is the oldest lineage in the group, besides also differing from Ahaetulla and other genera in morphological characters.
Distribution: The new taxon was found only in the far south of the Western Ghats. It occurs in Agasthyamalai hills, where it was recorded in the high elevation wet forests (> 1200 m asl) at Agasthyamalai peak and in Pandimotta (Fig 5). The new taxon’s range is likely to encompass other high elevation regions of Agasthyamalai. This taxon also broadly overlaps in its latitudinal distributional range with A. cf. dispar and A. cf. nasuta.
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| Fig 5. Habitat at the type locality of Proahaetulla antiqua gen. et sp. nov., showing montane rainforests atop Agasthyamalai hills, southern Western Ghats. |
Natural history: The holotype was collected from a tree branch inside a forest patch at an elevation of 1640 msl, on the way to Agasthyar peak. It was found resting coiled on a tree at ca. 1640 h about 2.5 meters above the forest floor. The paratype was sighted at night around 2200 h, sleeping on a shrub at a height of about 2 m from the ground, inside a dense forest patch at an elevation of 1224 msl in Pandimotta ca. 25 km north of the type locality. Members of Ahaetuliinae are mostly arboreal snakes with a few outliers such as Ahaetulla perroteti and A. dispar with environmental adaptations and a body plan suited for a life in open montane grasslands. The body morphology of this genus is similar to arboreal Ahaetulla and Dryophiops. Since both the specimens were found inside thickly wooded forests resting and showing activity on trees and shrubs, we speculate that this taxon is also adapted to a completely arboreal lifestyle.
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Key to Ahaetuliinae
1a) pupil horizontal; canthus rostralis strongly concave … 2a
1b) pupil rounded; canthus rostralis not strongly concave … 4a
2a) mid-dorsum with 13–15 rows of keeled scales …. Proahaetulla gen. nov. (1 species)
2b) mid-dorsum with not < 15 rows of smooth scales … 3a
3a) dorsal scales without apical pits; ventrals smooth … Ahaetulla (9 species)
3b) dorsal scales with apical pits; ventrals keeled … Dryophiops (2 species)
4a) mid-dorsum with 13–15 rows of scales; ventrals keeled … Dendrelaphis (45 species)
4b) mid-dorsum with 17 rows of keeled scales; ventrals notched … Chrysopelea (5 species)
Ashok Kumar Mallik, N. Srikanthan Achyuthan, Sumaithangi R. Ganesh, Saunak P. Pal, S. P. Vijayakumar and Kartik Shanker. 2019. Discovery of A Deeply Divergent New Lineage of Vine Snake (Colubridae: Ahaetuliinae: Proahaetulla gen. nov.) from the southern Western Ghats of Peninsular India with A Revised Key for Ahaetuliinae. PLoS ONE. 14(7): e0218851. DOI: 10.1371/journal.pone.0218851