Xenophrys

Perak horned frog Xenophrys aceras, copyright Roy Kittrell.

Belongs within: Megophryidae.

Xenophrys is a diverse genus of horned toads found in Southeast Asia, southern China and the eastern Himalayas (Chen et al. 2017).

Strange brows of south-east Asia
Published 8 August 2022

Among the characteristic inhabitants of the leaf litter carpeting the forest floor in south-east Asia are the horned toads of the subfamily Megophryinae. Though sometimes sizeable (the largest species grow to about twelve centimetres in length), the Asian horned toads often escape notice due to the way their typical mottled brown coloration and folded skin blends into the surrounding environment. As their vernacular name suggests, some species have extended points of skin surmounting the eyes. However, a quick scan over images of various exemplars suggests significant variation in just how ‘horned’ these horned toads are.

Short-legged toad Xenophrys brachykolos, copyright Thomas Brown.

The internal classification of the Megophryinae is currently contentious. Historically recognised genera have not been identified as monophyletic by recent studies and authors have differed on just how to resolve the issue. Mahony et al. (2017) supported rolling all megophryines into a single genus Megophrys, subdivided between several subgenera for those who were determined to use them. Conversely, Chen et al. (2017) proposed the recognition of five separate genera. Xenophrys, whose name happens to mean “weird eyebrows”, is the largest of these megophryine genera with close to fifty known species (new species continue to be described on a fairly regular basis). Physical characteristics of Xenophrys include a large head (more than a quarter of the snout-to-vent length) that is not majorly compressed and has the snout projecting over the lower jaw, a pointed snout lacking tubercles, smooth to tuberculate skin lacking horny spines, and long, comparatively thin hind legs lacking longitudinal ridges. Eyebrow projections may be present or absent; if present, they comprise a single conical projection or tubercle and not elongate ‘horns’. Xenophrys species are diverse in habits but are most commonly found hiding in leaf litter. During the breeding season, males may be found calling from branches and stones along mountain streams (Chen et al. 2017).

Perak horned toad Xenophrys aceras, copyright Psumuseum.

As adults, megophryines are mostly reasonably ordinary-looking frogs. As tadpoles, they are truly weird. Tadpoles of Xenophrys are most commonly found living in streams, in the calmer, shallow waters along the edge or buried among the leafy substrate. They have large, funnel-shaped mouths that are used to feed on debris floating on the water’s surface. They also have long tails containing numerous distinct, bony vertebrae.

Frog tails at maturity are usually much reduced (it’s kind of one of the things they’re known for), the tail bones being cut back to a single basal urostyle. Tadpoles, of course, have a more developed tail but usually still have a reduced skeleton comprising the urostyle, a couple of remnant neural arches, and a ventral rod called the hypochord. The presence of multiple ossified tail vertebrae in tadpoles of the family Megophryidae (that are then broken down and resorbed during metamorphosis) appears to be unique among frogs. Though megophryids are one of the more basal extant anuran lineages, they are nested deeply enough in the frog family tree that their youthful bony tails appear likely to be a secondary reversion rather than a retained ancestral character. Such an inference may also be supported by the fact that the exact manner in which the tail vertebrae develop varies between genera (Handrigan et al. 2007). In the genera Leptobrachella and Ophryophryne, each vertebra forms as a single ossification around the entire notochord, but in Xenophrys and Megophrys, they begin as separate dorsal and ventral elements that grow towards each other.

Tadpole of Xenophrys brachykolos, copyright Thomas Brown.

The presence of bony tails in megophryids may be related to their riparian habitat with stronger tails improving the tadpoles’ ability to swim against the current. Alternatively, they may assist the tadpoles in digging into the substrate. However, it is worth noting that other tadpoles with similar habits do not have similar tails. Is there some other impetus behind the extended megophryid skeleton, or do these other frogs just belong to lineages that have lost the ability to re-activate their ancestral abilities?

Systematics of Xenophrys

Characters (from Chen et al. 2017): Head large, somewhat narrow and comparatively non-compressed, angular, over than 25% of SVL; snout projecting above lower jaw, pointed in dorsal view, tubercles absent; palpebral projections (“horns”) absent or present as a single conical projection or tubercle; pupil vertical; no transverse skin fold at head basis; tympanum present; vomerine teeth present or absent; maxillary teeth present or absent; glandular supratympanic fold present or absent; no glandular projection on fold joining jaw angle and forelimb basis; dorsal skin smooth to tuberculate, no large horny spines on dorsum; dorsolateral folds distinct or indistinct or absent; two narrow glandular middorsal ridges present, forming X, H or Y-shaped figure; asperities or projections on dorsum present or absent; projections on lower abdomen and inner thighs present or absent; hind limbs long, comparatively thin, no longitudinal ridges; heels usually overlapping when hindlimbs are flexed, occasionally not overlapping; toe web absent or present; neural spines on vertebrae not enlarged; 2n=26 with varied karyotypes; 1 pair of Ag-NORs.

<==Xenophrys Günther 1864FG06 [incl. PanophrysCZ17]
|--+--X. pachyproctus [=Megophrys pachyproctus]CZ17
| `--+--X. aceras [=Megophrys aceras]CZ17
| `--+--*X. parva [=Megophrys parva]CZ17
| `--+--+--X. medogensis [=Megophrys medogensis]CZ17
| | `--X. zhangi [=Megophrys zhangi]CZ17
| `--+--+--X. glandulosa [=Megophrys glandulosa]CZ17
| | `--X. major (Boulenger 1908)CZ17, L50 [=Megalophrys majorL50, Megophrys majorCZ17]
| `--X. mangshanensis [=Megophrys mangshanensis]CZ17
`--+--+--X. minor (Stejneger 1926)CZ17, L50 [=Megophrys minorCZ17]
| `--+--+--X. brachykolos [=Megophrys brachykolos]CZ17
| | `--+--X. jinggangensis [=Megophrys jinggangensis]CZ17
| | `--X. lini [=Megophrys lini]CZ17
| `--+--+--X. boettgeri [=Megophrys boettgeri]CZ17
| | `--X. huangshanensis [=Megophrys huangshanensis]CZ17
| `--+--X. tuberogranulatus [=Megophrys tuberogranulatus]CZ17
| `--+--X. baolongensis [=Megophrys baolongensis]CZ17
| `--X. wushanensis [=Megophrys wushanensis]CZ17
`--+--X. daweimontis [=Megophrys daweimontis]CZ17
`--+--X. palpebralespinosa [=Megophrys palpebralespinosa]CZ17
`--+--X. wuliangshanensis [=Megophrys wuliangshanensis]CZ17
`--+--+--X. sangzhiensis [=Megophrys sangzhiensis]CZ17
| `--X. spinata [=Megophrys spinata]CZ17
`--+--X. jingdongensis [=Megophrys jingdongensis]CZ17
`--+--X. binchuanensis [=Megophrys binchuanensis]CZ17
`--+--X. binlingensis [=Megophrys binlingensis]CZ17
`--X. omeimontis (Liu 1950)CZ17, L50 [=Megophrys omeimontisCZ17, *Panophrys omeimontisCZ17]

Xenophrys incertae sedis:
X. acuta [=Megophrys acuta]CZ17
X. ancrae [=Megophrys ancrae]CZ17
X. auralensis [=Megophrys auralensis]CZ17
X. caudoprocta [=Megophrys caudoprocta]CZ17
X. cheni [=Megophrys cheni]CZ17
X. damrei [=Megophrys damrei]CZ17
X. kuatunensis [=Megophrys kuatunensis]CZ17
X. latidactyla [=Megophrys latidactyla]CZ17
X. lekaguli [=Megophrys lekaguli]CZ17
X. longipes (Mell 1922)CZ17, L50 [=Megalophrys longipesB89, Megophrys longipesCZ17]
X. maosonensis [=Megophrys maosonensis]CZ17
X. megacephala [=Megophrys megacephala]CZ17
X. monticola [=Megophrys monticola]L50
X. obesa [=Megophrys obesa]CZ17
X. oropedion [=Megophrys oropedion]CZ17
X. parallela [=Megophrys parallela]CZ17
X. robusta [=Megophrys robusta]CZ17
X. serchhipii [=Megophrys serchhipii]CZ17
X. shuichengensis [=Megophrys shuichengensis]CZ17
X. takensis [=Megophrys takensis]CZ17
X. vegrandis [=Megophrys vegrandis]CZ17
X. zunhebotoensis [=Megophrys zunhebotoensis]CZ17

*Type species of generic name indicated

References

[CZ17] Chen, J.-M., W.-W. Zhou, N. A. Poyarkov, Jr, B. L. Stuart, R. M. Brown, A. Lathrop, Y.-Y. Wang, Z.-Y. Yuan, K. Jiang, M. Hou, H.-M. Chen, C. Suwannapoom, S. N. Nguyen, T. V. Duong, T. J. Papenfuss, R. W. Murphy, Y.-P. Zhang & J. Che. 2017. A novel multilocus phylogenetic estimation reveals unrecognized diversity in Asian horned toads, genus Megophrys sensu lato (Anura: Megophryidae). Molecular Phylogenetics and Evolution 106: 28–43.

[FG06] Frost, D. R., T. Grant, J. Faivovich, R. H. Bain, A. Haas, C. F. B. Haddad, R. O. de Sá, A. Channing, M. Wilkinson, S. C. Donnellan, C. J. Raxworthy, J. A. Campbell, B. L. Blotto, P. Moler, R. C. Drewes, R. A. Nussbaum, J. D. Lynch, D. M. Green & W. C. Wheeler. 2006. The amphibian tree of life. Bulletin of the American Museum of Natural History 297: 1–370.

Handrigan, G. R., A. Haas & R. J. Wassersug. 2007. Bony-tailed tadpoles: the development of supernumerary caudal vertebrae in larval megophryids (Anura). Evolution and Development 9 (2): 190–202.

[L50] Liu, C.-C. 1950. Amphibians of western China. Fieldiana: Zoology Memoirs 2: 1–400.

Mahony, S., N. M. Foley, S. D. Biju & E. C. Teeling. 2017. Evolutionary history of the Asian horned frogs (Megophryinae): integrative approaches to timetree dating in the absence of a fossil record. Molecular Biology and Evolution 34 (3): 744–771.

2 comments

  1. I can’t see that Günther gave any explanation of the etymology of Xenophrys. Therefore ξένος could be implying any of its shades of meaning “strange, unusual, guest-friend, stranger, alien, foreigner, wanderer, refugee or mercenary”. Though I must admit strange or weird seems most likely, he did call the next genus “bad-foot”.

    1. I’ll admit, I did not actually check the original publication etymology-wise. “Strange eyebrows” just seemed a reasonable way to read the name of an animal that, well, has strange eyebrows.

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