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.
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).
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.
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?
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.
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.
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.