Once more, we return to those wonderful, phenomenally successful, charismatic beasts…. the toads. As you’ll know if you’ve read the previous articles in the toads series, it seems that most basal divergences within crown-Bufonidae happened in South America. So far as we can tell right now, crown-toads are ancestrally South American, and all of their early history happened on this continent [Rhaebo blombergi image below from here].
All of the basal toads looked at so far – the relatively small, slender-limbed, shallow-snouted members of the clades Melanophrynicus, Atelopus, Osornophryne, Dendrophryniscus, Truebella, Andinophryne, Oreophrynella and Frostius – look rather different from the ‘typical’ stout-bodied toads that we’re more familiar with. Indeed, some authors have suggested that most or all of these ‘basal toads’ should be grouped together in a toad subfamily termed Atelopinae (recent phylogenetic studies generally agree that no such monophyletic entity exists, however). Having gotten the ‘atelopines’ out of the way, we now start looking at the more ‘typical’ toads. As we’ll see below (and later), toads in this enormous clade are different in many important respects from ‘atelopines’ and might be regarded as ‘Bufonidae ver 2′.
Introducing the ‘Bufo guttatus group’
A group of nine (as of August 2010) South American toads traditionally included in Bufo and known as the ‘Bufo guttatus group’* share a number of morphological characters. These include absence of cephalic crests (for a discussion of cephalic crests in toads see the article on skulls, crests, snouts and giant poison glands), the production of yellowish-orange skin secretions, hypertrophied testes (!!) and an omosternum (Frost et al. 2006). The omosternum is an anteriorly projecting component of the pectoral girdle, located along the midline, anterior to the clavicles. A few other toads have an omosternum (including Nectophrynoides, Werneria, Capensibufo and species in the Incilius valliceps group), but none of them seem closely related to the ‘Bufo guttatus group’ [image below, from Kaplan (2004), shows the pectoral girdle (in ventral view) of a typical toad** (at left) - without omosternum - compared to an omosternum-bearing ranid frog at right (omosternum is marked 'o'). cl = clavicle; co = coracoid; e = epicoracoid; p = procoracoid; s - sternum].
* And sometimes as Cope toads (Frost 2009), presumably because Cope published the name Rhaebo.
** While labelled here as Bufo woodhousii, remember that the deconstruction of Bufo sensu lato requires that we use the name Anaxyrus woodhousii for this North American species. See The resurrection of Anaxyrus.
All species within this group have short, rounded snouts and broad skulls. Molecular studies have found them to be outside the clade that includes the Eurasian and American toads traditionally included in Bufo (Pauly et al. 2004, Pramuk 2006, Pramuk et al. 2008, Van Bocxlaer et al. 2009): this seems to confirm previous suggestions that the members of the ‘Bufo guttatus group’ are an ancient lineage that diverged from other toads early on in the Cenozoic (as in, in the Eocene).
Recognising the ‘Bufo guttatus group’ as a distinct lineage well removed from Bufo proper means that a new generic name is needed: Rhaebo Cope, 1862 is available and was used for these toads by Frost et al. (2006). Rhaebo might be particularly close to Peltophryne (see below), or it might be the sister-taxon to Andinophryne [the latter taxon was previously discussed here]. In fact, Mueses-Cisneros (2009) named the new Venezuelan species R. andinophrynoides on account of a strong similarity with Andinophryne [photo of R. guttatus used above from here] [R. caeruleostictus from Ecuador shown below, from Hoogmoed (1989). Scale bar = 1 mm].
While most Rhaebo species are ‘average’ in size for a toad (SVLs = approximately 70-80 mm), the Colombian giant toad or Blomberg’s toad R. blombergi [shown at very top] of Colombia and Ecuador is gigantic, with some specimens exceeding 240 mm in length. Some authors say that Blomberg’s toad is the largest known toad, but this isn’t right given that a few exceptional Cane toads have exceeded 300 mm SVL (unless there are some record-holding Blomberg’s toads that I haven’t read about). Despite its size, Blomberg’s toad was only named in 1951. John Funkhouse learnt of this species from Rolf Blomberg in 1950 (Blomberg had heard about it from local Ecuadorian people), and specifically went out in search of it. Obviously, he was successful in finding it (Shuker 2002).
West Indian toads
Eleven toad species from the West Indies were traditionally included in Bufo sensu lato and known as the ‘Bufo peltocephalus group’. These species are ecologically diverse, with some inhabiting dry scrubland and others being denizens of well-watered, forested habitats. They’re morphologically diverse too, with some being small (28 mm SVL) with weakly ossified, open-plan skulls, and others being large (190 mm SVL), with heavily ossified, box-like skulls [for pictures see skulls, crests, snouts and giant poison glands]. As Pramuk (2002) noted, marked morphological disparity among close relatives is often commoner in island-endemic amphibians and reptiles than it is in their mainland relatives.
Pregill (1981) was one of the first authors to properly argue that these toads formed a clade: a clade that could easily be distinguished from the other members of Bufo sensu lato. One of the strangest features of these toads is that their maxillae have extended anteriorly to fill in the space normally occupied by the premaxillae along the midline of the upper jaw. The premaxillae themselves have been displayed dorsoposteriorly (Pramuk 2000, 2002). Pregill (1981) argued that the old generic name Peltophryne Fitzinger, 1843 should be resurrected for these animals [adjacent photo of Puerto Rican crested toad P. lemur from wikipedia].
Phylogenetic work indicates that the divergence between Peltophryne and other bufonids happened early on within bufonid evolution, probably early in the Cenozoic (Pramuk 2006, Pramuk et al. 2008, Van Bocxlaer et al. 2010). Rhaebo may be equally old, and in fact Peltophryne and Rhaebo may be sister-taxa (Van Bocxlaer et al. 2010). An alternative hypothesis is that Peltophryne belongs in a clade with Pseudepidalea (the Green toad and kin), the Afro-Asian Duttaphrynus species and Schismaderma (the African red toads) (Frost et al. 2006). Peltophryne may first have colonised the Caribbean region during the Paleocene or Eocene, and apparently prior to the formation of the modern Caribbean islands.
While at least some Peltophryne species are terrestrial, burrowing toads (they use their heavily ossified heads to seal their burrow entrances), the Cuban long-nosed toad P. longinasus is unusual in being semi-arboreal (Pramuk 2002).
Bufonidae ver 2
There’s one more thing worth saying about Rhaebo and Peltophryne, and in fact it makes them rather significant. Most of the toads we’ve looked at so far have been relatively small anurans that produce tiny egg clutches and lack parotoid glands. In contrast, Rhaebo and Peltophryne are among the oldest members of the clade that I referred to above as ‘Bufonidae ver 2′: toads that are large to very large, produce large to stupendously large clutches, have exotrophous larvae (viz, larvae that feed from the environment, rather than rely on a maternally provided food source) and possess large to enormous parotoid glands [diagram below - from Van Bocxlaer et al. (2010) - shows how the appearance of several key traits triggered the evolution of the 'optimal range-expansion phenotype'. The major global expansion of 'ver 2' toads then followed].
We don’t know why toads evolved these novel features, but – in combination – they have allowed ‘ver 2′ toads to be supreme survivors, colonists and dispersers (Van Bocxlaer et al. 2010). Indeed, as discussed recently by Van Bocxlaer et al. (2010) – one of several new studies which shows how sexy toads are as research objects – ‘ver 2′ toads exhibit a ‘range-expansion phenotype’ that allows some species to be notoriously adaptive and exceptionally good at expanding their ranges. In very strong contrast, ‘atelopine’ toads generally have small ranges and are extremely vulnerable to extinction. We’ll be coming back to this story several times again. And, on that note, we’ll be coming back to toads in general some time soon – there is still a lot to get through.
For previous articles in the Tet Zoo toads series see…
- Toadtastic – the invasion begins!
- Bidder’s organ and the holy quest for synapomorphies
- Our sex lives in words and pictures (or, On the reproductive biology of the Bufonidae)
- Skulls, crests, snouts and giant poison glands: the heads of toads
- Toads of the world: first, (some) toads of the north
- The Natterjack, its life and times
- The resurrection of Anaxyrus
- South America, land of toads, part I: harlequins, redbellies and plump toads
- South America, land of toads part II: tree toads, Truebella, Frostius… oh, and did I mention the COMMUNAL NESTS?
For previous articles on hyloid anurans see…
- Britain’s lost tree frogs: sigh, not another ‘neglected native’
- Ghost frogs, hyloids, arcifery.. what more could you want?
- Green-boned glass frogs, monkey frogs, toothless toads
- It’s the Helmeted water toad!
- Horn-headed biting frogs and pouches and false teeth
- More wide-mouthed South American horned frogs
- We need MORE FROGS
Refs – –
Frost, D. R. 2009. Amphibian Species of the World: an Online Reference. Version 5.3 (12 February, 2009). Electronic Database accessible at http://research.amnh.org/herpetology/amphibia/ American Museum of Natural History, New York, USA.
– ., Grant, T., Faivovich, J., Bain, R. H., Haas, A., Haddad, C. F. B., De Sá, R. O., Channing, A., Wilkinson, M., Donnellan, S. C., Raxworthy, C. J., Campbell, J. A., Blotto, B. L., Moler, P., Drewes, R. C., Nussbaum, R. A., Lynch, J. D., Green, D. M. & Wheeler, W. C. 2006. The amphibian tree of life. Bulletin of the American Museum of Natural History 297, 1-370.
Hoogmoed, M. S. 1989. On the identity of some toads of the genus Bufo from Western Ecuador, with additional remarks on Andinophryne colomai Hoogmoed, 1985 (Amphibia: Anura: Bufonidae). Zool. Verh. Leiden 250, 1-32.
Kaplan, M. 2004. Evaluation and redefinition of the states of anuran pectoral girdle architecture. Herpetologica 60, 84-97.
Mueses-Cisneros, J. J. 2009. Rhaebo haematiticus (Cope 1862): un complejo de especies. Con redescripció def Rhaebo hypomelas (Boulenger 1913) y descripción de una nueva especie. Herpetotropicos. Mérida, Venezuela 5, 29-48.
Pramuk, J. B. 2000. Prenasal bones and snout morphology in West Indian bufonids and the Bufo granulosus species group. Journal of Herpetology 34, 334-340.
– . 2002. Combined evidence and cladistic relationships of West Indian toads (Anura: Bufonidae). Herpetological Monographs 16, 121-151.
– . 2006. Phylogeny of South American Bufo (Anura: Bufonidae) inferred from combined evidence. Zoological Journal of the Linnean Society 146, 407-452.
– ., Robertson, J. B., Sites, J. W. & Noonan, B. P. 2008. Around the world in 10 million years: biogeography of the nearly cosmopolitan true toads (Anura: Bufonidae). Global Ecology and Biogeography 17, 72-83.
Pregill, G. 1981. Cranial morphology and the evolution of West Indian toads (Salientia: Bufonidae): resurrection of the genus Peltophryne Fitzinger. Copeia 1981, 273-285.
Shuker, K. P. N. 2002. The New Zoo. House of Stratus, Thirsk, North Yorkshire.
Van Bocxlaer, I., Biju, S. D., Loader, S. P. & Bossuyt, F. 2009. Toad radiation reveals into-India dispersal as a source of endemism in the Wester Ghats-Sri Lanka biodiversity hotspot. BMC Evolutionary Biology 2009, 9:131 doi:10.1186/1471-2148-9-131
Van Bocxlaer I, Loader SP, Roelants K, Biju SD, Menegon M, & Bossuyt F (2010). Gradual adaptation toward a range-expansion phenotype initiated the global radiation of toads. Science (New York, N.Y.), 327 (5966), 679-82 PMID: 20133569