Tetrapod Zoology

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If you’ve been following the toad series, you’ll have read articles that introduce toads in general, discuss reproductive biology, and look at cranial anatomy. This can all be regarded as background introductory stuff. From hereon, we’re mostly going to look at toad diversity in rough phylogenetic order: that is, starting at the base of the clade and working up to the ‘top’ of the tree [European common toad Bufo bufo shown here, from wikipedia. This individual has really weird nostrils].

Basal toads are all South American, and include the South American redbelly toads (Melanophryniscus) and the stumpfoot or harlequin toads (Atelopus). The South American Rhaebo toads (conventionally included within Bufo) also seem to be basal within the bufonid crown. The rest of toad phylogeny is rather controversial: some studies recover a major Old World clade that includes several Asian, African and Eurasian clades as well as a clade of New World toads including the Cane toad group (Rhinella) and North American group (Anaxyrus) (Pramuk et al. 2008). If this topology is correct, crown-toads – ancestrally South American – radiated extensively in the Old World but re-invaded the Americas during the Eocene. Others studies find Rhinella and so on to be outside of the Old World clade (Van Bocxlaer et al. 2009). We’ll be coming back to the various details here later on.

While I mostly want to cover things in rough phylogenetic order, I’ve decided to start things by looking at the toads that – historically – have been most studied and most influential in terms of shaping our views on this huge amphibian group.

The Bufo bufo group

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It all started with Bufo bufo, the Common toad of Eurasia. It’s a mostly terrestrial animal with dry, warty skin, and its familiarity to European people explain why the term ‘toad’ later came to be used for any superficially similar anuran from elsewhere in the world, even those that are more closely related to Rana temporaria, the European common frog. B. bufo is tolerant of cool climates, it’s an explosive breeder, is reasonably large (males reach 50-60 mm and females 80-90 mm: as usual with anurans, all lengths given here are snout-vent lengths, or SVLs), and is sexually dimorphic (males are greyish, females are brownish). Its eyes are of an iridescent copper colour and its pupils are horizontal, and eyes like this are seen widely within Bufonidae [male Common toad shown here, from wikipedia].

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As is the case with so many species that have a large geographical range, the Common toad is a variable animal, and a consequence of this variation is that a number of subspecies have been recognised; furthermore, distinct populations in various regions have been identified as distinct species, though not without controversy. The Caucasian toad B. verrucosissimus (named in 1814) was generally regarded as a Common toad subspecies until 1987, and today there’s a lot of uncertainty over its status relative to the Common toad and to the Giant toad B. spinosus, a form usually regarded as a B. bufo subspecies but raised by some authors to species status [Caucasian toad pair shown here, from Nature of Georgia].

Natterjacks and the green toad group

Another toad is really familiar if you live in western Europe: the Natterjack. Frost et al. (2006) resurrected the generic name Epidalea Cope, 1865, the type species of which is the Natterjack (traditionally B. calamita), in the hope that they would be able to use it for a clade that included the European green toad (traditionally B. viridis) and all of its relatives (known conventionally as the ‘Bufo viridis group’) [an American toad, conventionally known as B. debilis, is sometimes called the Green toad too, though alternative names include Sonora toad and Dwarf toad].

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However, Frost et al. (2006) noted that Graybeal (1997) did not find the Natterjack and European green toad to form a clade and hence concluded that the hypothesis of a Natterjack + green toad group relationship was based entirely on general, phenetic resemblance, and was not supported by synapomorphies. According to Frost et al.’s (2006) favoured taxonomy, the Natterjack is on its own as Epidalea calamita while the former ‘Bufo viridis group’ is Pseudepidalea Frost et al., 2006. I’m not going to look at Pseudepidalea here, but will come back to it later (it’s a particularly confusing and complicated clade. How confusing and complicated? See the figure below, from Stöck et al. (2006)) [Green toad shown here. Ooh, pretty. Photo by Richard Bartz, from wikipedia].

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However, some authors have contested the distinction of the Natterjack relative to Pseudepidalea and have extended Epidalea to include the green toad group too: Speybroek & Crochet (2007) argued that Graybeal’s sequence data for the Natterjack appeared to contain errors, and they pointed to the data present by Stöck et al. (2006), some of which might support a Natterjack + green toad group (see also Sos 2008). Actually, Stöck et al. (2006) is controversial in that it advocates the recognition of three ‘new’ European members of the green toad group: B. balearicus Boettger, 1880 from southern Italy and the western Mediterranean islands, B. variabilis (Pallas, 1769) from Asia Minor, eastern Europe, Sweden, Denmark, Greece and Germany, and an unnamed taxon from Sicily. These authors weren’t the only ones to support the recognition of these possible species, and debate continues on the systematics and status of these toads. Genetic data indicates that the green toad group might be most closely related to Duttaphrynus and Peltophryne
(Frost et al. 2006), two genera that we’ll come to later on.

For previous articles in the toad series see…

For previous articles on hyloid anurans see…

Refs – -

Frost, D. R., 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.

Graybeal, A. 1997. Phylogenetic relationships of bufonid frogs and tests of alternate macroevolutionary hypotheses characterizing their radiation. Zoological Journal of the Linnean Society 119, 297-338.

Pramuk, J. B., 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.

Sos, T. 2008. Review of recent taxonomic and nomenclatural changes in European Amphibia and Reptilia related to Romanian herpetofauna. Herpetologica Romanica 2, 61-91.

Speybroek, J. & Crochet, P.-A. 2007. Species list for the European herpetofauna – a tentative update. Podarcis 8, 8-34.

Stöck, M., Moritz, C., Hickerson, M., Frynta, D., Dujsebayeva, T., Eremchenko, V., Macey, J. R., Papenfuss, T. J. & Wake, D. B. 2006. Evolution of mitochondrial relationships and biogeography of Palearctic green toads (Bufo viridis subgroup) with insights in their genomic plasticity. Molecular Phylogenetics and Evolution 41, 663-689.

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

Comments

  1. #1 John Harshman
    October 26, 2009

    Handsome toads indeed, and a truly admirable opera Bufo.

    However, I would like to launch a mild protest against the term “basal” as commonly used. A “basal taxon” is merely the less diverse of two phylogenetically equivalent sister groups. There’s constant temptation to take it as meaning something more, imputing, for example, primitiveness. So I try to avoid it.

  2. #2 David Marjanović
    October 26, 2009

    Green toad shown here. Ooh, pretty.

    Actually, the combination of that pink, that green, and that beige is indescribably ugly.

    In German, the green toad is called Wechselkröte, “change toad”… seems to fit its behaviour in phylogenetics!

  3. #3 johannes
    October 26, 2009

    > Actually, the combination of that pink, that green, and
    > that beige is indescribably ugly.

    De gustibus et coloribus non est disputandum, but I, for one, have always considered the green toad to be Europe’s most attractive coloured and patterned anuran.

  4. #4 David Marjanović
    October 26, 2009

    Can’t compete with Hyla arborea. Especially the blue variant.

  5. #5 Naraoia
    October 27, 2009

    Can’t compete with Hyla arborea. Especially the blue variant.

    H. arborea may be prettier, but green toads have the most beautiful eyes of the animal kingdom.

  6. #6 John Harshman
    October 28, 2009

    Beg to differ: tabanid flies.

  7. #7 John Scanlon FCD
    October 29, 2009

    Green toad: the only bufonid I’ve ever seen in its native habitat (a couple in a quarry at the base of the Akropolis), and much prettier than the feral giant Rhinella marina we get round here. (My nomination for most attractive anuran goes to Litoria citropa, the Blue Mountains tree frog. Looks a bit like Hyla arborea, iirc.)

  8. #8 Christian Fischer
    January 13, 2010

    The “weird nostrils” of the Common toad shown in the first picture on your site “Toads of the world: first, (some) toads of the north” are obviously caused by an infection with larvae of the Toadfly (see: http://en.wikipedia.org/wiki/Lucilia_bufonivora)! Poor toad…