After a little delay, it’s time to embark once more into the World of Toads!!! Having previously looked at toads in general, and at the toads of Europe, we here continue the series by looking at yet more familiar, northern toads: this time at those of North America.
As with some of the other ‘northern’ toads we’ve looked at (the Natterjack and the Green toad and its relatives), North America’s many toad species have conventionally been subsumed into the anuran genus-that-ate-the-world, Bufo. However, genetic studies have shown that the North American toad clade that includes the Common American toad (conventionally B. americanus) [shown here, from wikipedia] and all similar forms do not group together with Bufo sensu stricto (the B. bufo group) and are well away from them in cladograms (read on). Accordingly, they need their own generic name: Anaxyrus Tschudi, 1845 is available, and was used for this group by Frost et al. (2006). We’re going to look at the members of this clade here, but note that these aren’t the only North American toads: members of the genera Incilius and Rhinella inhabit the continent too.
Traditionally, the Anaxyrus toads – about 22 are currently recognised – have been classified into various species groups, as has been the tradition with members of Bufo sensu lato. As a European person I find it slightly surprising that a few toads inhabiting the USA weren’t named until as recently as the 1940s, 50s or even 60s: the species concerned are the Black toad or Deep Springs toad A. exsul (Myers, 1942), Sonoran green toad A. retiformis (Sanders & Smith, 1951), Houston toad A. houstonensis (Sanders, 1953) [shown here, from wikipedia] and Wyoming toad A. baxteri (Porter, 1968). In one case – that of the Wyoming toad – the animal had previously been regarded as a subspecies (of the Canadian toad A. hemiophrys), but the others really were new [for further thoughts on recently discovered North American tetrapods see The USA is still yielding lots of new extant tetrapod species].
Morphological synapomorphies uniting the Anaxyrus species have yet to be identified, but they do at least all look ‘rather similar’, and hence can be easily distinguished from their Middle American and South American relatives. Molecular data supports their monophyly and shows that they’re either the sister-taxon to a clade that includes both the Central American toads (Incilius) and the South American toads (Rhinella) (Pauly et al. 2004, Frost et al. 2006), or to the Central American toads alone (Pramuk 2006, Pramuk et al. 2008). Most are small, stocky, short-limbed toads that take refuge in burrows. Most are between 4 and 10 cm SVL, but some can be smaller (the Oak toad A. quercinus might be adult at 1.9 cm). Some (like the Texas toad A. speciosus) have enlarged, sharp-edged tubercles on their hind feet. The males of Anaxyrus species have vocal sacs that (when inflated) are large and rounded, but in some species the sac bulges upwards and forwards and is sometimes described as ‘sausage-shaped’. This is the case in the Great Plains toads A. cognatus, Texas toad and Oak toad.
The best known North American toads – species like the American common toad, Woodhouse’s toad A. woodhousii [shown here, from wikipedia] and Great Plains toad – have large ranges, in cases extending from Canada right down to the Gulf states. However, others have very restricted ranges and are highly vulnerable to environmental disturbance, pollution, and the spread of the Bd chytrid fungus. One of most restricted species is the Black toad, unique to Deep Springs Valley in Inyo County, California. Another – the Wyoming toad A. baxteri – is endemic to the Laramie Basin in Wyoming. It went into sharp decline during the 1970s and was thought to be extinct by 1980. It was rediscovered in 1987 near Mortenson Lake and has since been the subject of a reintroduction attempt in the Mortenson Lake National Wildlife Refuge, though this has, unfortunately, only had limited success (Dreitz 2006). Substantial declines of other North America toad species have been reported as well.
Natterjacks and Cane toads in the North American fossil record?
In the toad articles that have appeared so far, I haven’t said much about fossils. Partly this is because resurrected genera like Anaxyrus have been defined on molecular data, whereas fossil species (conventionally lumped into Bufo sensu lato) are of course classified on the basis of morphological characters. As a result, putting fossil species into the modern taxonomic framework is difficult or impossible. But mostly it’s because I’ve wanted to look at the diversity of the extant animals and to leave fossils out of it [cover of Holman's book shown here. I don't have it].
North America has a reasonably good fossil toad record: species like the Colorado river toad Incilius alvarius, Common American toad A. americanus, Great Plains toad A. cognatus and Woodhouse’s toad A. woodhousii have been reported from the Pliocene, Pleistocene and Holocene of the continent, but the presence of very weird species has also been claimed. ‘Bufo‘ alienus is a great example: it’s a Miocene fossil from the Ogallala Formation of Kansas, said to be so similar to the Natterjack Epidalea calamita that it evidences the presence of this group of toads in the Neogene Nearctic (Tihen 1962, Sanchiz 1998). The claimed presence of the Cane toad Rhinella marina in the Miocene of Kansas is also surprising. ‘Bufo‘ holmani and ‘Bufo‘ kuhrei, both from the Miocene of Nebraska, were rather large, robust species that seem to have been quite distinct from modern Nearctic toads, while ‘Bufo‘ rexroadensis from the Pliocene of Kansas (possibly a close relative of A. americanus) is notable for its thickened supraorbital and postorbital crests (Sanchiz 1998).
However, the great problem with fossil anurans is that their remains are, typically, highly fragmentary. In fact, most fossil anurans aren’t whole animals, but isolated ilia [like those in the adjacent image: from Rage (2003)]. As a result, palaeontologists have developed a tradition of heaping tons of significance on tiny details of ilium morphology. I’m reminded of the following quote about fossil horse teeth: “… the belief that characters in the dentition … are completely diagnostic of species-groups in the Equidae is, unfortunately, a myth: largely sustained, we suspect, by the wishful thinking of palaeontologists who often have nothing else to work with!” (Groves & Willoughby 1981, p. 341).
And, indeed, some recent studies have shown that the degree of intraspecific variation present within extant anuran ilia is such that fossil, ‘ilium-based’ species don’t stand out as distinct: rather, they fall within the range of variation present in extant taxa (Jones et al. 2002, Bever 2005). As Bever (2005) stated, this doesn’t demonstrate that any of the named fossil toads based on ilia are necessarily synonymous with extant species, but it does raise this as a possibility, and it does mean that neat and unusual ideas (like the presence of natterjack- or cane toad-like toads in the North American Miocene) cannot be endorsed.
The North American fossil toads do demonstrate one neat thing though: toads were definitely present in North America prior to the formation of the Central American land-bridge (there are North American bufonids from the Oligocene as well as from the Miocene). Given that toads are ancestrally South American, they must have gotten to North America via over-water dispersal (Pauly et al. 2004). As is increasingly well known (I’ve mentioned it enough times!), seawater is not an insurmountable barrier to anurans as was once thought, so it seems that ‘rafting’ allowed toads to island-hop across the developing land-bridge during the Miocene and earlier. Given that toads were present in Paleocene Europe (Rage 2003), they must have gotten out of South America and into Eurasia pretty early, and they can only have done the South-America-to-North-America bit of the journey by overwater dispersal.
Ok – that was a very brief rundown, and an awful lot more could be said. But, having gotten the ‘familiar’ European and North American toads out of the way, we need to move on…
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
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 – –
Bever, B. S. 2005. Variation in the ilium of North American Bufo (Lissamphibia; Anura) and its implications for species-level identification of fragmentary anuran fossils. Journal of Vertebrate Paleontology 25, 548-560.
Dreitz, V. J. 2006. Issues in species recovery: an example based on the Wyoming toad. BioScience 56, 765-771.
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.
Groves, C. P. & Willoughby, D. P. 1981. Studies on the taxonomy and phylogeny of the genus Equus. 1: Subgeneric classification of the recent species. Mammalia 45, 321-355.
Jones, M. E. H., Evans, S. E. & Ruth, B. 2002. Ontogenetic variation in the frog ilium and its impact on classification. Palaeontological Association Newsletter 51, 132.
Pauly, G. B., Hillis, D. M. & Cannatella, D. C. 2004. The history of a Nearctic colonization: molecular phylogenetics and biogeography of the Nearctic toads (Bufo). Evolution 58, 2517-2535.
Pramuk, J. B. 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.
Rage, J.-C. 2003. Oldest Bufonidae (Amphibia, Anura) from the Old World: a bufonid from the Paleocene of France. Journal of Vertebrate Paleontology 23, 462-463.
Sanchiz, B. 1998. Salientia. Handbuch der Paläoherpetologie, Teil 4. Verlag Dr. Friedrich Pfeil, München.
Tihen, J. A. 1962. A review of New World fossil bufonids. American Midland Naturalist 68, 1-50.