Anurans – frogs and toads – haven’t received enough coverage on Tet Zoo of late, so here’s one of several efforts to redress the balance. For no particular reason, in this article I want to talk about pelobatoids, also known as anomocoelans: the anuran group that (as conventionally conceived) includes spadefoot toads (Pelobatidae) and parsley frogs (Pelodytidae). Spadefoot toads are moderately familiar, squat-bodied, robust-limbed anurans that inhabit the arid and semi-arid regions of Europe, north Africa and Asia. There’s also a North American spadefoot group that's regarded by some authors as part of Pelobatidae and by others as a distinct ‘family’ (Scaphiopodidae).
The common name – ‘spadefoot toad’, that is – refers to the presence of a hard, keratinised flange present on the metatarsus and often supported internally by a rod-like or sickle-shaped bone. This is used as a digging tool and hence the animals dig by pushing aside sediment that’s underneath and slightly behind them. They thus ‘dig backwards’, sinking downwards into the ground when digging. This ‘digging flange’ is not present in all members of the group.
Pelodytids or parsley frogs are pelobatoids (or anomocoelans) of continental Europe and the Caucasian and Black Sea regions. All species lack a metatarsal flange but a number of other anatomical features – the presence of vertical pupils and an ossified, style-like sternum among them – link them to the other pelobatoid/anomocoelan lineages, as does molecular data. Key features unique to pelodytids include the presence of a very long, fused tibiale and fibulare (these are distal hindlimb elements that are uniquely elongate in anurans) and the presence of a parahyoid ossification. Numerous small teeth are present in both the premaxillae and maxillae, their number and exact position varying between the species (Sanchiz et al. 2002).
There are only three extant species: the Caucasian parsley frog Pelodytes caucasicus, which occurs from the western Caucasus across Georgia to Azerbaijan and Turkey, and the Common parsley frog P. punctatus and Iberian parsley frog P. ibericus of western Europe [photo above by Benny Trapp]. The Iberian parsley frog was only recognised as distinct in 2000 and shares more characters with P. punctatus than it does with P. caucasicus (Sanchiz et al. 2002). Genetic variation within European parsley frogs suggests that current taxonomy does not reflect phylogeny and that further changes will occur in future (Díaz-Rodríguez et al. 2014).
These extant members of the group have a classic ‘disjunct distribution’, since an extensive distance between them indicates local extinction across much of their presumed former range [range map below compiled by Carlos Bartolomé La Huerta].
All parsley frogs are small (c 40-50 mm SVL), brownish or greyish, secretive, nocturnal frogs with vertical pupils, a pointed snout and a preference for living close to water. The name ‘parsley frog’ is a reference to the greenish markings present across the body, somewhat reminiscent of a covering of chopped herbs. Mild sexual dimorphism is present: males have proportionally longer, more powerful forelimbs than females and (during the breeding season) have black tubercles on the ventral surface of the body (Arikan et al. 2007).
During the breeding season, strings of eggs are laid on aquatic plants. P. punctatus produces far bigger clutches than P. ibericus or P. caucasicus, laying 1000-1500 eggs while P. ibericus produces something like 350 and P. caucasicus 160-500 (Laňka & Vít 1986, Arikan et al. 2007). I wonder why there is such a disparity in fecundity. Maybe P. punctatus suffers higher mortality, or is subject to higher rates of predation, than the others. It should be noted that clutch size varies intraspecifically according to altitude, with those populations that live closer to sea level producing smaller clutches than those that live higher up (Arikan et al. 2007). The egg strings mass together, meaning that the resultant clutch takes the form of a vaguely rectangular mass about 12 eggs wide across its middle part.
Parsley frog tadpoles grow large (reaching total lengths of 65 mm) and sometimes over-winter and thus take two years to metamorphose. They possess a ventral sucker that allows them to attach themselves to stones when in flowing water.
Fossils reveal that parsley frogs were once far more widespread than they are today. European fossils referred to Pelodytes are known from the Lower Miocene onwards (the fossil species P. arevacus is from the middle Miocene of Spain) and there are even supposed to be Eocene records of this taxon (Rage 1988, 2006, Sanchiz 1998a, Rage & Roček 2003). It’s quite common for Paleogene amphibian and reptile fossils to be referred to extant taxa (something that’s generally never done with birds or mammals), this helping to perpetuate the general idea that herptile ‘genera’ are long-lived, relatively static entities. However, the fossils concerned are virtually always so fragmentary that the idea that they might be identifiable to ‘genus’ is typically misleading – they usually lack the diagnostic features that might allow confident placement and, in many cases, should only really be identified to ‘family’ (see, e.g., Good 1988). Anyway, I digress. European fossils referred to the extant P. punctatus have been reported from the Holocene, Pleistocene and even the Pliocene (Sanchiz 1998a).
Most surprising are the three North American fossil anurans thought to belong to this group: Miopelodytes gilmorei from the Middle Eocene Elko Formation of Nevada, Tephrodytes brassicarvalis from the Upper Oligocene of Montana, and Aerugoamnis paulus from the Lower Eocene Green River Formation of Wyoming. In contrast to the vast majority of fossil anurans (most of which consist of isolated and incomplete pelvic bones), all are known from articulated partial skeletons.
Miopelodytes seemingly differs from extant pelodytids in the form of contact between its urostyle and sacral vertebra, while the ilia of Tephrodytes are very different from those of virtually all other members of the spadefoot group (Holman 2003). In a recent phylogenetic analysis of morphological characters, Henrici et al. (2013) found Tephrodytes to be better placed within a clade that includes North American and Eurasian spadefoots, its pelodytid-like fused tibiale and fibulare therefore representing convergence (this character is also present in glassfrogs, meaning that it has evolved independently at least three times within Anura). Aerugoamnis lacks this character (despite possessing other osteological characters of the parsley frog lineage), indicating that tibiale-fibulare fusion characterises a subset of the pelodytid clade, not the group as a whole. Indeed, Henrici et al. (2013) opted to restrict the name Pelodytidae to the crown-group. They coined a new, branch-based name – Pelodytomorpha – for the whole lineage.
Incidentally, Miopelodytes was originally thought to be from the Miocene, hence its (now inappropriate) generic moniker.
I didn’t mean to take up so much of this article talking about fossils, but this former distribution of the parsley frog group is one of the most interesting things about them. While (as always) there’s more to say about them, this introductory article will do for now. What about the other pelobatoid/anomocoelan groups? Well, stay tuned...
For previous Tet Zoo articles on frogs and toads, see...
- In pursuit of Romanian frogs (part I: Bombina)
- In pursuit of Romanian frogs (part II: WESTERN PALAEARCTIC WATER FROGS!!)
- In pursuit of Romanian frogs (part III: brown frogs)
- The toads series comes to SciAm: because Africa has toads too
- 20-chromosome toads
- Glassfrogs: translucent skin, green bones, arm spines
- Everybody loves glassfrogs
- African tree toads, smalltongue toads, four-digit toads, red-backed toads: yes, a whole load of obscure African toads
Refs - -
Arikan, H., Tosunoğlu, M., Atatür, M. K & Göçmen, B. 2007. Some comments on the breeding biology of Pelodytes caucasicus Boulenger, 1896 (Anura: Pelodytidae) from Uzungöl, northeast Anatolia. Turkish Journal of Zoology 31, 53-64.
Díaz-Rodríguez, J., Gonçalves, H., Sequeira, F., Sousa-Neves, T., Ferrand, N. & Martínez-Solano, I. 2014. Molecular evidence for cryptic candidate species in Iberian Pelodytes (Anura, Pelodytidae). Molecular Phylogenetics and Evolution doi:10.1016/j.ympev.2014.12.007
Good, A. D. 1988. The phylogenetic position of fossils assigned to the Gerrhonotinae (Squamata: Anguidae). Journal of Vertebrate Paleontology 8, 188-195.
Henrici, A. C., Báez, A. M. & Grande, L. 2013. Aerugoamnis paulus, new genus and new species (Anura: Anomocoela): first reported anuran from the Early Eocene (Wasatchian) Fossil Butte Member of the Green River Formation, Wyoming. Annals of the Carnegie Museum 81, 295-309.
Holman, J. A. 2003. Fossil Frogs and Toads of North America. Indiana University Press, Bloomington and Indianapolis.
Laňka, V. & Vít, Z. 1986. Amphibians and Reptiles. Hamlyn, Twickenham.
Rage, J.-C. 1988. Le gisement du Bretou (Phosphorites du Quercy, Tarn-et-Gardonne, France) et sa faune de vertébrés de l’Eocène supérieur. I. Amphibiens et Reptiles. Palaeontographica Abteilung A 205, 3-27.
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Sanchiz, B. 1998a. Salientia. Handbuch der Paleäoherpetologie. Dr. F. Pfeil, Munich.
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- ., Tejedo, M. & Sánchez-Herráiz, M. J. 2002. Osteological differentiation among Iberian Pelodytes (Anura, Pelodytidae). Graellsia 58, 35-68.