The year 2009 is a double anniversary for Charles Darwin; the famous British naturalist was born in 1809 (i.e. 200 years ago) and published his major book “On the Origin of Species” in 1859 (i.e. 150 years ago). It is a priori not so easy to celebrate Darwin by speaking about amphibian evolution only, because he did not dissert a lot about this group in comparison with the other tetrapods: he did observe amphibians during his travel around the world, on the H.M.S. Beagle (from 27 December 1831 to 2 October 1836, cf. [13]), but these observations remain rare compared with those related to reptiles. He also described amphibian species (e.g., [7]), but few compared with reptile and especially bird species. However, considering the rare amphibians he crossed during his travel around the world, Darwin observed and described some of them for the first time: this is the case of the famous “Darwin's frog” (Rhinoderma darwinii), an amazing small (3 cm long) anuran which raises its tadpoles inside the vocal sac of the male. This taxon is indeed named after Darwin because he firstly discovered the species in Chile and Argentina.

Before developing further the relationships between Darwin and amphibians, it is interesting to note that there are, in the literature, at least 36 living and fossil species literally called darwinii (subspecies and amphibians excluded); two protists (Coccodiscus, Pediastrum), one mushroom (Cyttaria), 16 plants (Berberis, Gossypium, Maihueniopsis, Hoya, Eustephia, Lecocarpus, Senecio, Abutilon, Opuntia, Bonatea, Chaetomorpha, Tephrocactus, Calceolaria, Hymenophyllum, Nassauvia, Neosparton), eight “invertebrates” (Sapphirina, Cosmocalanus, Orchestia, Alleloplasis, Camponotus, Spirifer, Parahelops, Nettastomella), two “fish” (Marylina, Gephyroberyx), three reptiles (Liolaemus, Amphisbaena, Gymnodactylus), three birds (Rhea, Nothura, Tanager) and one mammal (Mylodon).

Darwin observed and collected amphibians (mainly frogs and toads), mostly in South America. He was very surprised about the peculiar way of life of some of them, which were living in very different environments than those he used to observe before, in Europe: for example, the famous “Darwin's toad” (“Phryniscus nigricans” or Melanophryniscus stetcheri or M. montevidensis) (Fig. 1, center) is a small buffonid found by Darwin himself in the dunes of the sandy shore of Maldonado (Uruguay). This subterrestrial species does not really like water. Darwin, surprised to find a living individual in a very dry environment, put it in a pond but, as it was unable to swim, he needed to take it back out… “At Maldonado, I found one in a situation nearly as dry as at Bahia Blanca, and thinking to give it a great treat, carried it to a pool of water; not only was the little animal unable to swim, but I think without help it would soon have been drowned.” ([13] pp. 101–102). Darwin attributed this bizarre amphibian's “terrestriality” to the capacity it has to store water in the bladder: “I believe it is well ascertained that the bladder of the frog acts as a reservoir for the moisture necessary to its existence…” ([13] p. 409).

But beyond the terrestriality of some amphibians, Darwin was more surprised by the euryhalinity of some of them: “Leiuperus salarius” (today Pleuroderma bufonina or P. buffoni) (Fig. 2, upper left) is indeed one of the rare living amphibians supporting salty waters. This amazing frog was first observed by Darwin at Port Désiré, Patagonia. Again, Darwin was surprised by its euryhaline habits “it is reproducing and living in waters too salted to be drunk” [4](Fig. 2).

This euryhalinity is very rare among living amphibians (lissamphibians), but it was more common among the fossil forms (e.g., stegocephalians). The amphibians (as nonamniotic tetrapods) emerged during the Late Devonian (if not during the Middle Devonian, [26]). It is now generally admitted that the first amphibians (e.g., Ichthyostega, Acanthostega) were mostly aquatic and euryhaline (e.g., [1] and [23]). This capacity to support high variations of salt concentrations within the body (which does not mean an obligatory life in a marine environment) could have been linked initially with polydactyly [27]: every fossil tetrapod showing more than five (true) digits (i.e., the Devonian amphibians, the Jurassic ophthalmosaurian ichthyosaurs and a Triassic marine reptile [28]) is indeed euryhaline (Fig. 3).

In living tetrapods, polydactyly is today exceptional but could still occur in lissamphibians, for example, and especially anurans (cf. prehallux, prepollex, e.g., [15]). However, morphogenetically speaking, it is often a pseudopolydactyly, i.e. the surnumerary “digit” is not a true digit, but is formed initially by dichotomy of a phalanx or a tarsal or carpal (e.g., [30]). However, even if most of the polydactylous living amphibians do not show any true extra digit like their Devonian relatives, this phenomenon was puzzling generations of naturalists, and Darwin himself, of course: “Additional digits have been observed in negroes as well as in other races of man, and in several of the lower animals. Six toes have been described on the hind feet of the newt (Salamandra cristata), and, as it is said, of the frog. It deserves notice from what follows, that the six-toed newt, though adult, had preserved some of its larval characters; for part of the hyoidal apparatus, which is properly absorbed during the act of metamorphosis, was retained.” ([9] p. 14). Interestingly, Darwin associated the occurrence of polydactyly with retention of larval characters in adults, i.e. paedomorphosis. He was, of course, aware of the works of his colleague (and strong defender) Ernst Haeckel (1834–1919), German embryologist who firstly created the terms “Ontogeny” and “Phylogeny” [17] to better study developmental heterochronies (reviewed in [16]). More importantly, the association made by Darwin of polydactyly (limb “supermorphology”) and rentention of the hyoidal apparatus (cranial morphology) is very pionneering, because it is confirmed today by molecular genetics: it has, indeed, been shown recently that the limb and digit morphogenesis is controlled by the same Hox gene groups (of types A and D) as the craniofacial morphogenesis (e.g., [29]).

After South America, the Beagle took cape toward the West and reached the Galápagos Archipelago, the 15 September 1835. The Galápagos visit of Darwin (until October 1835) is very famous for his “discovery” of the very endemic fauna composed of marine iguanas (Amblyrhynchus cristatus), giant turtles (Geochelone elephantopus) and numerous finches (again, the famous “Darwin's finches” such as Geospiza conirostris). However, Darwin was also surprised by the total absence of amphibians, despite humid and temperate upland forests on the Archipelago which could have well suited them. He opposed this amphibian absence with the reptile abundance on the oceanic islands, particularly of lizards that he considered a priori as similar morphotypes occupying the same ecological niches. Darwin indeed wrote (1890, p. 408): “The absence of the frog family in the oceanic islands is the more remarkable, when contrasted with the case of lizards, which swarm on most of the smallest islands. May this difference not be caused by the greater facility with which the eggs of lizards, protected by calcareous shells, might be transported through salt-water, than could the slimy spawn of frogs?” His ecological hypothesis, therefore, suggests that reptiles, thanks to their amniotic reproduction mode, benefit from a higher capacity of oceanic spreading than that of amphibians (as nonamniotic tetrapods). Following the systematic distinction between the classes Reptilia and Amphibia proposed by De Blainville [14], Darwin therefore put forward this higher spreading capacity of the reptiles which a priori involved a strong competition pressure on this poor amphibian oceanic distribution. Speaking about pressure, it is therefore possible that amphibians also correspond, for Darwin, to an inspiration source which might help him to write, later, his natural selection theory [8].

Darwin was therefore surprised by the wonderful amphibian world he discovered around the globe: he mentioned, for example, the extreme climbing hability of some specimens (Fig. 4) he collected in Rio de Janeiro from May to June 1832: “I had some difficulty in catching a specimen of this frog. The genus Hyla has its toes terminated by small suckers; and I found this animal could crawl up a pane of glass, when placed absolutely perpendicular.” ([6] footnote p. 33). He also enjoyed listening to the songs of the males during the reproduction period and even discussed later, in his second major book entitled “The descent of man, and selection in relation to sex” (1871), about this sexual communication between males and females: “These animals, however, offer one interesting sexual difference, namely in the musical powers possessed by the males […]; certain frogs sing in a decidedly pleasing manner. Near Rio de Janeiro I used often to sit in the evening to listen to a number of little Hylæ, which, perched on blades of grass close to the water, sent forth sweet chirping notes in harmony.” ([10] p.27) (Fig. 4).

Observing the terrestrial habit of Melanophryniscus (see above), he was also very surprised by its very colorful, flashy skin, black with flams of vermilion. Darwin ([13], p. 101) wrote, almost poetically: “If it had been an unnamed species, surely it ought to have been called Diabolicus, for it is a fit toad to preach in the ear of Eve.” Beyond poetry, Darwin pointed out the repulsive look of this amphibian and therefore the ecotrophic notion of passive defense… As for the active defense of the amphibians, Darwin made interesting observations (and experiments) on these animals once he had returned to Great-Britain, in the Zoological Gardens, from October 1836: the capacity of some frogs and toads to inflate their body in front of predators is carefully reported in his book “The expression of the emotions in man and animals” (1872): “When frogs are seized by snakes, which are their chief enemies, they enlarge themselves wonderfully; so that if the snake be of small size […], it cannot swallow the frog, which thus escapes being devoured.” ([11] p. 105). Whether passive or active, this self-defense notion in animals which represent possible prey for others, is therefore related in the natural selection theory of Darwin.

Darwin made also interesting comparisons during his stay at Maldonado (Uruguay), from July 26, 1832, where he collected many mammals and compared the blindness rodent Ctenomys brasiliensis (“tuco-tuco”) with the Spalax (the “mole rat”) and with the famous urodel called Proteus. He wrote: “Considering the strictly subterranean habits of the tuco-tuco, the blindness, though so common, cannot be a very serious evil; yet it appears strange that any animal should possess an organ frequently subject to be injured. Lamarck would have been delighted with this fact, had he known it, when speculating (probably with more truth than usual with him) on the gradually-acquired blindness of the Aspalax [as Spalax], a Gnawer living under ground, and of the Proteus, a reptile living in dark caverns filled with water; in both of which animals the eye is in an almost rudimentary state, and is covered by a tendinous membrane and skin.” ([13] p. 53). It is interesting to note that: •

Darwin cited Lamarck and his “Philosophie Zoologique” (see below), but he also strongly criticized him (cf. the parentheses): Darwin deliberately put the word “acquired” in italics in his text, by reference to the evolutionnary theory of the acquired character heredity of Lamarck he questioned (see also [12] for a discussion about progressive reduction of structures);

As mentioned above, Darwin ([13], p. 53) cited Lamarck. This allows me an interesting transition to discuss now about the relationships between Jean-Baptiste Lamarck (1744–1829) and amphibians. We are all aware that the year 2009 is a double anniversary for Darwin: everybody (especially the English speaking world) is celebrating the British naturalist, but one also need to celebrate the French one, who never read “On the Origin of the Species” but who published his major book “Philosophie Zoologique” in 1809 (Darwin's year of birth), two centuries ago.

Before Darwin, and thanks to amphibians (which were classified among reptiles at that time), Lamarck already had observed in 1802 that characteristics (i.e., gills and lungs) used in systematics to define two classes (Fish and Reptiles, respectively) are also visible during an anuran life: « Ainsi, non seulement la nature passe des branchies aux poumons dans des classes et dans des familles voisines comme l’indique la considération des poissons et des reptiles mais elle y passe même pendant l’existence d’un même individu, qui jouit successivement de l’un et de l’autre systême. On sait que la grenouille dans l’état imparfait de tétard, respire par des branchies, tandis que dans son état plus parfait de grenouille elle respire par des poumons. » ([21] p. 43) (“In this manner, nature is passing from gills to lungs in classes and in families which are close, as indicated by the consideration of fish and reptiles, but it is also passing even during the life of a single individual, which benefits successively to one and to the other system. One knows that the frog, in the imperfect stage of the tadpole, is breathing with gills, whereas, in its more perfect stage of the frog, is breathing with lungs.”). In other words, amphibian development, according to Lamarck, summarizes the tetrapod classification. This very interesting “evo-devo” remark was developed later by Lamarck himself in “Philosophie Zoologique” ([22], p. 245) but also by Haeckel [17] in his famous “biogenetic law” or his recapitulation theory (“ontogeny is a […] recapitulation of the phylogeny […]”).

Lamarck based his theory of acquired character heredity partly on amphibians: he illustrated his adaptationism by morphological characteristics such as the interdigital membranes of frogs: « Les mêmes efforts faits pour nager, c’est-à-dire, pour pousser l’eau, afin d’avancer et de se mouvoir dans ce liquide, ont étendu de même les membranes qui sont entre les doigts des grenouilles, des tortues de mer, de la loutre, du castor, etc. » ([22] p.249). (“The same efforts made to swim, that is to say, to push water, in order to go forward and to move in this liquid, have extended as identical the membranes which are between the digits of frogs, of sea turtles, of the otter, of the beaver, etc.”). Interestingly, Lamarck compared different and distant aquatic tetrapods showing the same swimming structure. This comparison is very pioneering, because it announced the notion of evolving convergence, as Darwin did later (see above). In his multiple comparisons, Lamarck was also aware of the ectothermism versus the endothermism which marks the difference between reptiles (incl. the “amphibious” – for the amphibians – at that time) and birds. He wrote: « […] et on verra, par exemple, que dans les amphibies où l’action organique est lente et pénible, puisque ces animaux, comme le crapaud, les serpents, etc., digèrent avec beaucoup de lenteur, la tendance à la décomposition s’effectue aussi très lentement ; de sorte que ces êtres animés font peu de pertes : aussi ont-ils peu de besoins. Les oiseaux offrent des faits très différens […] » ([20] p.218) (“[…] and one shall see, for example, that in the amphibious where the organic action is slow and difficult, as these animals, such the toads, the snakes, etc., are digesting with a lot of slowness, the tendency to the decomposition is also made very slowly; in consequence these animated beings are making few losses: they have also few needs. Birds show very different traits […]”).

At the time of Lamarck (1744–1829), almost no fossil amphibian (stegocephalian) record was published. In fact, the first stegocephalian to be described was the large temnospondyl Mastodonsaurus from the Triassic of Germany, published by Jaeger in 1828 ([19], p.35), the year before the death of Lamarck (although Damiani [5], mentioned a possible previous reference; [18]). The fossil specimens of Mastodonsaurus described by Jaeger were considered as fossil reptiles but were also compared (cf. Salamandroides [19] p.38) with Salamandra Daudin, 1803 (as Cryptobranchus), the giant salamander living in North America. Mastodonsaurus is no longer considered as ancestor of the (giant) urodels, it belongs to the stereospondyls, an extinct group of advanced temnospondyls mostly known in the Mesozoic, until the Cretaceous. However, some Palaeozoic representatives of temnospodyls could be more related to lissamphibians (as modern or living amphibians) but this is still much debated (e.g. [25])… Whatever their origin, since Lamarck and Darwin, the lissamphibians, therefore, played a major role in the conceptualization of evolutionary models.