October 1, 2013 | 86
Tet Zoo loves amphibians* (that’s anurans, salamanders, caecilians and their close relatives), and since 2008 I’ve been making a concerted effort to get through all the amphibian groups of the world. I’ve failed, and I’m blaming that entirely on the fact that I can’t put the time I need to into blogging. Sigh, always busy spending my time working my backside off for stupid money, sigh… Anyway, back in 2008 I did succeed in producing a couple of articles that review the caudates – the salamanders – of the world.
* I have to say at this point that I really prefer to use the term lissamphibian when referring to extant amphibian lineages. Mostly, this is because I somewhat dislike the use of the term ‘amphibian’: its perpetuation makes people think that non-amniote tetrapods – or anamniotes – are all close relatives, all closely related or ancestral to lissamphibians. However, since the term amphibian is in such wide use and is overwhelmingly popular among biologists and conservationists, I’ll stick with it here. Oh, and no, we haven’t forgotten you, albanerpetontids (round these parts, they’re affectionately known as albies).
There are about 655 recognised living salamander species, and many aspects of their diversity, biology and behaviour are fascinating. There are (or were) giant salamanders more than 2 m long, tiny, tree-climbing salamanders with prehensile tails, partially herbivorous salamanders that possess beaks, cave-dwelling salamanders, long-bodied, aquatic eel-like salamanders, and armoured salamanders with defensive spikes. Claws, intra-uterine cannibalism, neoteny, the defensive shedding of tails and limbs, weird developmental shifts in digital development, bizarre warning displays, lekking and nuptial dancing, facultative herbivory and fungivory, dedicated cave-dwelling… it’s all here. So shame on you if the thought ever occurred to you that salamanders might be plain or boring. A lot of new stuff on these animals has appeared since 2008, meaning that I’ve had to substantially update my original text. Without further ado, let’s get on with it…
The term caudate means ‘possessing a tail’ and, in contrast to anurans and caecilians, salamanders retain the body shape that seems to have been plesiomorphic for amphibians. Having said that, even salamanders exhibit a list of anatomical specialisations that make them unusual compared to other tetrapods: they’ve lost a long list of skull bones that are present in other anamniotes (including the postorbitals, jugals, tabular, supraoccipital and ectopterygoids), and also lack a middle ear (though they’re not deaf). Bizarrely, salamanders have disproportionately large amounts of DNA.
Salamanders are generally Laurasian and the groups that occur on the southern continents (there are plethodontids in South America and some salamandrids in northern Africa) have only gotten there comparatively recently. An important theme which has cropped up several times independently within different salamander clades is neoteny: the retention of juvenile characters into sexual maturity. It’s been widely suspected that neoteny results in the retention of confusing plesiomorphic characters, and in extensive convergence, so working out the relationships of neotenous salamander clades has been difficult (Wiens et al. 2005). Phylogenetic studies on salamanders have in fact differed pretty radically in the relationships they infer (particularly with respect to the position of sirens), though a rough consensus has emerged.
The oldest salamanders
The oldest salamanders we know of are Middle Jurassic forms from central Asia (Kokartus honorarius) and England (two species of Marmorerpeton). What we know indicates that these early forms looked superficially like stout-bodied living ones, but they lacked various bony and muscular characters present in the crown-group. The best known early salamander – Karaurus sharovi from the Upper Jurassic of Kazakhstan – has been compared to living mole salamanders (ambystomatids) and perhaps lived in a similar manner (Estes 1981). [Adjacent image by David Meloni.]
Whether the name Urodela is restricted to the crown-group, with Caudata used for the total-group, or whether the name Caudata is restricted to the crown-group, with Urodela used for the total-group, depends on which authors you pay attention to. Evans & Milner (1996) argued that it makes better sense to use Caudata for the total-group, and this seems to have been mostly followed and is used here. So Marmorerpeton and the karaurids Kokartus and Karaurus are stem-caudates but not urodeles. A few other Mesozoic taxa, like Pangerpeton and Jeholotriton from the Jurassic or Cretaceous of Liaoning Province in China, are regarded by some authors as additional stem-caudates (Wang & Evans 2006). [Image below by internet hero Nobu Tamura.]
Various fossil salamanders from the Middle and Upper Jurassic seem to be crown-group salamanders: that is, early members of the lineages that include the cryptobranchids (giant salamanders) or hynobiids (Asian salamanders) (together, cryptobranchids and hynobiids are termed cryptobranchoids), or members of Salamandroidea, the ‘advanced’, internally fertilising salamander clade. Chunerpeton from the Daohugou Beds of Inner Mongolia (Gao & Shubin 2003, Carroll & Zheng 2012), and – according to Carroll & Zheng (2012) – Pangerpeton, Jeholotriton, Regalerpeton, Liaoxitriton and Iridotriton are all Mesozoic cryptobranchoids. If this is correct, and if there’s a sister-group relationship between cryptobranchoids and salamandroids, then members of both of these major salamander clades had appeared before the end of the Jurassic. The recent description of Beiyanerpeton jianpingensis from the Jurassic Tiaojishan Formation (Gao & Shubin 2012) seems to confirm the predicted presence of salamandroids in the Jurassic. Iridotriton hechti from the Morrison Formation has also been interpreted as an early salamandroid (Evans et al. 2005) but this was contested by Gao & Shubin (2012) who found it to be a cryptobranchoid. By the Upper Cretaceous, members of most or all of the living salamander ‘families’ had probably appeared.
Cryptobranchoids: giant salamanders and Asiatic salamanders
Two particularly ancient groups of salamanders are still around today: the hynobiids and the cryptobranchids. Hynobiids, generally just called Asiatic salamanders, are a poorly known group of about 50 species that occur from Afghanistan and Iran eastwards to Japan, though between the Miocene and Pleistocene they also occurred in Europe (Venczel 1999). Some hynobiids occur in cold parts of northern Asia and are particularly cold-tolerant, being able to withstand freezing at temperatures below -50°C for months at a time (that is, they literally get frozen alive and can stay dormant in a block of ice). [Image of Onychodactylus above by Pierre Fidenci.]
Some hynobiids employ aquatic suction feeding while others have a projectile tongue. Some (like Onychodactylus) have evolved claw-like structures on their digit tips. A recent study of hynobiid phylogeny and biogeography indicated that the group originated in China, with the rest of its history and distribution being strongly influenced by local events like the desertification of Mongolia and the uplift of the Tibetan plateau (Zhang et al. 2006).
Grouped with hynobiids in the clade Cryptobranchoidea (or Cryptobranchiformes), are the giant salamanders, or cryptobranchids. There are only three extant species (the North American Hellbender Cryptobranchus alleganiensis, the Chinese giant salamander Andrias davidianus and Japanese giant salamander A. japonicus); all are salamanders of fast-flowing, well-oxygenated water (but this wasn’t the case for all fossil species). All possess dorsoventrally flattened bodies. Gills are absent in the adults and their lungs apparently don’t function in respiration, so all gas exchange occurs across the extensively folded, wrinkled skin. Eyelids are absent (a sure sign of aquatic habits in a caudate).
Giant salamanders are famous for, well, being giant, with record-holding specimens of the Chinese giant salamander reaching 1.8 m and 65 kg. Some fossil species were bigger, with A. matthewi from Miocene North America reaching 2.3 m. Little known is that cryptobranchids have particularly vicious teeth, exude a foul smell which has been partially likened to “the rankest public urinal crossed with that of stale sweat” (Brazil 1997, p. 64), and that the males brood the eggs. Giant salamanders can inflict massive wounds with their teeth: during territorial fights males frequently sever digits, limbs, and bits of tails of rivals, and massive fatal slices across the neck – sometimes resulting in decapitation – are apparently not uncommon.
Chunerpeton from the Jurassic of China (mentioned above) has been identified as a cryptobranchid, in which case both this group and its sister-taxon Hynobiidae have been around for a long time. Chunerpeton seems to have been reasonably large – about 200 mm long – but, of course, not ‘large’ by comparison with the Cenozoic species. For much more on those species see the Tet Zoo article on giant salamanders (link below).
Of batrachosauroidids and scapherpetontids (…. or scapherpetids)
Having discussed cryptobranchoids, I should say that there are a couple of entirely fossil salamander groups – naming Batrachosauroididae and Scapherpetontidae* – that have sometimes been regarded as part of this clade (Estes 1969). However, Estes (1981) later classified batrachosauroidids with proteids (olms and mudpuppies) and scapherpetids with ambystomatids. Batrachosauroidids have a fossil record that extends from the Upper Cretaceous to the Pliocene (a possible member of the group has been reported from the Jurassic-Cretaceous boundary, however) and they’re known from North America as well as Germany and France. They seem to have been large, long-bodied salamanders, probably with reduced limbs, with subtriangular, poorly ossified skulls superficially similar to those of amphiumas. Long ‘stalks’ on the occipital condyles suggest an ability to elevate the cranium extensively and hence open the jaws wide. Large size is indicated by the known skull and jaw remains: Batrachosauroides and Opisthotriton both have skulls about 30 mm long, which isn’t bad for a salamander.
* David Marjanović reminds me (see comments) that it should really be Scapherpetidae.
Scapherpetids are known from the Upper Cretaceous, Paleocene and Eocene and they’re mostly known from fragments (vertebrae and partial mandibles). They must have been large (vertebral centra are usually something like 20 mm long), and the shapes of their vertebrae and limbs show that they were another long-bodied, limb-reduced, mostly aquatic group: in Scapherpeton from the Upper Cretaceous, the limb bones are small compared to the size of the vertebrae, the proportions seemingly being about intermediate between Dicamptodon and amphiumas (Estes 1981).
The amazing sirens
Sirens (Sirenidae) are perhaps the strangest of salamanders. Indeed, they’re so strange that at times they’ve even been excluded from Caudata and put on their own as a ‘new’ sort of amphibian group termed Trachystomata or Meantes. Superficially eel-like, neotenic, and lacking a pelvis and hindlimbs, they possess external gills, lack eyelids, are adept at burrowing in mud, and reach 95 cm in the largest species [Image above by Stan Shebs]. The Cretaceous-Paleocene siren Habrosaurus reached 1.6 m, which is enormous and frightening. Habrosaurus, in fact, is so spectacular that I was half-expecting there to be some life restorations of it online (there are two species: H. prodilatus from the Campanian of Alberta, and H. dilatus from the Maastrichtian and Paleocene of Wyoming and Montana). Alas, that doesn’t seem to be the case, so I had to resort to creating this…
The big surprise is that sirens have a horny beak and pavements of teeth on the palate. The beak forms a broad platform inside the jaws, and the jaw joint is ventrally displaced relative to the rest of the skull. All of these features are adaptations for crushing, and field studies show that (as you might predict) sirens feed extensively on gastropods and bivalves. Sirens are unlike most salamanders in that their teeth are (usually) not pedicellate: that is, their teeth are not connected to the jaw bones by way of a flexible pedicle. Salamanders start their life as larvae without pedicellate teeth and normally develop the condition as they mature, so lack of pedicelly is seen as a neotenous condition. Proteids also lack pedicellate teeth as adults, as do batrachosauroidids and the Jurassic salamanders Kokartus and Beiyanerpeton. The fact that certain early salamanders (including stem members of the group) lack pedicellate teeth seems to challenge the hypothesis that pedicelly is a synapomorphy of Lissamphia and a primitive, inherited character for salamanders (Gao & Shubin 2012).
Sirens also appear to be partially herbivorous, ingesting vascular plants and algae. They have enlarged hindguts that may house symbiotic microbes (Pryor et al. 2006). If this is correct it’s a big deal and would make them unique among caudates. They are also unusual in that they can survive desiccation by forming a sort of mucus cocoon in the mud, a habit very similar to that better known for lungfishes. Oh yeah, and they emit a yelping noise when grabbed (Halliday & Verrell 1986).
Several features support the view that cryptobranchoids are anatomically primitive relative to the other crown-group salamanders, the salamandroids. Cryptobranchoids still possess an angular bone in the lower jaw, and they also practise external fertilization. Some studies indicate that cryptobranchoids are not alone in being outside of Salamandroidea, since it’s been argued that sirens belong here too (Larson & Dimmick 1993, Wiens et al. 2005, Pyron & Wiens 2011).
In a radical departure from this view, Frost et al. (2006) found sirens to be nested well inside Salamandroidea, being especially close to proteids (mudpuppies and olms). They used the name Perennibranchia Latreille, 1825 for this proposed siren + proteid clade. A position within Salamandroidea for sirens would be very surprising since sirens don’t possess any of the reproductive characters otherwise typical for this group, including internal fertilization, the possession of spermathecae or the production of spermatophores (read on for more on all of these features). If Frost et al. (2006) are correct, some pretty radical reversals have occurred in siren evolution. The hypothesis that sirens are not members of Salamandroidea, then, appears more parsimonious on face value.
Sirens have a fossil record extending back to the Upper Cretaceous, and while the extant species are all North American, fossil representatives have been described from Sudan, Germany, India and Bolivia. Given that salamanders are, as we’ve seen, mostly Northern Hemisphere animals, the presence of these animals in Gondwanan locations is significant. The identification of these fossils – some of which have often been united in a group termed Noterpetontidae – as sirenids has been challenged (Gardner 2003). One possibility is that noterpetontids are stem-sirens (Marjanović & Laurin 2007).
The wonder that is the internally fertilizing salamander clade
Salamandroidea – also named Salamandriformes or Diadectosalamandroidei – is also known as the ‘internally fertilizing salamander’ (or IFS) clade (Larson & Dimmick 1993). This is the clade that contains the vast majority of salamander species and lineages, including the (mostly) American lungless salamanders or plethodontids, the chunky mole salamanders (or ambystomatids) of the Americas, and the (mostly) Eurasian salamandrids. While its roots are in the Mesozoic, Salamandroidea is mostly a Cenozoic clade that exploded in diversity during the late Paleogene and Neogene.
How are these salamanders capable of ‘internally fertilizing’ when males don’t have an intromittent organ? As all amphibian fans will know, they produce an elaborately shaped sperm package (the spermatophore) that they deposit on the substrate. It’s then picked up by the female’s cloaca (some salamanders do all of this on land, others on the floor of a pond or stream. Some species are, err, well stocked and can produce multiple spermatophores in fairly rapid succession).
In order to get the female’s cloaca to make contact with the spermatophore, the male has to guide or place the female correctly, and salamanders have evolved all kinds of tricks to make sure this happens. Most remarkable (in my opinion) is the soft, dorsally projecting spike that some male salamandrids possess at the base of the tail. Once a female has expressed interest as a mating partner, the male manoeuvres himself to get beneath the female, and inserts the spike into her cloaca (Sever et al. 1997), presumably to get her into position for spermatophore collection.
Once a female has absorbed the spermatophore’s sperm-filled cap, she retains the sperm in special cloacal pockets called spermathecae, and it might be stored here for months or even years (by the way, at least one salamandrid breaks all the rules, and engages in cloacal contact during sperm transfer. What is it with evolution and its blatant disregard for rules and consistency?).
A moment ago, we looked at the bizarre sirens. Also long-bodied and super-weird are the amphiumas, or amphiumids. They’re represented by just three extant species in one genus (Amphiuma): all are restricted to the south-eastern USA, and fossil genera show that the group has been present in North America since the Upper Cretaceous at least (Gardner 2003).
Like sirens, amphiumas are eel-like, neotenic salamanders that lack eyelids, but unlike sirens they possess hindlimbs and don’t have external gills. They have rather long skulls with unusually textured bone in the facial region and, unlike sirens, practise internal fertilization. The alternative name ‘congo eel’ (often misunderstood as ‘conger eel’ by laypeople: the real conger eels really are, of course, eels) is particularly dumb, given that they aren’t eels and don’t come from the Congo or anywhere near it. Amphiumas are reportedly of unpleasant temperament and are said to bite savagely. [Photo below of amphiuma skull from Boneman_81's flickr site.]
Surprisingly perhaps, amphiumas have been found to be the sister-group to the plethodontids (Wiens et al. 2005, Pyron & Wiens 2011), and both groups were allied in the newly named clade Xenosalamandroidei by Frost et al. (2006). Incidentally, Edward Cope thought that amphiumas were ancestral to caecilians.
Mudpuppies, waterdogs and olms
Also aquatic and neotenic are the proteids: the mudpuppies, waterdogs and olms [adjacent image shows THE BLACK MUDPUPPY]. Like amphiumas, proteids practise internal fertilization and are definitely parts of the IFS clade. They have bushy external gills, laterally compressed tails and lack maxillae (sirens also have reduced maxillae). Only two extant genera are recognised – Necturus from North America (the mudpuppies and waterdogs) and Proteus from Europe (the olms). Fossil taxa extend the group’s history back to the Palaeocene (Estes 1981) [UPDATE: an Upper Cretaceous taxon - Paranecturus garbanii from the Hell Creek Formation - was published in 2013. See comments]. Proteid monophyly has been found to be questionable in some studies (Weisrock et al. 2005) but supportable in others (Trontelj & Goricki 2003, Wiens et al. 2005, Frost et al. 2006, Pyron & Wiens 2011).
Olms used to be regarded as uniquely cave-dwelling [adjacent image by Arne Hodalič], but we now know of a surface-dwelling form, the Black olm P. anguinus parkelj (olms have been covered on Tet Zoo before: see the links below). Proteids have been variously shuffled about the salamander family tree. Some studies have found them to be close relatives of sirens (Gao & Shubin 2001, Frost et al. 2006) whereas others find them to be the sister-group to the clade that includes amphiumas, lungless salamanders, mole salamanders and salamandrids (Pyron & Wiens 2011).
Axolotls and their friends and relatives
The 33-ish species of mole salamander, or ambystomatids, derive their name from their predominantly fossorial habits. These are robust-bodied North American salamanders, some of which – like the Tiger salamander Ambystoma tigrinum – are large (reaching 40 cm) and brightly coloured. Particularly well known is the fact that some, like the Axolotl A. mexicanum, are neotenous and aquatic. While the Axolotl is abundant as a pet and laboratory animal, the wild population – endemic to Mexico’s Lake Xochimilco – is in danger. Two mole salamander species are particularly odd in that they consist only of females. There are some detailed articles on ambystomatids in the Tet Zoo archives, see links below.
The four species of Pacific giant salamander (‘giant’ = 30 cm) – all united in Dicamptodon – are included by some workers in Ambystomatidae, but are otherwise regarded as worthy of their own ‘family’, termed Dicamptodontidae. All Dicamptodon species have particularly solid-boned skulls and blade-like teeth and are voracious predators of smaller salamanders, rodents and small snakes. Surprisingly, they’re pretty good climbers and some have been seen clambering about in vegetation 2.4 m off the ground (Stebbins 1966).
Fossil dicamptodontids are known from the Palaeocene onwards (in fact Dicamptodon itself goes back this far), with a couple of taxa being European: Rocek (1994) said that these are dicamptodontids “beyond any doubt” (p. 53), but there is in fact now a substantial amount of doubt about the alleged dicamptodontid affinities of the taxa concerned. Incidentally, one of the European taxa is Bargmannia Herre, 1955, but by googling this name I’ve learnt that it’s preoccupied by the siphonophore Bargmannia Totton, 1954 (ha – one year!).
Rhyacotritonidae, named only for the semi-aquatic torrent salamanders Rhyacotriton from the north-western US and previously grouped with Dicamptodon, might instead be closer to amphiumas and/or plethodontids (Pyron & Wiens 2011). In contrast to most ambystomatids and dicamptodontids, torrent salamanders are small (total length 10 cm or less) and with poorly ossified skulls, wrists and ankles. They’re animals of cold mountain streams and seepages, often found in splash zones or wet mossy places.
The lungless salamanders
Plethodontids – the mostly American lungless salamanders – are the most speciose (c. 380 species) and most diverse salamander group with aquatic, terrestrial, fossorial, cave-dwelling and even arboreal species. The smallest salamanders, those of the genus Thorius, belong to this group and may be adult at just 30 mm in total length (yes, including the tail… hence ‘total’ length). They lack lungs entirely, with all respiration occurring across the skin and membranes of the pharynx. Vertical grooves running the length of the body – the costal grooves – draw moisture up around the body, helping the skin remain moist. Phylogenetic studies show that plethodontids have done some freaky things in their evolution, with reversals and rampant convergence being well documented in some lineages.
Some plethodontids escape from predators by tucking in their limbs and rolling downhill, others have ballistic tongues or highly sensitive binocular vision. Plethodontids are not just really interesting; they have also proved really instructive in terms of what they’ve taught us about speciation, hybridisation, species concepts and how evolution works. Much of this research has been produced by University of California’s David B. Wake and his colleagues and students: his lab’s webpage (with many free pdfs) is here.
We saw above that some salamanders (the sirens) appear to be herbivores. Equally remarkable is the claim that some plethodontids are fungivores: if, that is, observations reported by Miller (1944) are correct. Miller wrote that the Santa Cruz black salamander Aneides flavipunctatus niger (recognised as a full species by some workers) ate the fruiting bodies of fungi, and it’s been suggested that other Aneides species might do likewise. However, there is some scepticism about this, and other salamander workers haven’t reported the same behaviour (to my knowledge).
Plethodontids are not uniquely American, as the European cave salamanders also belong to this group. Furthermore, a really amazing recent discovery is that this group also exists in Asia: to date, only one Asian species belonging to the group is known – the Korean crevice salamander Karsenia koreana Min et al., 2005 – but it’s possible and perhaps likely that additional Asian species await discovery. The plethodontid fossil record isn’t great, extending back to the Miocene in both North America and Europe.
Salamandrids: ribs as weapons, viviparity, sex aids
To many people, the most familiar of salamanders are the salamandrids: the mostly North American-Eurasian group that includes the newts and the familiar Fire salamander Salamandra salamandra and its relatives (note that hyper-variable ‘S. salamandra‘ of tradition is a species complex containing at least six different species-level units). There are about 75 living salamandrid species. Most studies have found salamandrids to be close kin of mole salamanders (Larson & Dimmick 1993, Frost et al. 2006, Pyron & Wiens 2011) though others have allied them with plethodontids (Gao & Shubin 2001).
Salamandrids have a fossil record that extends back to the Paleocene of Europe thanks to Koaliella from France and Germany: Zhang et al. (2008) posited the origin of the group as an Upper Cretaceous event. Numerous other fossil taxa are known from between the Middle Oligocene and Pleistocene (including Archaeotriton, Brachycormus, Chelotriton, Megalotriton, Oligosemia, Palaeopleurodeles and Carpathotriton). I always hoped that Megalotriton (which has sometimes been visualised as a giant version of the living Fire salamander) was 1 m long or more but… no. With vertebrae of – at most – 12.5 mm, it was perhaps as much as 40 cm long in total. Some phylogenetic studies find Salamandridae to consist of two major clades: Salamandrinae (containing Chioglossa, Lyciasalamandra, Mertensiella and Salamandra) and Pleurodelinae (containing all the others). [Newt images below by Neil Phillips of UK Wildlife.]
The very distinctive Spectacled salamander Salamandrina terdigitata and Northern spectacled salamander S. perspicillata, both endemic to western Italy, are generally recovered as forming the sister-group to the salamandrine + pleurodeline clade (Zhang et al. 2008, Pyron & Wiens 2011). These unique salamandrids are proportionally long-tailed, only have four toes on the hindfeet, and display their bright red ventral tail surfaces by forming a vertical loop with the tail when disturbed.
Salamandrids are generally amphibious, terrestrial outside of the breeding season, and often with poisonous skin glands and brightly coloured undersides. Some species (most famously the North American Taricha newts) are among the most poisonous of amphibians, and some perform a special contorted display – called an unkenreflex – to show off the vivid reds, oranges or yellows they have on their bellies. This is the group that include the species with the soft, spike-shaped ‘sex aid’ mentioned above: it’s present in the salamandrine species originally grouped together in the genus Mertensiella (they were originally grouped together due specifically to the presence of this structure). However, the species concerned are now known not to be especially close relatives (one of them – Luschan’s salamander – is now the type species of the genus Lyciasalamandra), meaning that some authors have suggested the possibility that tail spikes of this sort were originally more widespread in salamandrines, and lost in various lineages, including in Salamandra (Veith & Steinfartz 2004).
Another remarkable thing about salamandrids is that some species – and I’m thinking here of the Pleurodeles species (variously called sharp-ribbed newts or ribbed salamanders [adjacent photo by Peter Halasz]) – can (slightly) project the tips of their ribs out through their skin as a defensive tactic. Think about that for a minute. It’s badass. Neoteny occurs in some populations of some species, and viviparity has been evolved within two lineages.
Needless to say, there is tons more that could be said about the various salamander groups – there are hundreds of fascinating species that could all warrant detailed coverage on their own. But this very lengthy article is just meant to be an introduction to the group as a whole, not a comprehensive look at everything salamandry. I hope you enjoyed it, and there will be more amphibian-based goodness here at Tet Zoo in due time.
For previous Tet Zoo articles on salamanders, see…
Refs – -
Brazil, M. 1997. Mission massive. BBC Wildlife 15 (4), 62-67.
Carroll, R. & Zheng, A. 2012. A neotenic salamander, Jeholotriton paradoxus, from the Daohugou Beds in Inner Mongolia. Zoological Journal of the Linnean Society 164, 659-668.
Estes, R. 1969. The Batrachosauroididae and Scapherpetontidae, Late Cretaceous and Early Cenozoic salamanders. Copeia 1969, 225-234.
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Evans, S. E., Lally, C., Chure, D. C., Elder, A. & Maisano, J. A. 2005. A Late Jurassic salamander (Amphibia: Caudata) from the Morrison Formation of North America. Zoological Journal of the Linnean Society 143, 599-616.
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Gao, K. & Shubin, N. H. 2001. Late Jurassic salamanders from northern China. Nature 410, 574-577.
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Gardner, J. D. 2003. The fossil salamander Proamphiuma cretacea Estes (Caudata; Amphiumidae) and relationships within the Amphiumidae. Journal of Vertebrate Paleontology 23, 769-782.
Gayet, M., Marshall, L. G., Sempere, T., Meunier, F. J., Capetta, H. & Rage, J.-C. 2001. Middle Maastrichtian vertebrates (fishes, amphibians, dinosaurs and other reptiles, mammals) from Pajcha Pata (Bolivia). Biostratigraphic, palaeoecologic and palaeobiogeographic implications. Palaeogeography, Palaeoclimatology, Palaeoecology 169, 39-68.
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Larson, A. & Dimmick, W. W. 1993. Phylogenetic relationships of the salamander families: an analysis of congruence among morphological and molecular characters. Herpetological Monographs 7, 77-93.
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