An interpretation of the frog (anuran) fauna of Tasmania, w.r.t. evolutionary convergence

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INTRODUCTION

The frog (anuran) fauna of the southernmost island of Australia, namely Tasmania (https://en.wikipedia.org/wiki/Tasmania), is interest for two reasons, in terms of evolution and biogeography.
 
Firstly, the families Bufonidae (https://www.inaturalist.org/observations?subview=map&taxon_id=21359&view=species) and Ranidae (https://www.inaturalist.org/observations?subview=map&taxon_id=25473&view=species) are absent.

These constitute the classic toad and the classic frog respectively, and reach extremely far north in the Northern Hemisphere (beyond the Arctic Circle in the case of Rana at least). The southernmost anuran on Earth (https://www.inaturalist.org/taxa/66146-Nannophryne-variegata) belongs to the Bufonidae.

However, these lineages are absent from not only Tasmania, but all of southern Australia.

This raises the obvious question: which frogs, if any, take the places of Bufonidae and Ranidae in Tasmania?
 
Secondly, at similar high latitudes in the Northern Hemisphere, there is a great diversity and abundance of salamanders (Order Urodela, https://www.inaturalist.org/observations?subview=map&taxon_id=26718&view=species and https://en.wikipedia.org/wiki/Salamander).

Which frogs, if any, take the place of salamanders in Tasmania?
 
Tasmania would seem to be made for Bufonidae, Ranidae, and salamanders, but is a remote island. What has nature done in its abhorrence of this vacuum?
 
I can frame the central biogeographical question of Tasmania, in terms of amphibians, as follows:
 
The high latitudes of Tasmania seem suitable for ‘typical’ toads, frogs, and salamanders. However, these lineages have apparently not reached this isolated landmass. So, has the frog fauna of Tasmania produced forms evolutionarily convergent with Bufonidae, Ranidae, and salamanders?
 
AN OVERALL INTERPRETATION

Based on my survey of the literature, the following are my findings.
 
Firstly, the absence of Ranidae from Tasmania has been compensated by convergent evolution in a tree frog family, Hylidae (https://www.inaturalist.org/observations?place_id=6744&subview=map&taxon_id=23540).

This family occurs also in the Northern Hemisphere. However, it has strayed, evolutionarily, back on to the ground and into the water in Australia, in the absence of Ranidae. Tasmanian Ranoidea raniformis (https://www.inaturalist.org/taxa/517107-Ranoidea-raniformis) is even named after this resemblance.

I do not know if the fecundity of R. raniformis rivals that of Ranidae. However, in overall morphology this seems a satisfactory match.
 
Secondly, in the case of Bufonidae (which are morphologically more extreme than Ranidae, among anurans), the convergence is not close, but merely discernible. This is understandable given that true toads

• tend to be extremely toxic, and
• rely on this defence instead of speed, on landmasses where predation is far more intense than in Tasmania.

However, it is significant that ‘toadlets’ in the Myobatrachidae (https://www.inaturalist.org/observations?subview=map&taxon_id=25222&view=species), a family restricted to Australia and New Guinea, do resemble juveniles of Bufonidae in body shape, toxicity, and terrestriality.

Pseudophryne semimarmorata (https://www.inaturalist.org/taxa/25251-Pseudophryne-semimarmorata) is the species in question. The main difference from Bufonidae is the small body size (which makes sense in Australia with its limited productivity, hence growth-rates).

I suspect that Pseudophryne is far less fecund than Bufonidae as well, even if body sizes are corrected for. Again, the common name of Pseudophryne, viz ‘toadlets’, is suggestive.

As for adult Bufonidae, the closest form in Tasmania is Limnodynastes dumerilii (https://www.inaturalist.org/taxa/22038-Limnodynastes-dumerilii). The resemblance is not enough to earn this frog any name associated with toads.
 
Thirdly, there are obviously no close counterparts for salamanders. However, what Tasmania does have is a fauna of small, fairly short-legged frogs, which may not have any counterpart in the Northern Hemisphere at similar latitudes.

These small, dumpyish frogs are taken for granted by Australian herpetologists with no regard to salamanders. However, could it be that they are, in some sense, functionally comparable with salamanders?
 
To summarise, frogs in Tasmania have tended to fill in for

• salamanders in the form of small dumpy Myobatrachidae,
• toads in the form of two genera of myobatrachids, namely Limnodynastes (in body size) and Pseudophryne (in warty toxicity), and
• particularly, Ranidae, in the form of an aberrant tree frog (Hylidae) plus another (Limnodynastes) that bears no resemblance to a toad.

DETAILED EXAMINATIONS

Obviously convergent with Rana is the treefrog-gone-pondfrog Ranoidea raniformis (Hylidae, https://www.inaturalist.org/taxa/517107-Ranoidea-raniformis).

This species answers the question of convergence with a large Rana in the affirmative, in what seems to be an uncomplicated way. Please note the loss of the expanded discs on the digits, i.e. the reversion of the limbs to a non-arboreal form.
 
http://upload.wikimedia.org/wikipedia/commons/8/8f/Litoria_raniformis.jpg

Next is Hylidae: Litoria burrowsi/burrowsae (https://www.inaturalist.org/taxa/66027-Litoria-burrowsi). This frog is restricted to Tasmania and is particularly associated with BUTTONGRASS MOORLAND. It does seem somewhat convergent with Rana, and is essentially a treefrog gone back to the water. However, it retains expanded discs on the tips of the digits, suggesting that it may climb in the buttongrass.

https://frogs.org.au/frogs/species/Litoria/burrowsae/

Smaller than Litoria burrowsi/burrowsae, and remaining truer to its tree frog origins, is Litoria ewingii (https://www.inaturalist.org/taxa/23668-Litoria-ewingii), which occurs in southeastern Australia on the mainland as well.
 
https://www.reptiles.org.nz/herpetofauna/non-native/litoria-ewingii

In the family Myobatrachidae (https://www.inaturalist.org/observations?subview=map&taxon_id=25222&view=species) of non-arboreal frogs (all lacking expanded tips to the digits), which is restricted to Australia and New Guinea, is the small frog Crinia signifera (https://www.inaturalist.org/taxa/25231-Crinia-signifera), which eats tiny insects. I suspect it to be functionally comparable with certain abundant salamanders. In North America, Plethodon in particular tends likewise to eat ants.
 
http://en.wikipedia.org/wiki/Common_eastern_froglet#mediaviewer/File:Crinia_signifera.jpg
 
Also smallish is Crinia tasmaniensis (https://www.inaturalist.org/taxa/25238-Crinia-tasmaniensis), which is restricted to this island. I take this too as a ‘salamander you have when you have no salamanders.’
 
http://en.wikipedia.org/wiki/Tasmanian_froglet#mediaviewer/File:Crinia_tasmaniensis.JPG
 
Another small myobatrachid frog in Tasmania is Geocrinia laevis (https://www.inaturalist.org/taxa/25248-Geocrinia-laevis).

Genus Geocrinia is similar to genus Crinia, but is restricted to high latitudes, making it a particularly strong candidate for analogy with salamanders. This can be thought of as a ‘moss froglet’, respresented in South Africa by the frog Athroleptis (Arthroleptidae, https://www.inaturalist.org/observations?subview=map&taxon_id=24581&view=species).
 
http://en.wikipedia.org/wiki/Geocrinia_laevis#mediaviewer/File:Southern_Smooth_Froglet_(Geocrinia_laevis)_(8743396751).jpg
 
The frog most resembling a toad in Tasmania is the large myobatrachid, Limnodynastes dumerilii insularis (https://www.inaturalist.org/observations?taxon_id=893959), which burrows during the off-season.

However, as the following photo shows (http://en.wikipedia.org/wiki/Limnodynastes_dumerilii#mediaviewer/File:Lim_dumerilii_Insularis.jpg), the similarity to toads is not enough to call this frog convergent with Bufo.

The main elements of convergence are

• the unwebbed feet and an ability to dig with the hind feet,
• the stout form and large head, and
• a prominent poison gland, which in the case of Limnodynastes is not parotoid (as in Bufo) but located on the hind leg.

Limnodynastes is fecund for an Australian frog. However, I doubt that this rivals Bufo.
 
A rather nondescript myobatrachid frog, associated with grassland near water, is Limnodynastes tasmaniensis (https://www.inaturalist.org/taxa/22036-Limnodynastes-tasmaniensis). Like its congener, it is relatively fecund. This species seems to 'fall among stools', not resembling any particular type of frog in the Northern Hemisphere.
 
http://en.wikipedia.org/wiki/Spotted_grass_frog#mediaviewer/File:Limnodynastes_tasmaniensis_orange.jpg
 
Limnodynastes peronii (https://www.inaturalist.org/taxa/22039-Limnodynastes-peronii) is widespread in eastern Australia, but occurs in Tasmania only in the extreme north. This species resembles Rana much more than Bufo. However, it lacks webbed feet.

http://en.wikipedia.org/wiki/Striped_marsh_frog#mediaviewer/File:Limnodynastes_peronii.jpg

So, two families in Tasmania have converged with the missing Rana. However, it is the Hylidae which has produced the stronger convergence, as well as the wider distribution, in Tasmania.
  
A myobatrachid resembling a juvenile toad (Bufo) is Pseudophryne semimarmorata (https://www.inaturalist.org/taxa/25251-Pseudophryne-semimarmorata), which also resembles toads in having toxic skin as indicated by its ventral aposematism. This is the closest convergence with toads to be found in the frog fauna of Tasmania. However, it leaves much to be desired in body size.
 
http://fr.wikipedia.org/wiki/Pseudophryne_semimarmorata#mediaviewer/File:CSIRO_ScienceImage_7816_Southern_Toadlet.jpg

Posted on 15 de julho de 2022, 06:56 AM by milewski