Salamanders vs skinks

(writing in progress) 
 
One of the most obvious similarities in the vertebrate world is the superficial similarity between lizards and salamanders.

Although this similarity is familiar, has anyone actually acknowledged how remarkable it is that a lineage of reptiles should be so similar to a completely unrelated lineage of amphibians? The resemblance is more one of convergent evolution than one of shared ancestry.
 
To help to explain what is remarkable, please consider bony fishes and cartilaginous fishes, which are separate classes of fishes. I do not know of any bony fish that would be confused, even at a glance, for any cartilaginous fish, or vice versa. However, skinks (Reptilia: Squamata: Scincidae) and salamanders (Amphibia: Urodela: e.g. Plethodontidae) belong to different classes too. And it would be easy to mistake one for the other at a glance, would it not?
 
Can readers think of any other examples of vertebrates in which the members of one class are similar enough in overall appearance to be confused with the members of another class?

I would not say that is true for e.g. dolphins and ichthyosaurs; they are too different-looking, despite being depicted in the textbook entries on evolutionary convergence.

Nor would I say that one could ever confuse a penguin with a seal or otter. They share some adaptations but remain different enough to be unmistakable as birds and mammals respectively, not so?

Sea snakes may resemble eels in the most superficial of ways but the body plans are quite different. The tail, for example, is long in eels and short in sea snakes, and the larval stages of eels are categorically different from anything seen among reptiles.
 
Since lizards are the more widespread group, it is normal to refer to lizard-like amphibians rather than salamander-like reptiles. But either of these ways of framing the similarity is worth thinking about. This is because of a strange quirk of global biogeography: that salamander-like reptiles are extremely widespread whereas salamanders are largely restricted to the Northern Hemisphere mainlands.
 
In this Post, I would like to begin to explore the incidence of skinks in particular, because to my mind they are the lizards most resembling salamanders. What I think this will show is that salamanders have by no means excluded skinks from their habitats, indicating that there is no real competition between skinks and salamanders. Therefore, one cannot explain the absence of salamanders from the southern lands and the Antilles as the result of their niches having been usurped by skinks.
 
Let us start in North America, the global headquarters for salamanders. The diversity and abundance of salamanders in the USA makes one’s head spin.

One would think that if skinks have been excluded by salamanders anywhere on Earth, it would be in North America. And yet such is not the case. Not only is there a decent radiation of skinks in the USA, but three species approach or cross the Canadian border.

Two good examples are Plestiodon fasciatus and P. anthracinus. Far from avoiding salamander habitat, Plestiodon anthracinus, found e.g. in western New York State, lives near springs and does not hesitate to take refuge in shallow water, going to the bottom and hiding under stones or debris while holding its breath! This is amphibious behaviour in a skink, and the point is that it is found in salamander habitat, not as a replacement for salamanders in some area not reached by salamanders.
 
I know that skinks reach Argentina. However, I have been unable to find out how far south this family reaches in South America. I have not yet tracked down any skink in Patagonia, but I will keep looking. It seems that skinks cut out at a certain latitude, so that no skinks make it into the coldest parts of South America. If so, this would be understandable, given that only a few skinks make it as far as Canada in North America. So in its own way South America again shows that there’s no tendency for the niches of salamanders to be usurped by skinks. This is because, in the case of Patagonia, neither salamanders nor skinks occur.
 
In New Zealand, again there seems to be a pattern in which skinks are mainly restricted to the North Island. There are plenty of species of skinks in northern New Zealand, which superficially resemble salamanders (which are of course absent from New Zealand).

Although geckos in New Zealand extend surprisingly far south, skinks do not seem to do likewise. This leaves the ‘salamander niches’ of the South Island empty, with neither salamanders nor salamander-like lizards, with the possible exception of certain aberrant geckos restricted to New Zealand - which are a whole topic of their own.
 
Salamanders and skinks have a superficial resemblance which breaks down on closer scrutiny.

I suspect that both lineages have similar diets. The fact that both lineages can autotomise and regenerate their tails is impressive, and both groups show some climbing ability and some affinity with water as a refuge. Both prefer cool conditions, the skinks being relatively cold-tolerant for lizards.

However, salamanders are more cold-tolerant than any skink (some salamanders being able to move slowly under freezing conditions, and most salamanders breeding in early spring whereas skinks presumably breed in summer), and the lesser aerobic capacity of salamanders than of lizards is indicated by the fact that most terrestrial species of salamanders in North America lack lungs completely.

There are also reproductive differences despite the internal fertilisation shared by the two lineages. Skinks copulate whereas salamanders go to remarkable lengths not to copulate, the female instead self-inseminating by picking up a spermatophore, deposited by the male during courtship, with her cloaca.

Of course, salamanders have larvae whereas skinks do not. Most salamanders lay eggs, while skinks at high latitudes give birth to active offspring. Salamanders tend to have toxic skins, whereas I know of no skink that has a toxic skin. Skinks, particularly the North American ones, go in for bright blue tails at the juvenile stage, distracting potential predators from the head. This is not a tactic familiar in salamanders.

Salamanders seem to live longer than skinks despite being the more fecund in terms of clutch/litter sizes.
 
Where do these findings leave us?
 
The picture emerging is that, despite their superficial similarities, salamanders and skinks do not seem to affect each other biogeographically. There is no evidence that the plethora of salamanders in North America has usurped the niches of skinks there. Although skinks occur in New Zealand, their presence there does little to explain the absence of salamanders. Skinks depend more than salamanders on warmth, as reflected by the greater latitudinal penetration of North America by salamanders than by skinks.
 
The bottom line for now is that the similarity between salamanders and skinks seem to have no explanatory power for the puzzle of northern and mainland restriction in the distribution of salamanders. The similarities between the two groups, while remarkable in their own way, seem to boil down to a single body shape being applied to two different ecological roles.

(writing in progress)

Publicado el 21 de junio de 2022 por milewski