Tetrapod Zoology

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I’ve been ill, and pressing deadlines for book chapters and other projects have kept me busy. An inability to post stuff on Tet Zoo always frustrates me, as there’s just so much Tet Zoo-relevant stuff to get through. And, on that note: I must have said on many occasions that there are whole tetrapod groups, consisting of hundreds or even thousands of species, that I’ve either never mentioned at all, or have only touched on in passing. I aim to get through as many as I can while the going’s good, but there are just so many constraints and distractions. Anyway…

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Among the many incomplete chunks of text I have lying around is the one on skinks. The recent article about a small number of Emoia skinks inspired me to add a few bits to it, but it’s nowhere near completion. To whet your appetite for the wonder that is the World of Skinks, here’s a short article on one of many, many weird ones: the near-limbless, burrowing worm skinks, snake skinks or isopachys skinks (Isopachys) of Thailand and Myanmar [photos of I. gyldenstolpei above and below by F. Schäfer, from Holtmann (2007). If you have additional photos I can use, please contact me!].

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These slim-bodied, burrowing skinks have been known to science since 1914 when Georges Boulenger described I. anguinoides from Thailand. Boulenger didn’t regard this animal as generically distinct, but included it in the enormous genus Lygosoma (during much of the 19th and 20th century, Lygosoma served as the ultimate taxonomic wastebasket among skinks, containing species that have since been split into at least 16 separate genera… and with much more work left to do). A second Thai species, I. gyldenstolpei, was named in 1916 by Swedish zoologist Axel Johan Einar Lönnberg, and this is where the new generic name Isopachys came into being (could Lönnberg have named it for Nils Philip Gyldenstolpe (1734-1810), Marshal of the Royal Court of Sweden and member of the Swedish Academy? UPDATE: nope! See comments).

A third species, now I. roulei, was first described as Typhloseps roulei by M. F. Angel in 1920. Some confusion followed in the 1930s and beyond when all three of these skinks were placed in the Australian genera Ophioscincus and Rhodona. Heyer (1972) showed, however, that all three shared characters and were similar enough to warrant inclusion in the same genus, so he resurrected Isopachys. This has generally been accepted since, and a fourth species – I. borealis from central Thailand and southern Myanmar – was described by Lang & Böhme (1990).

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It’s said that Isopachys skinks can reach lengths of about 300 mm, but most individuals are much smaller than this. Indeed, the largest known specimen of I. anguinoides is just 75 mm long (Pauwels et al. 2003). They lack external ears, are often striped in yellow and brown, and have blunt, rounded tail-tips (the tail is readily autotomised). They appear limbless from the outside but, as discussed below, still possess remnants of the forelimb skeleton at least. A particularly large retroarticular process* in I. roulei [skull shown here, from Heyer (1972)] and I. gyldenstolpei suggests that these species might be particularly good at opening their jaws and grabbing prey while burrowing through substrate. As is typical for burrowing squamates, their scales are smooth and shiny, and various of the normally separate head scales have fused together to form large shields.

* A prong on the rear part of the lower jaw that projects backwards and provides the anchor point for some of the jaw muscles.

What few records are known indicate that Isopachys skinks prefer loose, sandy soils, though I. borealis has been found in damp soil. Stomach contents from I. borealis show that it had eaten termites and earthworms. Like caecilians, scolecophidian snakes and amphisbaenians, Isopachys species may, if ever present in sufficient numbers, be locally significant predators of those organisms known collectively as soil ecosystem engineers.

Like Emoia, Isopachys is one of lygosomine skinks: a widespread and speciose group (over 600 species… that’s about half of all skinks) that occurs across Asia, Africa and Australasia, and on many island groups in the Indo-Pacific region. Unlike other skinks, lygosomines have an extensive bony secondary palate and fused frontal bones. Within Lygosominae, Honda et al. (2003) found Isopachys to be part of the sphenomorphine clade, and – within it – close to Sphenomorphus. Must avoid urge to expand discussion of lygosomine phylogeny…

No limbs and no limb girdles?

Skinks have lost their limbs (or, alternatively, have evolved limblessness) on numerous separate occasions and are far from unique among lizards in doing so. What makes I. borealis so remarkable is that it was reported to not only lack all of its limbs, but also all of its limb girdles (Lang & Böhme 1990). While limb loss is not uncommon among lizards, loss of the limb girdles definitely is. Excluding snakes and some amphisbaenians, all squamates (even limbless ones) have pectoral girdles. And, excluding snakes alone, all squamates (even limbless ones) have pelvic girdles. I. borealis would thus be truly unique, if its describers were correct. It could perhaps be regarded as the most snake-like of all non-snakes (in terms of pelvic and pectoral reduction, at least), and its morphology could even be used to help bolster the hypothesis that snakes are more closely related to skinks than they are to any other squamates (Conrad 2008).

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Greer (1997) was curious about this alleged lack of limb girdles and obtained x-rays of five I. borealis individuals [x-ray above from Greer (1997). The arrows point to the tiny pectoral and pelvic girdle remnants. In the pectoral region, the body is just 9.2 mm wide]. These showed that the girdles were present after all, albeit highly reduced, and so was a probable remnant of the humerus. This new data makes I. borealis similar to the other Isopachys species: they also have strongly reduced girdles, and I. roulei at least may still retain a strongly reduced humerus.

Well, there we have it – just about everything I know about Isopachys. As I’ve said, it’s far from unique – there are many, many, many limbless and reduced-limbed skinks, some of which are a great deal weirder. One day, my friends… one day. Oh yeah, keep an eye out for the new ish of Scientific American.

For previous Tet Zoo articles on neat squamates see…

And don’t forget the (still unfinished) series on gekkotans…

Refs – –

Conrad, J. L. 2008. Phylogeny and systematics of Squamata (Reptilia) based on morphology. Bulletin of the American Museum of Natural History 310, 1-182.

Greer, A. (1997). Does the Limbless Lygosomine Skink Isopachys borealis Really Lack Pectoral and Pelvic Girdles? Journal of Herpetology, 31 (3) DOI: 10.2307/1565684

Heyer, W. R. 1972. A new limbless skink (Reptilia: Scincidae) from Thailand with comments on the generic status of the limbless skinks of southeast Asia. Fieldiana Zoology 58, 109-129.

Holtmann, T. 2007. Isopachys gyldenstolpei – a mysterious skink from Thailand. Aqualog News 74, 10-11.

Honda, M., Ota, H., Köhler, G., Ineich, I., Chirio, L., Chen, S.-L. & Hikida, T. 2003. Phylogeny of the lizard subfamily Lygosominae (Reptilia: Scincidae), with special reference to the origin of the New World taxa. Genes & Genetic Systems 78, 71-80.

Lang, M. & Böhme, W. 1990. Description and phylogenetic position of a new species of Isopachys from central Thailand and southern Burma (Squamata: Scincidae). Bulletin de l’Institute Royal des Sciences Naturelles de Belgique Biologie 60, 231-240.

Pauwels, O. S. G., David, P., Chimsunchart, C. & Thirakhupt, K. 2003. Reptiles of Phetchaburi Province, western Thailand: a list of species, with natural history notes, and a discussion on the biogeography at the Isthmus of Kra. The Natural History Journal of Chulalongkorn University 3, 23-53.

Comments

  1. #1 C. M. Kosemen
    November 27, 2010

    Limbless skinks are some of my favorite animal groups. Lerista, Aconita and Anomalopus all deserve their own Tet Zoo article (Lerista, maybe more than one.) How many times have limbs been lost in skinks? (let alone lizards?)

  2. #2 Jerzy
    November 27, 2010

    Perfectly camouflaged, these pests can wreak havoc in any shoelace factory!

  3. #3 Sven DiMilo
    November 27, 2010

    Nice vestigials!
    I take it the squamate phylogeny is not well enough established to permit an accurate count of independent limb-losing lineages. But I bet the minimum number is a lot.
    Top of my head: snakes, amphisbaenians, dibamids, anniellids, anguids, pygopodids, skinks…and some of those multiple times.
    a few more

  4. #4 llewelly
    November 27, 2010

    Darren:

    It’s said that Isopachys skinks can reach lengths of about 300 mm, but most individuals are much smaller than this. Indeed, the largest known specimen of I. anguinoides is just 75 mm long (Pauwels et al. 2003).

    Er, if the largest known specimen is 75 mm, where did 300 mm come from? Is there a (different) species in Isopachys for a which 300 mm specimen is known?

  5. #5 heteromeles
    November 27, 2010

    I think this falls into the category of: how many times have “snakes” evolved convergently from lizards? The answer appears to be “lots,” and the fun part is that only the macrostomatans really underwent a true adaptive radiation.

    I’m with the first commenter: I think limbless lizards are cool.

  6. #6 Allen Hazen
    November 27, 2010

    By my rather minimal Greek, “Isopachys” would mean something like “equal thickness”: so the generic name is in honor of the lack of tapering at tail and neck? … Lack of tapering would, I suppose, be adaptive in a burrowing animal: taper and you need a bigger-bore tunnel to fit the same total volume through. Cylindrophis is a burrower too, isn’t it?
    (Or: a little Greek can inspire big biomechanical speculations!)

  7. #7 Tim Morris
    November 28, 2010

    Oh my goodness, that thing is just adorable, it’s overbite jaw makes it look so dejected!

  8. #8 Gunnar
    November 28, 2010

    Yay Squamates

  9. #9 John Scanlon FCD
    November 28, 2010

    Not so much ‘worm-like’ as ‘uropeltid-like'; many limbless skinks are more wormy than that. The skull isn’t highly modified at all (near-complete postorbital bar, postorbital-squamosal contact, long epipterygoid, and there’s nothing really remarkable about the length of that RAP), suggesting a relatively recent origin of fossoriality.

    Skinks would include the majority of origins of (near-)limblessness in squamates, though none of the limb-reduced clades (of which there are quite a few independent ones within the large ‘genus’ Lerista) is as speciose or ecologically diverse as, say the Pygopodidae. Maybe it’s juat not a long-run successful way to be a skink?

    As for non-‘macrostomatan’ snakes (uh, did you know that Macrostomata isn’t really the clade we thought it was a few years ago?) not having any ‘true’ adaptive radiations… OK I’d better not get started on madtsoiids (until a few more have been published)

  10. #11 Allen Hazen
    November 29, 2010

    Philippe–
    Keeping to the Yglesias post itself, and not getting into the comments thereon (which quickly get far from science: more people seem to have opinions about Teddy Roosevelt and politics than do about cladistics)…

    This is a big subject on which a lot of the regular contributors to Tetrapod Zoology discussions have strong opinions. If you are new to the field, maybe an “elementary” discussion will help you orient yourself. So…

    This is old news. Reptiles, in the traditional sense (=lizards, snakes, turtles and crocodilians, + some selection of fossil types), do not form a CLADE: they are not all the descendants of a common ancestor. (Most recent common ancestor of lizards and alligators was also an ancestor of birds.) There are lots of zoologists who, at least in their official public statements, hold that we should only use terms for a “kind” of organism when the organisms of that kind form a clade.

    (I remember Gareth Nelson, who was a paleoichthyologist at the American Museum of Natural History, giving a talk on … this and related matters .., back in the late 1980s: one of his repeated slogans was “There is no such thing as a reptile.”)

    Some of us are a bit sceptical, and would like to argue that there IS sometimes a good reason to group organisms into non-clades, on the basis (e.g.) of general overall similarity. Alligators may be more closely related to birds than to lizards (reckoning “relatedness” in genealogical terms: more closely related = having a more recent common ancestor), but birds have evolved feathers and warmbloodedness, and in these respects alligators haven’t changed as much. So, we might suggest, there might be useful biological generalizations we could make about “reptiles” which would justify continuing to use the concept REPTILE.

    To which the cladists would answer (and Gary Nelson, for example, did answer in the talk I remember) that you are much more likely to be misled than helped by using a concept, REPTILE, based on a few superficial similarities. Alligators have been evolving just as long as birds have since their common ancestor lived roughly 250 million years ago, and have been changing all the time. It isn’t a NECESSARY truth, any more than it is a necessary truth that if you toss a fair coin a hundred times it won’t come up heads every time, but the ODDS are that they have changed about as much from what their common ancestor was like. Their common ancestor had a lot in common with the contemporary common ancestor of lizards (the common ancestor of alligators, birds and lizards probably lived not too many millions of years before the common ancestor of alligators and birds), but alligators have — on the odds — lost about as many “reptile” features as birds have.

    And, in fact, there are LOTS of details of anatomy (e.g.: number of chambers in the heart) where alligators and lizards are different. So, if you were led by the idea that they are both “reptiles” to think that lizard-anatomy is a good guide to alligator-anatomy, you’d go wrong very often. So, the cladists plausibly argue, thinking in terms of the REPTILE concept will often be misleading. Often enough that we would be better off abandoning it.

    Hope that is helpful.

  11. #12 Dartian
    November 29, 2010

    Darren:

    Axel Johann Einar Lönnberg

    I noticed that the English-language Wikipedia article on Lönnberg says ‘Johann’, but according to the Swedish Wikipedia it should actually be ‘Johan’ – which would make sense as that is by far the more common way to spell that name in Swedish; ‘Johann’ is German. (In his publications the man usually only referred to himself as ‘Einar Lönnberg’.)

    could Lönnberg have named it for Nils Philip Gyldenstolpe (1734-1810), Marshal of the Royal Court of Sweden and member of the Swedish Academy?

    Surely he could have, but he didn’t; Lönnberg named the species after his colleague, the zoologist Nils Gyldenstolpe (1886-1961). And I strongly suspect that it was Gyldenstolpe who actually discovered the skink – he was in Thailand in 1911-1912 (Lönnberg, IIRC, was in Africa at the time).

  12. #13 Darren Naish
    November 29, 2010

    Many thanks for comments. As seems widely recognised, yes, limb reduction and limblessness has definitely evolved independently on numerous separate occasions within Squamata; as Sven notes (comment 3), I’m not sure that phylogeny is well reconstructed enough for us to give a precise number across the whole clade, but it’s gonna be something like 30 times or more. It might have evolved three times in Anguidae alone, as for skinks… yikes, I’m not on top of this yet, though some limbless and near-limbless taxa (the acontines Acontias and Typhlosaurus, and Typhlacontias, Melanoseps and Feylinia, for example) do group together in phylogenies. Note that reversal of digit loss has been suggested for Lerista. Remember that there are reduced-limbed cordylids.

    Some select responses. llewely (comment 4) asked…

    if the largest known specimen is 75 mm, where did 300 mm come from? Is there a (different) species in Isopachys for a which 300 mm specimen is known?

    There are four species: the 75 mm max length applies to I. anguinoides while the 300 mm applies to I. gyldenstolpei.

    And John (comment 9) is of course right about Isopachys not being particularly snake-like: I was only emphasising the fact that the claimed absence of limb girdles in I. borealis would be uncannily snake-like. I am compelled to cover Feylinia next – does anyone have any good photos? Photos are always a problem for these ‘obscure herp’ articles.

  13. #14 heteromeles
    November 29, 2010

    Thanks for the comments on the Macrostomatans. I’m not a herpetologist, and I wanted to be efficient rather than checking up. Madtsoiids are neat, though, I agree.

    Re: Cladistics. When I was in grad school, I proposed this thought experiment to demonstrate the limits of cladistic thinking:

    Let’s assume that aliens come down and kidnap an entire classroom of cladistics students somewhere (feel free to nominate targets). They take this sample of humanity to an extremely Earth-like planet around another star, and use them to start another colony of humanity.

    That colony then undergoes allopatric speciation, exactly as one would expect from a small group colonizing a novel environment.

    The thing is, humans on Earth are no longer a unified clade, as there’s another species that is not human that descended from humanity’s common ancestor.

    However, there’s no way for us Earth humans to know this. It may have already happened, repeatedly.

    That’s the problem with strict cladistics. Allopatric speciation happens all the time (especially with the plants I’m more familiar with), and it almost never comes from a convenient edge of the cladogram where the new allo-species (or allo-genus) can be pinched off. Usually the new species appears right in the center of the parental tree.

    Obviously, cladistics deals with this by developing hypotheses based on best available information. However, names and categories really should be useful. If a vagrant population has descended into an orgy of adaptation or adaptive radiation, it shouldn’t negatively affect how we classify the ancestral group, especially when those ancestors are still around.

  14. #15 John Scanlon FCD
    November 29, 2010

    heteromeles, cladists do not (typically) insist that species be monophyletic, because of the kind of example you mention. Do note, however, that a species is (approximately, depending on what definition you are using on the day) the most inclusive group that can regain monophyly over time, whether it’s established by plucking a sample from an existing species, introduction of a new geographic barrier, or hybridization of distant relatives. A ‘genus’ or ‘order’ that is para- or polyphyletic today will never become monophyletic through ordinary sexual reproduction, but a species can.

  15. #16 Adam F
    November 30, 2010

    Ah, but what if I have a genus of skinks, all quite similar to one another. And what if I pluck a few skinks from some unassuming member in the center of this genus, and place them on a distant island. Wait a million years or so, and the island skinks have undergone massive radiation, becoming predators and herbivores, losing their legs several times! A passing naturalist would class the island skinks into several genera, while the stay at home skinks still plod along relatively unchanged.

    So, how do you describe this situation, where a group too diverse to be a genus falls out phylogenetically right in the middle of an otherwise perfectly normal and conservative genus? How do you describe the true relationship AND the true character of the organisms involved?

  16. #17 John Scanlon FCD
    December 1, 2010

    How do you describe…?

    With a map, a timeline, a phylogeny, and published data on morphology, ecology and behaviour.

    The names are just handles.

  17. #18 David Marjanović
    December 2, 2010

    What John Scanlon said.

    As always, be careful not to confuse the science of phylogenetics with nomenclature. Cladistics reconstructs the tree; what labels we tie to the branches of the tree is a completely separate question, and it’s one of convention, not of science.

    Whether species have to be clades depends on your species concept. (Most concepts allow species to be paraphyletic.) Whether phylogenetics is at all relevant to what is and is not a species depends on your species concept.

    Finally, do we really need to refer every organism to a species? The codes of rank-based nomenclature say so (at least implicitly), but under many species concepts there’s no scientific reason for this. Accordingly, the PhyloCode will allow us to use specimens that do not (as we may claim) belong to a named species as specifiers for naming clades, and it will not require that we name a species in the process.

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