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…
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!].
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).
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).
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…
- Mosasaurs might have used the same microscopic streamlining tricks as sharks and dolphins
- Tongues, venom glands, and the changing face of Goronyosaurus
- Dinosaurs come out to play (so do turtles, and crocodilians, and Komodo dragons)
- Tell me something new about basilisks, puh-lease
- ‘Cryptic intermediates’ and the evolution of chameleons
- The Great Goswell Copse Zootoca
- Of giant plated lizards and rough-necked monitors
- Ermentrude the liolaemine
- Evolutionary intermediates among the girdled lizards
- Hell yes: Komodo dragons!!!
- Amazing social life of the Green iguana
- Arboreal alligator lizards – yes, really
- Pompey and Steepo, the world-record-holding champion slow-worms
- Mystery emo skinks of Tonga!
And don’t forget the (still unfinished) series on gekkotans…
- The Tet Zoo guide to Gekkota, part I
- Gekkota part II: loud voices, hard eggshells and giant calcium-filled neck pouches
- Squirting sticky fluid, having a sensitive knob, etc. (gekkotans part III)
- Lamellae, scansor pads, setae and adhesion… and the secondary loss of all of these things (gekkotans part IV)
- The incredible leaf-tailed geckos (gekkotans part V)
- 300 years of gecko literature, and the ‘Salamandre aquatique’ (gekkotans part VI)
- Whence Uroplatus and… there are how many leaf-tailed gecko species now?? (gekkotans part VII)
- Ptychozoon: the geckos that glide with flaps and fringes (gekkotans part VIII)
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.