ADVERTISEMENT
  About the SA Blog Network













Tetrapod Zoology

Tetrapod Zoology


Amphibians, reptiles, birds and mammals - living and extinct
Tetrapod Zoology Home

Hammer-toothed skink SMASH!

The views expressed are those of the author and are not necessarily those of Scientific American.


Email   PrintPrint



This sequence of photos – taken by my good friend Markus Bühler – shows snail-crushing behaviour in a captive individual of the Australian scincid lizard Hemisphaeriodon (read on) gerrardii, popularly known as the Pink-tongued skink. Unique to the coastal eastern strip of Queensland and New South Wales, it’s a predominantly terrestrial skink of damp sclerophyll forests and rainforests, though it also occurs in drier forests in the northern part of its range (Cogger 2000).

The Pink-tongued skink is large (up to 45 cm in total length), viviparous, and has a relatively broad head. Its tail is prehensile and is used as a climbing aid when the animal clambers about among low branches, but – in view of what seems like an obvious arboreal adaptation – it seems surprising that the animal is otherwise (apparently) mostly ground-dwelling.

Marsupials and skinks and convergent evolution of ‘hammer teeth’

Cogger (2000) said that the Pink-tongued skink feeds on “insects and other small arthropods”; more recently, however, the snail-crushing abilities of the species have become well known thanks in part of the description of the remarkable ‘hammer-toothed’ Miocene marsupial Malleodectes, described in 2011 as a mammalian analogue of these skinks (Arena et al. 2011). Derrick Arena and colleagues drew attention to the general similarity present between the upper jaw dentition of Malleodectes with that of the Pink-tongued skink: a massively enlarged, rounded maxillary tooth, surrounded posteriorly by smaller, rounded molariform teeth and anteriorly by low-crowned, far smaller teeth, forms the primary shell-crushing adaptation of these remarkable hammer-toothed lizards (Hutchinson 1992, Arena et al. 2011).

Palates of (a) Pink-tongued skink and (b) Miocene marsupial Malleodectes to show similar mollusc-crushing teeth. From Arena et al. (2011).

Malleodectes and Pink-tongued skinks both, it seems, specialised to feed on the same prey (terrestrial gastropod molluscs), and both occur in the same habitat, and same approximate place. However, hammer-toothed skinks and hammer-toothed marsupials do not occur together, leading Arena et al. (2011) to propose that the skinks took over the role from the marsupials as climatic conditions became more difficult for the marsupials within or after the Late Miocene. A fascinating hypothesis.

But what sort of skink?

As you might be able to guess from the animal’s general appearance, Hemisphaeriodon is a member of the lygosomine skink radiation, and it’s a close relative of the blue-tongue skinks (Tiliqua) and slender skinks (Cyclodomorphus).

All species in these groups have a reduced phalangeal formula compared to other skinks: they have one or two less phalanges in manual digit IV than is usual, and one phalanx less in pedal digits IV and V than is usual. Some lineages in Tiliqua have gone further, with additional phalangeal reductions being present in manual digits III and V, and in pedal digits II-V. The stumpiest digits of course belong to the amazing Shingleback, Bobtail, Sleepy lizard, Pinecone skink or Stump-tailed skink Tiliqua rugosa (formerly Trachydosaurus rugosus).

After being given its own genus – Hemisphaeriodon – in 1867, the Pink-tongued skink was sunk into Tiliqua in 1950. Wells & Wellington (1983, 1985) then resurrected the name Hemisphaeriodon Peters, 1867 without discussion. While this has been followed by some authors (e.g., Cogger 2000), it hasn’t gone unchallenged.

A skink you know and love: the Blotched blue-tongue (Tiliqua nigrolutea). Photo by JJ Harrison, licensed under Creative Commons Attribution-Share Alike 3.0 Unported license.

The Pink-tongued skink is specifically Cyclodomorphus-like in its pattern of supraocular scalation, in the arrangement of its skull bones, and even in the sort of tongue-flicking behaviour it practises. For these reasons and others, Shea (1990) argued that Hemisphaeriodon should be subsumed into Cyclodomorphus, with the Oak skink C. casuarinae being its closest relative. This is definitely the ‘most popular’ hypothesis of its affinities today. However, compared to other Cyclodomorphus species, the Pink-tongued skink is especially long-bodied (it has 26-34 midbody scale rows, whereas other Cyclodomorphus species have 20-26); it also has a greater number of lamellae beneath each of its digits (14-17 vs 8-14) (Shea & Miller 1995). [Adjacent image of Tiliqua scincoides nigrolutea (see comments) by JJ Harrison].

And so ends another entry in the Tet Zoo archives, and another entry intended to be a picture-of-the-day-type article stating merely “here are some photos of skinks”. I’ve written about some skinks before – see the links below – but, my god, there is still so much to do…

Refs – -

Arena, D. A., Archer, M., Godthelp, H., Hand, S. J. & Hocknull, S. 2011. Hammer-toothed ‘marsupial skinks’ from the Australian Cenozoic. Proceeding of the Royal Society B doi:10.1098/rspb.2011.0486

Cogger, H. G. 2000. Reptiles & Amphibians of Australia (Sixth Edition). New Holland Publishers, Sydney.

Hutchinson, M. 1992 Origins of the Australian scincid lizards: a preliminary report on the skinks of Riversleigh. The Beagle 9, 61-70.

Shea, G. M. 1990. The genera Tiliqua and Cyclodomorphus (Lacertilia: Scincidae). Generic diagnoses and systematic relationships. Memoirs of the Queensland Museum 29, 495-519.

- . & Miller, B. 1995. A taxonomic revision of the Cyclodomorphus branchialis species group (Squamata: Scincidae). Records of the Australian Museum 47, 265-325.

Wells, R. W. & Wellington, C. R. 1983. A synopsis of the Class Reptilia in Australia. Australian Journal of Herpetology 1, 73-129.

- . & Wellington, C. R. 1985. A classification of the Amphibia and Reptilia of Australia. Australian Journal of Herpetology, Suppl. Ser. 1, 1-61.

Darren Naish About the Author: Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at darrennaish.wordpress.com. He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at tetzoo.com! Follow on Twitter @TetZoo.

The views expressed are those of the author and are not necessarily those of Scientific American.





Rights & Permissions

Comments 16 Comments

Add Comment
  1. 1. Gigantala 2:22 pm 11/15/2012

    Ah, Hemisphaeriodon skinks. The poor man’s Sphenodon.

    Link to this
  2. 2. ayates 6:19 pm 11/15/2012

    Just a nerdy point of correction and no consequence. The Tiliqua you feature as T. scincoides is infact T. nigrolutea: note the absence of elongate temporal scales and the colour pattern on its back.

    Link to this
  3. 3. naishd 6:38 pm 11/15/2012

    ayates: thanks, I copied from wikipedia without checking. The very short temporal scales do indeed make it look very different from a T. scincoides (to those who don’t know, T. scincoides is unique among blue-tongue skinks in having those long anterior temporals).

    Darren

    Link to this
  4. 4. Hornemadness 11:51 am 11/16/2012

    They tend to be far more arboreal when they are young. As they grow larger they venture onto the ground more and more. Their life style and prehensile tail are somewhat reminiscent to me of the alligator lizards we have here in the US.
    On a side note I would be interested in finding out how animals that feed on terrestrial gastropods deal with the high parasite loads their prey tend to carry.

    Link to this
  5. 5. naishd 11:57 am 11/16/2012

    Interesting stuff, thanks – I haven’t read about the ontogenetic niche shifting but that fits with data from other skinks. Do you know it from observation or is it in the literature?

    The parasite load question is a good one. I don’t think this has been studied.

    Darren

    Link to this
  6. 6. BrianL 12:23 pm 11/16/2012

    What sort of marsupial is *Malleodectes*? Is it a dasyuromorph, a peramelimorph or something different?

    Link to this
  7. 7. naishd 12:36 pm 11/16/2012

    Good question, Brian – I meant to mention this. Aaand, the answer is – - – we don’t know. Arena et al. (2011) put Malleodectes as Marsupialia incertae sedis. They note that posterior molar reduction is elsewhere seen in thylacoleonids (which are, of course, vombatiform diprotodontians), but this isn’t used to support any idea of affinity. For more on thylacoleonids, there’s the Tet Zoo vombatiform articles.

    Darren

    Link to this
  8. 8. Hornemadness 1:05 pm 11/16/2012

    The change in habit with maturity is something I have read in literature and have witnessed with the pink tongue skinks that I keep. Many different kinds of lizards follow a same pattern (monitors are one that comes to mind) of being arboreal when young and terrestrial when older.
    The parasite thing was just something I was thinking about. After all snails seem to be intermediate hosts for all kinds of parasites.

    Link to this
  9. 9. David Marjanović 7:00 pm 11/17/2012

    is especially long-bodied (it has 26-34 midbody scale rows, whereas other Cyclodomorphus species have 20-26)

    :-) I’m so culture-shocked by lengths being given as numbers of scale rows instead of vertebrae. :-)

    Link to this
  10. 10. naishd 6:33 am 11/18/2012

    Ha ha. It’s easy to find scale-row counts in the extant squamate literature – less easy (or impossible) to find vertebral counts.

    Darren

    Link to this
  11. 11. Sebastian Marquez 1:20 pm 11/18/2012

    Wasn’t that mekosuchine from New Caldeondia (inexpectus?) thought to be a mollusc eater too? The teeth on that look similar to these in any regard? Is it still a plausible hypothesis for the croc?

    Link to this
  12. 12. John Scanlon FCD 1:51 am 11/19/2012

    Darren: “…less easy (or impossible) to find vertebral counts.”

    So you still haven’t got a copy of Greer (1989)! I only refer to my copy a few times a year, but there are certain kinds of information that exist nowhere else. [Maybe somebody should just scan the thing and set it free?] Surrey Beatty’s website just crashed my browser, but I’m sure anyone can find the book with a few keystrokes.

    Re Mekosuchus inexpectatus: the teeth are not nearly as inflated or unequal in size as in the Pink-tongue, but seem comparable to cochleophagous African varanids. Australian Mekosuchus have proper blade-like teeth, so I guess the Riversleigh crocs left the snails to Malleodectes, and maybe Tiliqua pusilla (the whole Tiliqua group is more or less durophagous, gerrardii is just the most extreme in dentition).

    Link to this
  13. 13. John Scanlon FCD 2:00 am 11/19/2012

    Incidentally, Pink-tongues have an odd resemblance to the African water cobra Boulengerina annulata, but I doubt it’s functional mimicry.

    And about Hemisphaeriodon: is there any reason not to believe the results of Gardner et al. 2008, which place gerrardii not only within Cyclodomorphus, but possibly within Tiliqua as well?

    Link to this
  14. 14. John Scanlon FCD 5:38 am 11/19/2012

    For reference, Greer 1989 Ch 5 Table 1 (p171), Tiliqua s.l. presacral vert counts (range, mean, n):
    multifasciata 33-34, 33, 4
    adelaidensis 33-35, 34, 6
    gerrardii 34-36, 34.9, 10
    occipitalis 34-35, 35, 4
    rugosa 35-37, 36, 26
    nigrolutea 36-38, 37/38, 15
    scincoides 37-39, 39, 12
    casuarinae 37-44, 40.6, 32
    maxima 41-42, 42, 5
    branchialis 40-44, 42.1, 67

    Egernia s.l. nearly always have the ancestral 26.

    But I just noticed Darren referred in the post to the ranges of midbody scale rows, which (as in snakes) are counted around the body, not along it, i.e. a measure of (roughly) body diameter and distensibility, not elongation.
    To put it another way, the Pink-tongue is not on a continuum between other morphological extremes of the Tiliqua group, it just sticks out orthogonally.

    Link to this
  15. 15. naishd 5:54 pm 11/20/2012

    John – thanks for correcting errors and adding extra info. It’s true, I still don’t own Greer’s book, it’s not cheap enough. Taxonomy of Cyclodomorphus: aww, nuts – I specifically checked Gardner et al. (2008) while writing this article, since I wanted to see what they did with C. gerrardii. I missed their comment about how the paraphyly of Tiliqua means that Cyclodomorphus should be ‘sunk’.

    As for mid-body scale counts, I’m an idiot.

    Darren

    Link to this
  16. 16. David Marjanović 10:54 pm 11/20/2012

    Thanks for the vert counts! That’s a unit I can deal with :-)

    Link to this

Add a Comment
You must sign in or register as a ScientificAmerican.com member to submit a comment.

More from Scientific American

Scientific American Holiday Sale

Give a Gift &
Get a Gift - Free!

Give a 1 year subscription as low as $14.99

Subscribe Now! >

X

Email this Article

X