November 15, 2012 | 16
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).
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.
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 – -
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.