This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Skinks (properly Scincidae... though read on) are one of the most successful of squamate groups, accounting for approximately 1500 species – in other words, for about 25% of all lizards. Skinks occur on all continents (except Antarctica) as well as on numerous island groups. Extant species range from less than 10 cm in total length (as in the North American Little brown skink Scincella lateralis [adjacent photo by Dawson]) to over 70 cm in the case of the arboreal Solomon Islands monkey-tailed skink Corucia zebrata. They exhibit a range of body shapes, proportions, and head and tooth morphologies.
By the way, I thought long and hard about the title for this article.
An ‘average’ member of the group (example: the North American Five-lined skink Plestiodon fasciatus) is a mid-sized (c 12-20 cm long), strong-limbed, brownish lizard (boldly marked as a juvenile), covered in smooth, shiny scales. It's predominantly a terrestrial predator of arthropods (though it can also tackle small vertebrates, including frogs, smaller lizards and rodent nestlings); it also climbs readily [adjacent photo by Patrick Coin]. Viviparity is widely present in skinks as is copious interesting social behaviour. Pair-bonding and monogamy (in cases extending for over 20 years), extended care of juveniles, egg-brooding, colonial nesting and co-operative burrow building and colonial burrow-dwelling are all known for skinks. There are burrowers, climbers, clamberers, leaf-litter dwellers, grassland specialists, semi-aquatic skinks (including Parvoscincus and Amphiglossus), and one that forages in tidal pools and remains submerged in seawater for minutes at a time (this being Emoia atrocostata; Brown 1991).
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This is also the group that contains the heavy-bodied, broad-headed and proportionally short-tailed blue-tongued skinks and shinglebacks (Tiliqua), and the amazing spiny, armour-plated crocodile skinks (Tribolonotus) [adjacent Tiliqua photos by Benjamint444 and Jarrod]. Muscular, prehensile tails are present in some scansorial and arboreal taxa and some Australian taxa (like some Egernia species) have a spine-covered tail used in self-defence. Numerous skink lineages have reduced or even lost their limbs as they have adapted to fossorial life, their bodies being longer and their limbs and limb girdles smaller than those of close relatives. There are even genera where some species are fully pentadactyl, other species possess three, or two digits, or one digit, and yet other species in the genus lack digits or limbs entirely (examples: the Eurasian Chalcides and the Australian Lerista). [Chalcides striatus and C. ocellatus photos below by Benny Trapp.]
As with any enormous, complex group, numerous different ideas have been put forward as to how the different skink species might be classified and placed in a phylogeny. The more work that gets done, the more obvious it becomes that various of the groups that have been proposed historically (including ‘subfamilies’ and ‘genera’) are almost certainly not monophyletic. This goes for the familiar multi-species genera Emoia, Mabuya, Eumeces, Lygosoma and others: those who follow squamate taxonomy will know that various chunks of these speciose giant genera have recently been split off and now go by different names. Despite these problems, what might be a general consensus on skink phylogeny has emerged in recent years, as we'll see below. [Mabuya photo below by Mark Stevens.]
Fossils show us that skinks as a whole are a fairly old group of lizards, the oldest specimens attributed to the group dating to the Lower Cretaceous. Alas, the vast majority of early fossil representatives of the group are jaw fragments alone (Estes 1983). These are certainly from skink-like lizards (from the major lizard group termed Scincomorpha, that is), but they might not all be from skinks proper, and some have been suggested to actually represent other groups (like the armadillo lizards or cordylids, a scincomorph group that also has a possible Cretaceous fossil record). Definite fossil members of modern groups – like blue-tongued skinks – are present in the Miocene.
Let’s kill Scincidae!
Traditionally, Scincidae has been divided into four 'subfamilies': Acontinae, Feylininae, Lygosominae, and Scincinae (Greer 1970), though the last of those has virtually always served as a wastebasket group for taxa that lack the peculiarities of the others. Some efforts have also been made to break up ‘Scincinae’: Griffith et al. (2000), for example, named Eumecinae for the part of 'Scincinae' that contains Eumeces and close kin.
But, more recently, some authors have proposed that we should do the same thing to the massive, unwieldy version of traditional Scincidae that’s already been done to Iguanidae and Gekkonidae – that is, recognise its major constituent clades as ‘families’ in their own right and elevate the concept of ‘skinks’ to what used to be termed an ‘infraorder’. Within a newly elevated Scincomorpha*, Hedges & Conn (2012) proposed the recognition of seven ‘family-level’ skink groups: Acontidae, Egerniidae, Eugongylidae, Lygosomidae, Mabuyidae, Scincidae, and Sphenomorphidae. Mabuyidae itself got three new ‘subfamilies’ (Chioniniinae, Dasiinae and Trachylepidinae). The idea that this new taxonomy was at all necessary has been contested (Pyron et al. 2013), and of course this view has been counter-contested (Hedges 2014). That last study saw the addition of two more ‘families’ (Ateuchosauridae for the very poorly known, east Asian short-limbed Ateuchosaurus skinks, and Ristellidae for the Indian cat skinks and Sri Lankan lanka skinks).
* Hedges (2014) used Scincomorpha as an ‘elevated’ version of Scincidae – that is, as a name that can be used exclusively for the clade that contains the skink lineages alone. Alas, he did not credit the fact that Scincomorpha is already in widespread use for the squamate clade that (in some phylogenies) includes skinks, teiioids, lacertids and cordyliforms. We thus have the name Scincomorpha being used in two very different contexts. Not good. Scincomorpha was originally named by Charles Camp in 1923.
This idea of an 'exploded Scincidae' is all very similar to the debate that surrounds those ‘family-level’ names created for the groups traditionally included within Iguanidae (see links below for Tet Zoo coverage of this issue). Do we really need a new taxonomy when we all agree that the lineages concerned form a clade? (that is, that Scincidae of tradition is monophyletic). Well: maybe, maybe not. Does creating new ‘family-level’ names really help as goes communication and research focus, and does it allow us to better appreciate the diversity, enormity, and evolutionary history of the group concerned? Again: maybe, maybe not. My bias is to think that splitting up these giant groups is a useful and worthy thing, in part because it helps emphasise the disparity and history of the groups concerned. But I don't expect everyone to agree with that perspective.
And to non-tetrapod workers who point to the longevity and taxonomy of their favourite copepod 'family' or midge 'family' or whatever... look, this isn’t about standardising Tetrapoda with the rest of Biota. We totally get the fact that your taxa are useless sprawling monsters with no anatomical homogeneity.
In the rest of this article and the following ones that make up the series, I’ve mentioned the new taxonomy where appropriate but haven’t adopted it wholesale.
Acontines and feylinines: weird and limbless
Acontines (or acontids) are named for the lance skinks (Acontias), a group of limbless, sand-swimming, viviparous skinks of sub-Saharan Africa, the biggest of which (A. plumbeus) reaches 49 cm. There are species that inhabit grasslands, coastal places and mountainous regions. Acontines have sometimes been classified according to the nature of their eyelids. Those with normal, mobile eyelids are termed Acontias, those with immovable, semi-transparent eyelids are termed Acontophiops, and those with the full transparent, lidless ‘spectacle’ condition are termed Typhlosaurus. Several small-bodied species previously included in Acontias were given a new name – Microacontias – in 2006. Lamb et al. (2010) analysed acontine phylogeny, however, and concluded that only Typhlosaurus and Acontias should be recognised, some of their species having previously been allocated to the wrong genus. Recent phylogenies find acontines to be the sister-group to the rest of the skink clade (Whiting et al. 2003, Skinner et al. 2011, Brandley et al. 2012, Pyron et al. 2013, Hedges 2014). [Acontias image below by Onagro.]
Feylinines are named for Feylinia, a limbless, viviparous, tropical African skink about which little is known (there are six species, one of which is endemic to Príncipe Island). Feylinia is said to be a termite predator and to reach lengths of 35 cm. One of the most memorable facts about it is that “There is a local superstition that Feylinia can enter the human body whenever it desires and, when it leaves again, the person dies” (Whitfield 1983). I’m not quite sure how it supposedly enters the body, but I can guess. The idea that Feylininae might be a ‘subfamily’ of course assumes that the lineage falls outside the other recognised skink groups. This view has been favoured by some authors simply because Feylinia is so weird (it has fused nasal bones, lacks a jugal, has less than eight cervical vertebrae... these are all peculiar features for a skink). It’s even been suggested that it might have some sort of affinity with gekkotans (Rieppel 1981).
But these ideas aren’t supported by molecular phylogenies, since Feylinia is consistently found to be deeply nested within Scincinae. A position close to Melanoseps and Typhlacontias within a sub-Saharan clade dominated by Scelotes skinks and the mostly Madagascan Amphiglossus group has been recovered in some studies (Whiting et al. 2003, Pyron et al. 2013). Brandley et al. (2005) found it to be part of a clade that also included the mostly Eurasian Chalcides and Sphenops and kin, and the Amphiglossus group. It’s part of Scincidae sensu stricto in the ‘new’ taxonomy (Hedges 2014).
Oh, Amphiglossus of tradition is another of those non-monophyletic groups, and it’s now been split up. Those species that are less elongate, and have higher premaxillary tooth counts than Amphiglossus proper are grouped together in Madascincus (Schmitz et al. 2005, Pyron et al. 2013). Amphiglossus proper includes species that vary substantially in body length (there are species with just 30 presacral vertebrae versus others with 59) and habitat choice: there are rainforest species that hide in leaf litter, montane species that shelter beneath rocks, and a few that spend time sitting in forest streams. Glaw & Vences (2007) mention a specimen of the robust-headed A. reticulatus that was found, half-submerged, in a stream at night. “Upon capture it inflicted heavy bites that caused a numb feeling of the hand of the collector for at least one hour” (p. 336). Some are nocturnal or crepuscular on occasion.
The weird, pink Madagascan skink Sirenoscincus – named in 2003 for the species S. yamagishi and notable for its retention of flipper-like forelimbs and lack of hindlimbs – belongs somewhere within the Amphiglossus group. A second Sirenoscincus species that lacks pigmentation and has reduced eyes covered by scales – S. mobydick – was named in 2012 (Miralles et al. 2012). Yes, Sirenoscincus mobydick. It gets better, since another recent discovery has shown that weirdass skinks with small forelimbs and no hindlimbs evolved elsewhere, in another skink lineage...
And we’ll pick that story up later. My original plan was to write one brief article on skinks. Buuut it ended up as a monster that I’ve had to split into several chunks. So -- more skinks coming real soon!
Tet Zoo now features some fairly reasonable coverage of squamate diversity, but there is still so much to do.
Dibamids and amphisbaenians
Amphisbaenians and the origins of mammals (April 1st article!)
Gekkotans
Gekkota part II: loud voices, hard eggshells and giant calcium-filled neck pouches
Squirting sticky fluid, having a sensitive knob, etc. (gekkotans part III)
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)
Lacertids
The New Forest Reptile Centre (on Zootoca and Lacerta)
Iguanians
Harduns and toad-heads; a tale of arenicoly and over-looked convergence
The Squamozoic actually happened (kind of): giant herbivorous lizards in the Paleogene
Australia, land of dragons (by which I mean: agamids) (part I)
Scincomorphs (conventional sense)
Anguimorphs
Pompey and Steepo, the world-record-holding champion slow-worms
Dinosaurs come out to play (so do turtles, and crocodilians, and Komodo dragons)
What I saw at the zoo yesterday... (more brief comments on Komodo dragons)
Perentie tries to swallow echidna. Echidna too spiky, Perentie gets horribly injured. Dies.
“Lean, green and rarely seen”: enthralling prasinoid tree monitors
Snakes
"What was that cute little Mexican snake?", and other musings...
Snake 195 mm long eats centipede 140 mm long. Centipede too big. Snake dies.
Monster pythons of the Everglades: Inside Nature's Giants series 2, part II
The more you know about colubrid snakes, the better a person you are
Love for Mastigodryas, Tomodon, Sordellina and all their buddies: you know it’s right
Refs - -
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Brown, W. C. 1991. Lizards of the genus Emoia (Scincidae) with observations on their evolution and biogeography. Memoirs of the California Academy of Sciences 15, 1-94.
Estes, R. 1983. Sauria Terrestria, Amphisbaenia. Handbuch der Paläoherpetologie, Part 10A. Gustav Fisher Verlag, Stuttgart.
Greer, A. E. 1970. A subfamilial classification of scincid lizards. Bulletin of the Museum of Comparative Zoology 139, 151-183.
Griffith, H., Ngo, A. & Murphy, R.W. 2000. A cladistic evaluation of the cosmopolitan genus Eumeces Wiegmann (Reptilia: Scincidae). Russian Journal of Herpetology 7, 1-16.
Lamb, T., Sayantan, B. & Bauer, A. M. 2010. A phylogenetic reassessment of African fossorial skinks in the subfamily Acontinae (Squamata: Scincidae): evidence for parallelism and polyphyly. Zootaxa 2657, 33-46.
Miralles A., Anjeriniaina M., Hipsley C. A., Müller J., Glaw F. & Vences M. 2012. Variations on a bauplan: description of a new Malagasy “mermaid skink” with flipper-like forelimbs only (Scincidae, Sirenoscincus Sakata & Hikida, 2003). Zoosystema 34, 701-719.
Rieppel, O. 1981. The skull and the jaw adductor musculature in some burrowing scincomorph lizards of the genera Acontias, Typhlosaurus and Feylinia. Journal of Zoology 1954, 493-528.
Schmitz, A., Brandley, M. C., Mausfield, P., Vences, M., Glaw, F., Nussbaum, R. A. & Reeder, T. W. 2005. Opening the black box: phylogenetics and morphological evolution of the Malagasy fossorial lizards of the subfamily “Scincinae”. Molecular Phylogenetics and Evolution 34, 118-133.
Skinner, A., Hugall, A. F. & Hutchinson, M. N. 2011. Lygosomine phylogeny and the origins of Australian scincid lizards. Journal of Biogeography 38, 1044-1058.
Whiting, A. S., Bauer, A. M. & Sites, J. W. 2003. Phylogenetic relationships and limb loss in sub-Saharan African scincine lizards (Squamata: Scincidae). Molecular Phylogenetics and Evolution 29, 582-598.