This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
I like iguanian lizards – who doesn’t? Among the enormous number of taxa that you hardly ever hear anything about is the endemic Argentinean taxon Leiosaurus, type species of Leiosauridae. I did a quick check online and was surprised to find that there’s hardly any information out there on these neat little South American lizards, most sources saying that little is known about them. Well, ain't that helpful.
All Leiosaurus species are stout-bodied, insectivorous, terrestrial iguanians, easily identifiable due to their proportionally big, wide, well-muscled heads, a tail that is slightly longer than the body, relatively smooth-looking skin (hence the name: it means ‘smooth lizard’) and a unique dorsal pigmentation pattern that involves large spots or blotch on or on either side of the vertebral midline. These blotches form so-called ‘shark tooth’ or ‘fleur-de-lis’ patterns in some species. They are lizards of arid and semi-arid places.
Four Leiosaurus species are currently recognised (L. bellii, L. caramarcensis, L. jaguaris, and L. paronae). L. jaguaris is the newest, having been named in 2007 (Laspiur et al. 2007a). Its name reflects the superficial similarity between its bold dorsal patterning and that of its namesake, the big cat Panthera onca. Several additional species once included within Leiosaurus seem to be part of a closely related taxon, Diplolaemus. Some phylogenies indicate that Diplolaemus is closer to Pristidactylus than to Leiosaurus (Frost et al. 2001) though these authors recovered some topologies where Leiosaurus was paraphyletic with respect to the Diplolaemus + Pristidactylus clade. Abdala et al. (2009) incorporated a lot more data, however, and supported the monophyly of Leiosaurus. Good support for the monophyly of Leiosaurus comes from its complicated gular musculature (Abdala et al. 2009). The name Aperopristis Peracca, 1897 is available for the L. caramarcensis + L. paronae clade, should anyone want to use it (at the moment, they don’t). Leiosaurus and its relatives are closely related to the enyaliines, a group I’ll ignore for now.
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Incidentally, an alleged fossil species of Leiosaurus (L. marellii Rusconi, 1937), based on vertebrae from the Pleistocene of San Isidro in Argentina, is not a leiosaurid at all but a misidentified member of Amphisbaena (Torres & Montero 1998).
These lizards possess a large sesamoid on the palm of the hand, embedded in the tendinous sheet associated with the flexor tendons (funny how you don’t ever hear much about sesamoids in lizards). In Leiosaurus, the palmar sesamoid is especially large, perhaps because the terrestrial, arid-land lifestyle of these lizards has encouraged the evolution of features that enhance the rigidity of the hand. Climbing leiosaurid species have smaller palmar sesamoids, presumably because this permits an enhanced ability to flex the fingers (Abdala et al. 2009).
Another neat (and poorly known) thing about these lizards is that they vocalise, making distinct warning noises and combat noises. The warning vocalisations are associated with postural behaviour: an animal jerks backwards and erects its tail while making these sounds (Laspiur et al. 2007b). Leiosaurus is not unique among iguanians in making noises of this sort, since Pristidactylus species do as well. Vocalisations have not been much reported across Iguania but there are indications that they’re more widespread than currently thought.
The whole ‘Iguanidae Thing’, again
Where and how these lizards are classified varies depending on which source you consult. Tradition has it that pleurodont iguanians (that is, those iguanians that are outside of Acrodonta, the acrodont-toothed clade that includes chameleons and agamas) are included within a super-inclusive Iguanidae, carved up into numerous ‘subfamilies’. Leiosaurus used to be included in Polychrotinae, a group imagined (prior to the late 1980s) to include an array of arboreal, chameleon-like and anole-like iguanians – sometimes termed para-anoles – as well as the terrestrial, shorter-bodied, cryptically coloured leiosaurines. The Leiosaurus species were sometimes called either ‘leiosaur polychrotids’ back in those days, or ‘pristidactylines’ due to the (probably correct) idea that Leiosaurus forms a clade with Pristidactylus and also Diplolaemus.
Nowadays, Leiosaurus and a few close relatives are generally classified within a group variously termed Leiosaurinae or Leiosauridae and taken to include both pristidactylines and enyaliines (Frost et al. 2001, Abdala et al. 2009, Pyron et al. 2013). Some authors argue that we should emphasise monophyly and stick with a conservative taxonomy where possible: according to this view, it’s wise to maintain a super-inclusive Iguanidae that contains a Leiosaurinae and numerous other subfamily-level divisions. Other authors argue that maintaining a super-inclusive Iguanidae is misleading, since it downplays the diversity and disparity of these lizards (some authors have also wanted to avoid using ‘Iguanidae’ in the grand, inclusive sense since there were concerns that it referred to a paraphyletic grade. That no longer seems to be the case though, since modern phylogenies find pleurodont iguanians to be a clade).
I think that the decision to avoid the old, super-inclusive version of ‘Iguanidae’ is desirable, since including all of these many lineages within one ‘family’ is inappropriate when we compare their diversity with that of mammals and birds (where just about every distinct taxon is considered special enough to get its own higher-level taxonomic entity). Of course, invertebrate workers usually make witty retorts at this stage, saying that all of tetrapod diversity could hypothetically be included within a single nematode genus or whatever. Well, screw that: we’re not talking about making tetrapod taxonomy consistent with that of midges or snails, but consistent with that of other tetrapods. Some say that this stuff doesn’t matter as long as we know which taxa we’re talking about. I can agree with that, but I also think that it does matter since we’re still lumbered with a wholly anachronistic and inaccurate view of biodiversity whereby people think that living amphibians and non-avian reptiles are substantially less diverse than mammals and birds.
One last thing. Why did I write this article? Because my colleague Oliver Rauhut happened to share some nice photo of Leiosaurus on facebook. Thank you, Oliver. I love writing about obscure squamate taxa, so – dear readers – please keep me in mind if you have good photos that you’re prepared to let me use.
For previous Tet Zoo articles on iguanians, see…
Harduns and toad-heads; a tale of arenicoly and over-looked convergence
The Squamozoic actually happened (kind of): giant herbivorous lizards in the Paleogene
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Frost, D. R., Etheridge, R., Janies, D. & Titus, T. A. 2001. Total evidence, sequence alignment, evolution of polychrotid lizards, and a reclassification of the Iguania (Squamata: Iguania). AmericanMuseum Novitates 3343, 1-38.
Laspiur, A., Carlos Acosta, J. & Abdala, C. S. 2007a. A new species of Leiosaurus (Iguania: Leiosauridae) from central-western Argentina. Zootaxa 1470, 47-57.
Torres, S. E. & Montero, R. 1998. Leiosaurus marellii Rusconi 1937, is a South American amphisbaenid. Journal of Herpetology 32, 602-604.