One of the greatest fallacies held about evolutionary theory is that fossils are essential in demonstrating the existence of change (don’t believe me? Look at ‘creation science’ books like Duane Gish’s Evolution: the Challenge of the Fossil Record and Evolution: the Fossils Say No!). Of course, fossils do indeed show how characters were accrued and modified over time, and it’s that ‘time’ aspect of the data that they shed crucial information on. But we most certainly do not need fossils to demonstrate the fact of evolution, as we are surrounded by evolutionary intermediates right here in the modern world. In fact, if we didn’t have any fossils at all we would still conclude – from the living organisms that surround us – that evolution happens…
I was hoping, back in 2006 when writing for Tet Zoo ver 1, to cover the full diversity of the agamid lizards, or agamas. Agamids belong to the group of iguanian lizards called acrodonts, and most of the world’s 400-odd agamid species belong to the Australo-Papuan amphibolurine clade, to the southern Asian draconine clade, or to the Afro-Asian agamine clade. For further introductory stuff on agamids please visit the ver 1 post here. In that ver 1 post, I spent some time looking at the toad-heads or toad-headed agamas Phrynocephalus, a mostly Asian, mostly desert-dwelling group of short-headed agamine agamas [P. arabicus shown below].
It’s reasonably well known, and well established, that the closest relatives of agamids are the chameleons, and traditionally both groups have been regarded as separate, closely related ‘families’. However, some workers have found evidence indicating that agamids are paraphyletic with respect to chameleons (Estes et al. 1988): in other words, that chameleons are actually a specialised agamid sub-group. This is not, however, the currently accepted ‘consensus’ view. In view of this it’s of special interest that some characters generally thought unique to chameleons are seen in agamids, and specifically in toad-heads.
Schwenk & Bell (1988) showed that one of the toad-heads, Phrynocephalus helioscopus, practises an extreme form of tongue protrusion highly similar to that of chameleons [image at top, from Schwenk & Bell (1988), shows feeding P. helioscopus compared to the chameleon Chamaeleo zeylanicus]. P. helioscopus is a small Chinese species restricted to the north-west Xinjiang Uygur Autonomous Region. Individuals were observed and filmed extending their tongues to procure insect prey in what was, surprisingly, an essentially chameleon-like manner: the dorsal, glandular part of the tongue was extended and formed a large ‘pad’ wrapped around the tongue’s muscular stem, and this was then shot forward in a fraction of a second. Sure, no toad-head has a tongue that can be extended for a length equivalent to anything like its own body length (as can the tongues of chameleons), but Schwenk & Bell (1988) argued that the tongue morphology and feeding behaviour of P. helioscopus is homologous with that of chameleons. That’s neat, as chameleons have always been thought to have a totally unique tongue.
If what P. helioscopus does is homologous with what chameleons do (and not just convergent with it), then there are two interpretations of this evidence. One is that this sort of tongue protrusion and prey apprehension is primitive for the agamid-chameleon clade: in other words, that tongue protrusion of the P. helioscopus type was present in the common ancestor of both agamids and chameleons. If this is true, then what P. helioscopus does with its tongue should be typical for agamids (unless there are species that have secondarily reverted to the typical form of prey apprehension used by lizards). To test this idea we need more information on the tongue anatomy and feeding behaviour of other agamids.
The second interpretation is that P. helioscopus is particularly closely related to chameleons, and that the form of tongue protrusion it shares with chameleons is a late-evolved novelty restricted to an agama subgroup that includes chameleons. For this to be correct, agamas (and, perhaps, toad-heads) have to be paraphyletic with respect to chameleons. As noted above, the idea that chameleons are a specialised agama sub-group has been proposed on occasion. In their mtDNA-based phylogenetic study of the Chinese toad-heads, Pang et al. (2003) found P. helioscopus to be the most recently evolved member of a small clade that was itself part of the oviparous ‘Clade B’ toad-head clade. Could chameleons actually be members of the toad-head ‘Clade B’? Given the large number of unique morphological characters that differentiate toad-heads from other agamas (Arnold 1999), it is actually highly unlikely that chameleons (which lack these characters) fit in there.
So if chameleons aren’t modified toad-heads, the (apparently homologous) form of tongue protrusion shared by some toad-heads and chameleons must, presumably, be more widely distributed within agamids: again, we need more information on the tongue anatomy and feeding behaviour of agamids to test this further. Has anyone studied tongue protrusion in agamids to see how widespread this behaviour is? While writing this article I learnt about Smith’s (2005) study of agamid tongue morphology and function (I wrote the article you’re reading now in 2006 and, here in 2008, am updating and modifying it). In agreement with the data on P. helioscopus, Smith (2005) showed that the mechanism of tongue protrusion present in chameleons is not unique but should be considered an exaggerated version of the style of tongue protrusion present in agamids [adjacent image shows Mary Blanchard with Parson's chameleon Calumma parsonii].
If current phylogenies positing a sister-group relationship between agamids and chameleons are correct, then the common ancestor of the two groups exhibited an unusual protrusible tongue. What would be really neat now is if we could show how the remarkable arboreal features of chameleons evolved in concert with their increasingly modified tongue apparatus – but for this of course we do need to look at the fossils for our data. Alas, I don’t have time now to embark on a discussion of chameleon evolutionary history, but I must do so at some time.
Coming soon: squirrels of the world unite, a good year for phorusrhacids, and the history of tree-climbing dinosaurs!
Refs – -
Arnold, E. N. 1999. Phylogenetic relationships of Toad-headed lizards (Phrynocephalus, Agamidae) based on morphology. Bulletin of British Museum of Natural History (Zoology) 65, 1-13.
Estes, R., de Queiroz, K. & Gauthier, J. 1988. Phylogenetic relationships within Squamata. In Estes, R. & Pregill, G. (eds). The Phylogenetic Relationships of the Lizard Families. Stanford University Press, Palo Alto, pp. 119-281.
Pang, J., Wang, Y., Zhong, Y., Hoelzel, A. R., Papenfuss, T. J., Zeng, X., Ananjeva, N. B. & Zhang, Y.-p. 2003. A phylogeny of Chinese species in the genus Phrynocephalus (Agamidae) inferred from mitochondrial DNA sequences. Molecular Phylogenetics and Evolution 27, 398-409.
Schwenk, K. & Bell, D. A. 1988. A cryptic intermediate in the evolution of chameleon tongue projection. Experientia 44, 697-700.
Smith, K. K. 2005. Form and function of the tongue in agamid lizards with comments on its phylogenetic significance. Journal of Morphology 196, 157-171.