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. 2015 Sep 18;15:184. doi: 10.1186/s12862-015-0464-4

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© Diaz and Trainor. 2015

Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

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Fig. 17 — Evolutionary trends in chameleon autopodial morphology. Outgroup comparison was conducted with the extant Rhynchocephalian Sphenodon punctatus (a) and 2 outgroups within Squamata (Teiidae [Aspidoscelis; (b)], Agamidae [Pogona; (c)]). Fusion of proximal tarsal elements arose during amniote evolution by formation of the Astragalus (tibiale + intermedium) and the Calcaneum (fibulare) (see [87] for review). Aspidoscelis (b) and Pogona (c) both show an increase in fusion of mesopodial elements relative to Sphenodon (a). The small and early diverged chameleons (Brookesia (d), Palleon) show the distal clefting of the autopodia between the forelimb digits III and IV and hindlimb II and III characteristic of chameleons. These two genera also present the greatest modified proximal autopodial skeleton within chameleons. The larger Rieppeleon and Rhampholeon (with shorter tails; (e, f)) presents a sesamoid in the fibularis brevis tendon of the heel which is maintained in all higher chameleons. Bradypodion (g), Chameleo (h-j), Furcifer (k, l) and Trioceros (m, n) (‘true chameleons’) sampled in this paper showed an ascending phylogenetic trend toward increased body size and an associated increase in skeletal elements present in the mesopodium, leading to a wrist with increased flexion capability as well as being the first amniote group to form an independently ossified tibiale and intermedium which is only known to be present in amphibians. Thus, in early chameleon lineages there appears to be a skeletal bottleneck relative to outgroup squamates with a subsequent phylogenetic trend of ‘re-evolving’ mesopodial elements present in outgroup taxa. The red gradient shows a trend toward reduction of elements from outgroup taxa to chameleons while the red gradient shows a relative increase in mesopodial skeletal elements in advanced chameleon genera studied. Genera in “<Genus>” are currently recognized taxa but were not examined in this study. Letters within figure correspond to the species with the same letters in Additional file 1: Figure S1