The Evolution of the Subdigital Pad of Anolis 2. Comparisons among the Iguanid Genera Related to the Anolines and a View from outside the Radiation

Abstract

The gross and fine structure of the subdigital scales are examined in the iguanid genera thought to most closely resemble the ancestors of anolines: the basal iguanids (Morunasaurus, Hoplocercus and Enyalioides), Polychrus and the pre-anoline genera (Diplolaemus, Leiosaurus, Aperopristis and Pristodactylus). The basal iguanid genera share a number of features not found in the anolines: a large, asym- metrical keel that creates pre- and postaxial surfaces of the toe, and a bare or mi- croarchitecture over the scale surface. The honeycomb microarchitecture may be primitive for ig- uanids. The pre-anoline genera exhibit a number of clear similarities with the anolines, including 1) wide subdigital scales with little intrinsic contour and 2) a spinate microarchitecture. Based on stalk diameter and density, the spines of pre-anolines are homologous and appear to be homologous with those of the anoline genera, Anolis, Chamaeleolis and Phenacosaurus. In a morphological series consisting of Diplolaemus, Pristodactylus achalensis and P. torquatus, 1) lamellar-like scales appear under the second phalanx and the proximal portion of the first phalanx (the location of the pad in most anolines), and 2) variation in fine structure models the spinule, spine, spike, prong, and perhaps even the seta-prong portion of the anoline morphotypic series in fine structure. The spine-seta morphotypic series may be an evolutionary series. Pristodactylus torquatus is the most arboreal form examined and exhibits the closest approximation to the anoline pad, but it is not clear that the initial steps in pad evolution, for example, the origin of a spinulate fine structure, represent arboreal adaptation.