The incredible, enigmatic, fantastical distribution of iguanas, and the meaning of allopatry among high-level squamate clades

Iguanas and their relatives comprise a group, Pleurodonta, that includes over 1 000 species. Nearly all are restricted to the Americas, but there are also species in Fiji and in Madagascar. Recent studies have discussed various dispersal theories for iguanas, but they have not considered a simple vicariance model. This proposes that a global ancestor differentiated, in situ, into Pleurodonta (mainly in the Americas) and their sister, Acrodonta (mainly in Africa, Asia, and Australia). The distribution of iguanas is often regarded as anomalous and puzzling.But its components, including trans-tropical Pacific connections, are quite normal and are repeated in different, unrelated groups of plants and animals. The pattern in iguanas is the result of a phylogenetic break in the south-west Pacific in a global ancestor, resulting in Pleurodonta and Acrodonta. This was followed by differentiation of their families and by large-scale extinction in the central Pacific. The extinction was caused by the cooling and subsidence of the seafloor by thousands of meters, during which large numbers of high, forested islands were reduced to low atolls and submerged seamounts.