The historical biogeography of the lizard clade Iguanidae is complicated. In addition to difficulties within the New World, where most of the more than 900 living species are found, two extant iguanid clades, Brachylophus and Oplurinae, occur well outside it. Moreover, there is a small set of Eocene species in Europe, most notably those placed in the genus Geiseltaliellus. To examine the relevance of Geiseltaliellus to iguanid biogeography, I redescribe several well-preserved specimens (nearly complete skeletons with epidermal scales) from the middle Eocene lake deposits of Messel, Germany. These specimens were previously referred to the type species, G. longicaudus, but comparison with the type material reveals differences that warrant specific distinction. Messel Geiseltaliellus resembles extant Basiliscus in squamation and parietal growth. Phylogenetic analysis of morphological data using Bayesian and parsimony methods suggests the following about the evolution of pleurodont iguanians: (1) Iguanidae is monophyletic, its members united by unique features of the snout; (2) Iguanidae is divided into two major clades, one consisting of Polychrotinae* Corytophaninae, Iguaninae Hoplocercinae and Crotaphytinae (Clade A), the other of Phrynosomatinae, Tropidurinae* and Oplurinae (Clade B); (3) Polychrotinae* and Corytophaninae are sister taxa; and (4) Geiseltaliellus is on the stem of Corytophaninae. The presence of Geiseltaliellus in Europe during the warm, humid Eocene suggests dispersal from North America and a more northerly distribution of the corytophanine stem than the crown. Geiseltaliellus represents a separate invasion by Iguanidae of the Old World and an evolutionary dead end. On the basis of its fossil record and modern distribution, Clade A is interpreted as ancestrally North American. Persistent conflict of morphological with molecular genetic data on iguanid relationships remains to be resolved.
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