Recent mousebirds are the smallest of the living neognathous bird orders – just two genera comprising six species. Understanding the evolutionary dynamics of this avian lineage is important because these birds occupy a critical position within Neoaves and can be used as a model for patterns of morphological diversification in other taxa. We present a review of all known fossil mousebirds (Aves: Coliiformes), which formed a diverse assemblage in the Paleogene (65–23.8 ma). We also present a new phylogenetic analysis including the living and fossil representatives of the order, on the basis of a larger character/taxon matrix. This analysis suggests that the Lower Eocene Eocolius is the most basal member of clade Coliiformes, while the Lower–Middle Eocene Sandcoleidae form a basal clade with respect to all other known coliiforms. We show that the Eocene taxon Chascacocolius is the sister group of the crown Coliidae, and that the Middle Eocene taxon Selmes is basal to a clade comprising Chascacocolius and Coliidae. On the basis of this phylogenetic analysis, we propose a new classification for the order Coliiformes including two new families, Chascacocoliidae and Selmeidae. For all families (clades) of mousebirds we provide descriptive osteological diagnoses and discuss a number of functional considerations related to the evolution of their hindlimbs; these considerations can be extended across all extant and fossil perching birds because we show that the hindlimb morphology of Coliiformes is strongly related to their lifestyle, as reflected in their evolutionary history.
Palaeontology. 2008. V. 51(6): 1403-1418.