A review of crocodylian phylogeny reveals a more complex history than mighthave been anticipated from a direct reading of the fossil record withoutconsideration of phylogenetic relationships. The three main extant crocodylianlineages—Gavialoidea, Alligatoroidea, Crocodyloidea—are known fromfossils in the Late Cretaceous, and the group is found nearly worldwide duringthe Cenozoic. Some groups have distributions that are best explained by thecrossing of marine barriers during the Tertiary. Early Tertiary crocodylianfaunas are phylogenetically composite, and clades tend to be morphologicallyuniform and geographically widespread. Later in the Tertiary, Old Worldcrocodylian faunas are more endemic. Crocodylian phylogeneticists face numerouschallenges, the most important being the phylogenetic relationships and time ofdivergence of the two living gharials (Gavialis gangeticus andTomistoma schlegelii), the relationships among living true crocodiles(Crocodylus), and the relationships among caimans.