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SYNOPSIS Despite two decades of research into the earliest Eocène fossils of the Rocky Mountain interior of North America, almost nothing is known of the squamates (lizards and snakes) from this time, nor of the response of this component of the terrestrial biota to climate change near the PalEocène/Eocène boundary. The present study addresses this lacuna. Sixteen squamate species are described in the earliest Eocène (Wao) Castle Gardens local fauna of the lower-most Willwood Formation of the Bighorn Basin, Wyoming. These include: two new iguanids, a species similar to Geiseltaliellus on the stem of Corytophaninae (basilisks and relatives) and a polychrotine (anoles and relatives); new stem representatives of the xantusiid (night lizard) clades Lepidophyma and Xantusia; a 'higher' amphisbaenian (worm lizard) of indeterminate affinities; six anguids, including a new species on the stem of Anniella (California legless lizards), a ?stem diploglossine (galliwasps), a ?stem gerrhonotine (alligator lizards) and a new glyptosaurine morphologically intermediate between 'melanosaurins' and Glyptosaurini; a new anguimorph of undetermined affinities; and two species compared to the varanid (monitor lizard) taxon Saniwa.At least seven species at Castle Gardens were recent immigrants. The nearest relatives of one of these is obscure, but the other six (three iguanids, stem Lepidophyma, stem Anniella, ?stem Diploglossinae) have living relatives exclusively or almost exclusively confined to warm and frost-free (megathermal) parts of the New World. Among lizards, dispersal coincident with climatic warming near the PalEocène/Eocène boundary was predominantly intracontinental. It is argued that when habitat tracking occurs and phylogenetic niche conservatism obtains, species within a clade could migrate in broad concert in response to climate change. Mid-latitude fossil groups may have contributed directly to the modern tropical biota, making palaeontological data of direct relevance to evaluating historical influences on modern biodiversity patterns.

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