The past decades have witnessed considerable multidisciplinary attention to climate change around the Eocene/Oligocene boundary and the causes and consequences of associated biotic change. This is partly due to a re-calibration of the North American terrestrial record (Prothero and Swisher 1992; Swisher and Prothero 1990), extensive study of deep-sea cores (e.g., Thomas 1992; Lear et al. 2000), and debate over bolide impacts (see discussion in Prothero 1994).

Wolfe (e.g., 1978, 1992) argued early on from paleobotanical data that climatic deterioration was not limited to the Eocene/Oligocene boundary interval: the middle/late Eocene boundary was also marked by a decrease in mean annual temperature (MAT) and an increase in mean annual range of temperature (MART) in many areas on the fringe of the North American continent. According to his studies, amelioration during the late Eocene was followed by another marked deterioration in the earliest Oligocene.

How apparent climatic change along the North American periphery was reflected in its interior is a matter of some contention. Hutchison (1982, 1992) could accept a moderate drop in MAT (<5°C) as an explanation for the observed decrease in maximum carapace length in testudinid turtles; but he also argued that in the early Oligocene, mean winter temperature could not have been below 13°C, because the relatively large terrestrial turtles (>30 cm carapace length) known from that time could not burrow to escape lower temperatures. Similarly, Evanoff et al. (1992) concluded that there was a minor drop in MAT in the early Oligocene (from 16.5°C in the late Eocene) based on the terrestrial gastropod fauna near Douglas, Wyoming. Contrariwise, Wolfe (1992) found that, between the late Eocene and early Oligocene in Colorado, MAT decreased from ~12.5 to ~4.5°C; the magnitude of this drop (8°C) is comparable to what he found in the Pacific Northwest and elsewhere in North America (Wolfe 1978). The late Chadronian Ruby flora of Montana had a great many conifers and indicates a MAT of ~12°C and a probable MART of <16°C (Wolfe 1992). These pre-deterioration values already exceed the limits suggested by Hutchison (1982) for the early Oligocene.

A change in mean annual precipitation (MAP) across the Eocene/Oligocene boundary seems less controversial. Studies of palaeosols (e.g., Retallack 1992; Sheldon and Retallack 2004) imply that MAP gradually dropped by ~50% between 36 and 27 Ma, by which time MAP values in Nebraska likened modern-day values.

The transformation of North American squamate assemblages during the Tertiary has not been investigated in detail (Hutchison 1992), but broad shifts in assemblage composition may relate to climate change. Gauthier (1982) noted a general replacement of Paleogene mesic-adapted lizards (especially anguimorphs) by Neogene xeric-adapted lizards (especially iguanids). Holman (2000) described an analogous shift from booid- to colubroid-dominated assemblages around the same time. Many of the common Paleogene squamate taxa moved south and are found today in the southeastern United States and Central America (Estes 1970; Savage 1960).

Despite increasing aridity and cooler temperatures, squamate diversity appears to increase across the Eocene/Oligocene boundary in central North America (10 to 17 species: Sullivan and Holman 1996)¹. A temperature decrease of only ~5°C is related to a moderate decrease in lizard species richness in modern North American deserts (Pianka 1967). The pattern seen in the North American fossil record, however, may be vitiated by differences in sampling, indicated by Lazarus taxa in the Chadronian (Sullivan and Holman 1996).

In this paper, I report on a new and very diverse fauna from the Chadronian of North Dakota, the Medicine Pole Hills local fauna (l.f.). Many new lizard morphotypes are identified, and three are formally named. Extensive comparisons are made with the disarticulated skeletons of extant taxa in order to estimate their phylogenetic position. I then evaluate the effects of this new fauna on our understanding of squamate species richness across the Eocene/Oligocene boundary.