The evolution of mid-latitude faunas during the Eocene: late Eocene lizards of the Medicine Pole Hills reconsidered
Bulletin of the Peabody Museum of Natural History
2011
Journal Article
52
1
3-105
Dipsosaurus dorsalis
The Medicine Pole Hills local fauna of the late Eocene (Chadronian) of North Dakota affords an exceptional view of a terrestrial community in central North America prior to the climatic deterioration of the earliest Oligocene. The recovery of hundreds of new squamate specimens -particularly abundant dermal skull bones - gives occasion for a re-evaluation of that portion of the assemblage. This work represents the first attempt to associate significant amounts of cranial material from a large number of closely related species. New material of Polychrus charisticus suggests it lies outside the crown of Polychrus (monkey lizards). One previously known but unnamed species is a crown iguanine (true iguana) related to Dipsosaurus dorsalis (the Desert Iguana). Another lies within crown Corytophaninae (basilisks) on the stem of Laemanctus + Corytophanes. New material of Tuberculacerta pearsoni indicates that its proposed relation to Phrynosomatinae (fence lizards, horned lizards, and others) was probably in error; however, a well-supported alternative hypothesis has not emerged. Unusual similarities are reported between Hoplocercinae (spiny-tailed and dwarf iguanas) and Cypressaurus, but more material from the latter taxon is needed for a firm phylogenetic determination. The presence of a diploglossine (galliwasp) in the assemblage is confirmed and two additional small anguids are reported: an annielline (the last known central North American record of the California limbless lizard lineage) and a gerrhonotine (alligator lizard). On the basis of the new specimens, the number of independent iguanid lineages is reduced from eight to five. However, the higher-taxonomic diversity of Iguanidae in the late Eocene is strengthened. Eocene lizard faunas in central North America show considerable faunal continuity, consistent with the notion that middle and high latitudes were warmer in the late Eocene than previously thought. Late Eocene faunas are still dominated by lineages with exclusively tropical living representatives but also include several lineages that today remain extratropical.