© Springer Science + Business Media B.V. 2008. Teeth of primate-like mammals from the Paleogene (“plesiadapiforms”) have been known for at least 130 years (Gervais, 1877). These fossil taxa are generally recognized as being closely related, but not monophyletic (e.g., Gingerich, 1976; Szalay et al., 1987; Beard, 1993a; Silcox, 2001; Bloch and Boyer, 2002a; Bloch et al., 2007) and we maintain that view here. Thus “plesiadapiforms” are referred to with quotation marks throughout the text to reflect that status. Vertebrate paleontologists have struggled to understand the nature of the phylogenetic relationship of “plesiadapiforms” to the extant and extinct members of crown group Primates [= Euprimates (Hoffstetter, 1977)], since Plesiadapis was first described by Gervais in 1877 (e.g., Lemoine, 1887; Stehlin, 1916; Teilhardde-Chardin, 1922; Gidley, 1923; Simpson, 1935; Russell, 1959; Simons, 1972; Szalay, 1973; Gingerich, 1975, 1976; Szalay et al., 1975; MacPhee et al., 1983; Gunnell, 1989; Kay et al., 1990; Beard, 1993a; Silcox, 2001; Bloch and Boyer, 2002a). The strongest support for a close relationship between “plesiadapiforms” and Euprimates, specifically, has historically come from the excellent fossil record of teeth known for “plesiadapiforms,” first in Europe and then in North America (e.g., Gidley, 1923). Cranial and postcranial fossils were relatively rare and fragmentary initially, such that the first researchers were unable to evaluate the presence of non-dental euprimate features in “plesiadapiforms.” As nondental fossils of “plesiadapiforms” were recovered there was some disagreement as to whether they suggested treeshrew or euprimate affinities. A humerus from the San Juan Basin (at the time attributed to Nothodectes) was figured by Gregory (1920) and interpreted to fit the “tupaioid” pattern. Later, a crushed skull and additional postcranial material associated with that specimen (and now referred to Nannodectes gidleyi Gingerich) was interpreted by Simpson (1935) as being similar to both lemurs and treeshrews. An implication of this acknowledged similarity was that Nannodectes spent time in the trees, as expected for the early forebears of the euprimate clade. However, Simpson (1935) discounted the similarities to treeshrews as being primitive (plesiomorphic), while he emphasized perceived shared-derived (synapomorphic) characters with lemurs as supporting a relationship with Euprimates. At the same time, he rejected any special relationship to Daubentonia, an idea that had been seriously considered based on the shared presence of procumbent incisors (Stehlin, 1916; Teilhard-de-Chardin, 1922). Instead, Simpson (1935) attributed this similarity to convergence. He interpreted the procumbent incisors of Nannodectes as feeding specializations for a way of life that likely differed in significant respects from that of the euprimates to which he had compared it (Lemur and Notharctus). He also interpreted the differences between Nannodectes and euprimates in other parts of the skeleton to reflect differing ecological specializations. These differences indicated to Simpson that Nannodectes could not be the direct ancestor to later occurring euprimates. Description of two skulls of Plesiadapis tricuspidens, from Cernay and Berru, France, appeared to support a euprimate relationship in certain characteristics. These included a bony auditory bulla continuous with the petrosal bone, and a thin, ring-like intrabullar component to the ectotympanic bone (Russell, 1959, 1964; Szalay, 1971; Gingerich, 1976; Szalay et al., 1987). Additional cranial material also supported a “plesiadapiform”-euprimate link. Szalay (1972b) described Phenacolemur jepseni, and found that it too had a petrosal bulla (although, see MacPhee et al., 1983; Bloch and Silcox, 2001), and additionally, that it had a large posterior carotid canal for the internal carotid artery. He concluded that it would have had a transpromontorial bony tube for this vessel, as in many early Eocene adapoid and omomyoid euprimates. Additional evidence for an arboreal lifestyle began to accumulate with fragmentary postcranials (e.g., Szalay and Decker, 1974).