Foot morphology and evolution in early Eocene Cantius

More than one hundred undescribed foot elements of Cantius and other closely related notharctine adapids (7 species in all) from throughout the early Eocene (Wasatchian) Willwood Formation of the Bighorn Basin, Wyoming, have been analyzed. For four species (Cantius ralstoni, “Copelemur” feretutus, Cantius frugivorus, and? Pelycodus jarrovii) these represent the first described postcranial material. The clade of Cantius and allied genera in the Bighorn Basin includes several speciation events and both increases and decreases in body size. The largest species represented by tarsal remains (? Pelycodus jarrovii) is several times larger than the smallest, Cantius ralstoni. However, despite the number of species, temporal duration, and range of body size represented, there are no obvious changes in foot morphology as measured by various indices or by observation of qualitative features. This contrasts with the more obvious changes in dental features (e.g., development of hypocone and mesostyle) in the same group of species described by earlier authors. The fossil record of Cantius thus illustrates different kinds of evolutionary change in the dentition and tarsus. The dentition changes in both size and shape while the tarsus changes only in size. We describe significant differences in foot anatomy which distinguish notharctines from extant strepsirhines, including the nature of the articulations between the navicular and cuboid and the distal tarsus with the metatarsus. These features indicate that the fossil taxa are not particularly closely allied phylogenetically to either extant strepsirhines in general or lemurids in particular.