Phylogenetic Relationships of Adapidae Family

In addition to their ancientness (Middle-to-Late Eocene), adapids have been sought as potential ancestors of modern strepsirhines because of features that have been presumed to be primitive. Adapids lack a tooth comb of the sort seen in modern lemurs and lorises; they typically have a greater number of premolars (four as opposed to three in each quadrant of the jaw); and they have a “lemurlike” bulla, which, because it is similar to that in Lemur, was seen, almost by definition, as primitive. Aside from the occasional inconsis tency, such as having a fused mandibular symphysis, Adapis especially could fulfill the role of ancestor to the modern strepsirhines. Gregory even argued that dental similarities between the fossil form and the extant Malagasy lemur, Lepilemur, demonstrated the primitiveness among the living taxa of Lepilemur and thus the descent from Adapis of other lemurs via Lepilemur. Just over 50 years later, P.D.Gingerich thought the dental similarities were greater between Adapis and the extant Hapalemur and thus suggested that this genus, rather than Lepilemur, was the link between the extinct taxon and the other modern strepsirhines, a view not accepted here. In 1979, J.H.Schwartz and I.Tattersall turned the argument around and suggested that the distinctiveness of the compressed cusps and shearing crests of the molars of Adapis, as well as Hapalemur and Lepilemur, indicated that these taxa were closely related and specialized members of Strepsirhini, forming a separate clade; these authors included the Notharctus group in Adapidae. Subsequently Schwartz pointed out that there really are no features that would unite a Notharctus group with an Adapis group, and he and Tattersall presented dental and some cranial evidence suggesting a relationship between Adapidae, in the restricted sense of Adapis plus those few forms sharing derived characters with it, and a particular group of Malagasy primates, the indrioids.
During this latter review, Schwartz and Tattersall failed to discover any derived characters that would unite with Adapis those taxa traditionally placed into Adapidae. As Robinson had suggested about North American fossils included in the (primarily) Eocene family Omomyidae, it seemed that taxa had been placed in Adapidae because they were Eocene in age and European in location. An appraisal of the spectrum of so-called adapids led to the suggestion that some were actually related to Notharctus or Pelycodus, such as Cercamonius and Protoadapis, and Pronycticebus and Agerinia, respectively; others were linked to extant taxa, such as the fossil genus Huerzeleris to the living Malagasy primate, Phaner, and yet others were lorisoids of uncertain affinity, such as Anchomomys and Periconodon.
Adapidae seemed, therefore, to be a group of few members (Adapis and Leptadapis, as well as the recently proposed genera Simonsia and Paradapis) related to a small number of specialized extant primates. Pelycodus-also emerged as sharing some potential derived features with Notharctus, as well as others with Smilodectes. Although not contributing to a resolution of its relationships, this does indicate that Pelycodus could not have been ancestral to both a Notharctus group and an Adapis group. More recently, postcranial evidence has been brought to bear on the relationships of the Adapis group to the Notharctus group and of each of these groups to extant taxa. Studies by K.C.Beard and colleagues of wrist and ankle bones attributed to Adapis, Notharctus, Cantius, and Smilodectes indicated that there were distinct differences between Adapis and the three taxa representative of the Notharctus group. In a comparison with a diversity of extant primates, Beard et al. concluded that the Adapis group was more closely related to extant lemurs than to the Notharctus group because Adapis shared with extant lemurs a unique articulation between the ulna and the small pisiform bone of the wrist. This feature is not found in Smilodectes (the only taxon of the Notharctus group for which the appropriate bones are known) and is apparently not characteristic of the anthropoid primates analyzed. Thus, Beard et al. concluded that certain aspects of wrist morphology corroborated the interpretation based on craniodental features: The Notharctus group and the Adapis group are not sister taxa. Beard et al. did not, however, find support for the suggestion that Adapis may be closely related to only a few of the extant lemurs. Rather, these authors argued that another feature of the wrist—an os centrale that overlaps the capitate and makes contact with the hamate—is found uniquely in extant lemurs to the exclusion of Adapis. Although research being conducted by Schwartz and Yamada indicates that some features of wrist and ankle morphology require further documentation, it is apparent that the traditional phylogenetic and systematic schema involving Adapidae are in need of revision.

Family Adapidae
Subfamily Adapinae
†Adapis
†Leptadapis
†Simonsia
†Paradapis
†Cryptadapis
†Alsatia
†extinct