1288


Journal of Paleontology 91(6):1272–1295


Table 4. Measurements (mm) of the metapodials of AMNH FM 28895. APD = anteroposterior diameter; diap = diaphysis; dist = distal; L = length; px = proximal;W = width. Dashes represent not measured dimensions.


L Wpx Wdist


APD px APD dist Wdiap


APD diap


Mc III 34.5


6.5 7.0 6.3 4.5 6.5 -


Mc II -


6.1 -


8.2 - - -


Mc V 15.9


2.6 3.9 3.7 2.7 2.4 1.7


AMNH FM 28895, the isolated bone is here tentatively con- sidered a right McV(Fig. 6.4–6.7). It is complete, but fractured; the body is nearly straight and quadrangular in section (Fig. 6.4, 6.5); the proximal epiphysis is laterally compressed and the distal epiphysis is anteroposteriorly flattened (Fig. 6.6). The proximal facet is rectangular in shape, anteroposteriorly very convex, and laterally inclined (Fig. 6.7); posteriorly, the surface extends downwards in a small fan-shaped area (Fig. 6.5). Internally, just below the surface, there is a thin ridge that borders a concavity. The external face is convex and has no distinctive facet. The distal articular surface is anteriorly convex and has a moderately developed keel on the posterior face (Fig. 6.7); in anterior view, a semi-circular sulcus separates the articulation from the shaft. The ulna fragment (Fig. 6.8, 6.9) has a moderately concave


and nearly vertically oriented articular facet for the humerus, similar to the ulna of Notopithecus adapinus (Vera, 2012b) and differing from the curved surface observed in Protypotherium (YPM-PU0-15828, YPM-PU 0-15341; also see Sinclair, 1909).


Discussion


Variation throughout ontogeny and systematic implications.— It is widely known that wear modifies the dimensions and morphology of the dentition of herbivorous mammals through- out ontogeny. In particular, among native ungulates from South America, ontogenetic sequences based on extreme morpholo- gical changes on dentition were described for Archaeohyracidae (Croft et al., 2003; Billet et al., 2009; Cerdeño et al., 2010), the notopithecid Transpithecus (Vera, 2012a), and other groups of notoungulates (Francis, 1960; Madden, 1997; Billet et al., 2008; Cerdeño et al., 2008). Concerning archaeopithecids, the morphological changes


on the occlusal surface of upper and lower dentition became really noticeable as wear progresses. However, these changes were originally assumed to represent taxonomic differences, therefore Ameghino (1897, 1901, 1902, 1903) erected at least six species of archaeopithecids. Simpson (1967b) regarded the same criteria, but he reduced the number of species and recognized variation. On this aspect, Croft et al. (2003) provided the basis for interpreting wear-related metric variation in archaeohyracids tooth dimensions and demonstrated that most cheek teeth decrease in length and increase in width through


Figure 6. Archaeopithecus rogeri. AMNH FM 28895: (1–3) right Mc II-III, (1)anterior, (2) proximal (anterior face to bottom) and (3) lateral views; (4–7) right Mc V, (4) anterior, (5) posterior, (6) internal, (7) proximal and distal views; (8, 9), fragment of left ulna, (8)anteriorand (9) lateral views. Scale bar is 5mm.


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