On the Individual Variations of Upper Molars of Howlers and the Phyletic Relations among South American Monkeys Based on Molar Structure

Abstract

HURZELER (1948) reported an individual of red howler monkey on which second upper molar the direct connection between the protocone and hypocone is not seen; the Nannopithex-f old (this structure must be regarded as the postprotocrista as discussed in the text) runs from the protocone postero-buccally and extends between the hypocone and metacone. 70 individuals of howlers were examined; all of them show the direct connection between the protocone and hypocone, and thus it is concluded that the specimen shown by HURZELER is an example of specifically rare case. Because the Miocene Stirtonia which gave rise to extant howlers (Alouatta) shows no direct connection between two cusps mentioned above, the connection between the protocone and hypocone seen in almost all howlers is safely regarded as a derived or secondary character.Based on the hypothesis that the Eocene Omomyidae and its allies might be an ancestral stock to whole South American monkeys, and compared them with omomyids, the following primitivecharacters of molar structure among platyrrhines are discriminated;1. The hypocone is present.2. The entoflexus is deep and long transversely.3. The postprotocrista extends to the base of the metacone.4. Each cusp is distinct and rather tall.5. Upper molars are wider than long.6. The third molar is present.Among extant platyrrhines, only squirrel monkeys (Saimiri) have these six primitive characters. Cebus, which is usually treated as an unspecialized monkey among platyrrhines and thought to be closely related with Saimiri, attaines several derived characters. Here, it is concluded that Saimiri retains primitive characters which were attained by the ancestral stock of platyrrhines. Based on these points of view, a provisional phyletic tree is introduced.