Conclusion Tupaiids as Sister Group of Primates

In summary, it appears that tupaiids share a series of derived skull characters with primates, among which are basicranial characters supported by ontoge-netic studies (Wible and Martin, 1993) and mutually agreed upon characters linked with crucial primate visual apomorphies. Several other cranial characters also proposed in recent studies lend support to this hypothesis (Kay et al., 1992; Wible and Covert, 1987). Among those cranial characters, the most intriguing is the complete postorbital bar, which may require an explanation beyond its role in living primates. Some soft anatomical characters may add support to this view (olfactory bulbs, uterus, Luckett, 1980b). Finally, tarsal characters offer strong support in favor of the same view, and evidence against a closer affinity with plesiadapiforms. Thus, tupaiids appear as the best living or fossil sister group of primates, and they remain so even in comparison with Paleocene-Eocene plesiadapiforms (Figure 7). Plesiadapiforms appear as a likely North American monophyletic group; tupaiids are Asiatic and do not show, at least for Ptilocercus, specializations (other than dental and carpal), which would exclude them from being the best model for primate ancestry. Their behavior, especially their manual insect seizing (Le Gros Clark, 1926; Sargis, 2001), may be considered particularly well-suited to lead to the acquisition of primate characteristics (see later section).

If the hypothesis of a close tupaiid-primate relationship is true, it should be corroborated in the future by dental characters. Dental characters have proven to be of little use as there is no Paleocene tree shrew, which would show more primitive tupaiid dental characters. From the dentition of living tupaiids, one would easily infer that all of them have derived characters that prevent them of being ancestral to primates, among which specializations in their anterior dentitions or molar characters in tupaiines (see Butler, 1980). This is not astonishing, knowing that teeth have continuously evolved in most mammalian groups. The genus Ptilocercus is dentally more primitive than other tupaiids, and it also has a few derived characters that would exclude it as a possible ancestor; however, these characters do not appear important. The absence of conules in tupaiids is sometimes mentioned as an obstacle; however, Butler (1980) mentioned that the preprotocrista sometimes develops a paraconule in Ptilocercus, and the postprotocrista, abruptly interrupted on the M2/ figured by Hooker (2001, Figure 33), which would also appear unlike that of primitive primates, is in fact extended past the base of the meta-cone in some specimens (Butler, 1980). The hypocone is very small. What is

Figure 7. Cladogram summarizing the main characters, which unite primates and scandentians to the exclusion of plesiadapiforms. 1: Derived dental characters of pur-gatoriids excluding possible ancestry to primates, or cross-specializations: enlarged procumbent I/1, P4/ having a metacone, high P/4 with high metaconid and talonid, lower molars with some anteroposterior trigonid compression, some paraconid reduction. 2: Accentuated plesiadapoid specializations, including reduction of the teeth located between I/1 and P/4, strong postprotocingulum on upper molars, long muzzles, etc. 3: Possible homologous synapomorphies shared by tupaiids and primates, including basicranial characters (enlarged tegmen tympani, bony canals around middle ear arteries, and others), postorbital bar, a series of tarsal characters (see text). 4: There are autapomorphies of living tupaiids, including several dental characters.

Figure 7. Cladogram summarizing the main characters, which unite primates and scandentians to the exclusion of plesiadapiforms. 1: Derived dental characters of pur-gatoriids excluding possible ancestry to primates, or cross-specializations: enlarged procumbent I/1, P4/ having a metacone, high P/4 with high metaconid and talonid, lower molars with some anteroposterior trigonid compression, some paraconid reduction. 2: Accentuated plesiadapoid specializations, including reduction of the teeth located between I/1 and P/4, strong postprotocingulum on upper molars, long muzzles, etc. 3: Possible homologous synapomorphies shared by tupaiids and primates, including basicranial characters (enlarged tegmen tympani, bony canals around middle ear arteries, and others), postorbital bar, a series of tarsal characters (see text). 4: There are autapomorphies of living tupaiids, including several dental characters.

attractive in the dentition of Ptilocercus is that the upper and lower P4 are simple, not molarized, and close in shape to those of primitive primates. Importantly, the lower molars are relatively low but retain an anteroposteri-orly elongated trigonid—a condition that must have been present in the ancestral primate, but that was lost in a number of primitive proteutherians and insectivores. Hence, it appears that a more primitive ptilocercine, which from common evolutionary trends can be predicted to have had upper molars more transversely elongated than those of Ptilocercus, without a hypocone, with small conules and more usual protocristae, and with a more primitive anterior dention, could come close to the (problematic) primate morphotype. The reality of these trends is documented in tupaiines by the isolated teeth of the Chinese Eocene Eodendrogale, which has transversely elongated upper molars without hypocone (Tong, 1988). Even if this may appear quite speculative, the dentition of Ptilocercus in spite of its autapomorphies, presents basic similarities with primates, which probably contain some phyloge-netic signal. Due to the temporal gap, it seems difficult to be more precise at this moment. Early tupaiid dentitions should be found in the future, and provide a test of these ideas.

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