Plesiadapiform Dental Characters and Primate Origins

Many plesiadapiforms have enlarged anterior incisors and a degree of reduction of the posterior teeth, between I/1 and P/4. It seems clear that the increase of the anterior incisor results in a crowding of the following teeth, which leads in many advanced plesiadapiforms to a diastema between the large incisor and the remaining teeth, or to a series of tiny crowded teeth as in carpolestids. Such specializations have commonly been recognized as preventing any possible ancestral relationship between advanced plesiadapiforms and primates. However, many authors have considered that the most primitive members of the group would be primitive enough to root primates within them (e.g., Simons, 1972; Szalay, 1975; Szalay and Delson, 1979; Van Valen, 1994; Van Valen and Sloan, 1965). Nevertheless, the possibility that Purgatorius might be ancestral to primates has also been questioned (Gunnell, 1989; Rose et al., 1994). Having done a very complete survey of plesiadapi-form material, Silcox (2001) writes that "every plesiadapiform for which the lower central incisor is known has an enlarged, procumbent I/1." She was able to verify that this is also true for Purgatorius janisae. Incisor size increase is common in plesiadapiforms, and it suffers rare exceptions [e.g., the lower incisor decreased in size between Elphidotarsius and Carpolestes in relation with a shift in function involving the hypertrophied fourth premolars (Biknevicius, 1986)]. However, even in such an exception, the lower incisor stayed procumbent and did not reevolve a more primitive shape. A size decrease of the anterior incisor accompanied by increased orthality seems very unlikely. The reduction of the following teeth also started early in the group; most plesi-adapiforms have lost P/1, which is retained only in Purgatorius, Palenochtha weissae, Anasazia williamsoni, and one other undetermined specimen (Silcox, 2001; Van Valen, 1994). Because early primates have a P/1, they could be rooted only in one of these earliest plesiadapiforms. The P/4 of P. janisae has a relatively large paraconid, which is located quite high on the protoconid. This morphology is very unlikely ancestral to the small incipient paraconid of the earliest primates, which is set at a lower level on the protoconid (Figure 4). P. unio has a smaller P/4 paraconid; however, it is tall and the P/4 seems to already display some increase in height in comparison with the P/4 of the earliest primates (see also Rose et al., 1994). The upper P4/ of P. janisae has a distinct metacone, whereas early primates have a simple P4/ (Figure 4). The very simple upper and lower P4 of Donrussellia and Teilhardina are suggestive of an ancestry in a group having simple fourth premolars, and not the higher

Figure 4. Isolated teeth of Purgatorius (A-D) compared with those of early primates (E-H). Drawn to same length or same transverse width (A and E); scale bar in H is 1 mm. All are occlusal views except B and F, which are lingual. On the P4/ (A and E), Purgatorius (A) has a metacone (arrow) and a more developed trigon basin than Teilhardina (E). On P/4, Purgatorius (B) has higher metaconid, paraconid, and talonid summit, than Donrussellia (F). On M/1, Purgatorius (C) has a reduced trigo-nid due to its reduced paraconid (arrow) in comparison with Eosimias (G). On M/3, the trigonid is also anteroposteriorly narrower in Purgatorius (D), with a slightly reduced paraconid (arrow) and moderately elongated posterior lobe, in comparison with Eosimias (H), which has a broad trigonid, broad paraconid, and a relatively short posterior lobe. All the character states in Purgatorius are interpreted as more advanced in comparison with the states in the various primitive primates. (Redrawn from Buckley, 1997; Kielan-Jaworowska et al., 1979; Rose et al., 1994; Szalay and Delson, 1979; Tong, 1997.)

Figure 4. Isolated teeth of Purgatorius (A-D) compared with those of early primates (E-H). Drawn to same length or same transverse width (A and E); scale bar in H is 1 mm. All are occlusal views except B and F, which are lingual. On the P4/ (A and E), Purgatorius (A) has a metacone (arrow) and a more developed trigon basin than Teilhardina (E). On P/4, Purgatorius (B) has higher metaconid, paraconid, and talonid summit, than Donrussellia (F). On M/1, Purgatorius (C) has a reduced trigo-nid due to its reduced paraconid (arrow) in comparison with Eosimias (G). On M/3, the trigonid is also anteroposteriorly narrower in Purgatorius (D), with a slightly reduced paraconid (arrow) and moderately elongated posterior lobe, in comparison with Eosimias (H), which has a broad trigonid, broad paraconid, and a relatively short posterior lobe. All the character states in Purgatorius are interpreted as more advanced in comparison with the states in the various primitive primates. (Redrawn from Buckley, 1997; Kielan-Jaworowska et al., 1979; Rose et al., 1994; Szalay and Delson, 1979; Tong, 1997.)

and more complex ones of P. janisae [it is possible that the P4/ metacone secondarily decreased and disappeared in some plesiadapiforms, Silcox (2001); however, this is unlikely to have happened in the primate ancestral group, because some primates later develop a large P4/ metacone. When this happens, it typifies whole groups, such as adapines or sivaladapines, and does not show high plasticity]. The lower molars of Purgatorius have a trigonid that is antero-posteriorly compressed, with a relatively reduced paraconid; the paraconid and paracristid are even smaller in P. unio than in P. janisae. On the contrary, the earliest primates have a trigonid, which is longer antero-posteriorly because they have a large paraconid (Figure 4). This clearly is a character for which both species of Purgatorius and all other plesiadapiforms are more derived than the earliest primates are and cannot be ancestral to them (Buckley, 1997, also mentions the incipient postprotocingulum of Purgatorius and the "strongly mesiobuccally shifted and mesially inclined molar protocone," valid if it did not revert in plesiadapoids). Considered in isolation, some of these characters could be debated and suspected of some possible degree of reversion. Taken together, however, they show that the typical plesiadapiform specializations were well under way in the earliest known members of the group. These characters, especially the procumbency of the large I1 and the shorter lower molar trigonid, exclude any possible ancestral link between purgatoriids or other plesiadapiforms, and primates.

Despite the absence of an ancestral relationship of plesiadapiforms to primates, the former are still in part dentally primate-like, and a better understanding of the polarity of other dental characters would be important to flesh out the preceding point of view, and to enhance our understanding of early archontan relationships. Silcox (2001) mentions that many plesiadapiforms have the trigonid mesially inclined, excluding Palenochtha and plesiadapoids. Purgatorius has a small degree of trigonid mesial inclination (Buckley, 1997; Silcox, 2001), and this could be a further indication of divergence from primates, which do not have it. The place of the protocone on the upper molars, relatively mesial in many genera but central in P. janisae and plesiadapoids (Beard and Wang, 1995; Szalay and Delson, 1979; Van Valen, 1994) should be scrutinized. Silcox (2001) codes the protocone "skewed mesiobuccally" in tupaiids and Donrussellia, whereas it is coded as central in other primates. On the described upper molar of Eosimias (Tong, 1997), the protocone seems quite mesial; however, it is not particularly labial (= buccal) in position. An understanding of the polarity of this character would be important. It is likely to be complex; the labial shift and long lingual slope of the protocone is one consideration (discussed by Godinot, 1994, and poorly understood functionally), and the mesiodistal place of the protocone is another, hopefully to be distinguished from the simple posterior extension of the posterior face, which makes the protocone appear "mesially shifted" in relation to the whole mesiodistal diameter; another aspect is the distolingual angle or shift of the lingual part of the upper molars, precisely defined by Van Valen (1994). Hence, at least three characters should be evaluated to better describe the complexity of the upper molar "basic" structure.

For upper molar and other dental characters, it must be stressed that the primate ancestral dental morphotype has not been elucidated. The description of eosimiid dentitions (Beard et al., 1994, 1996; Tong, 1997) has introduced new primitive dental character states that need to be taken into account in deciphering the primitive primate morphotype. As in Tarsius, eosimiids have a trigonid on the lower molars, which does not become more compressed antero-posteriorly from M/1 to M/3 (judged from Beard et al., 1994, 1996; the isolated lower molars figured by Tong, 1997, suggest a low degree of trigonid compression and paraconid labial shift on M/2-3, derived in comparison to the other specimens). On the contrary, trigonid compression markedly increases from M/1 to M/3 in other primates and a number of plesiadapiforms, again demonstrating convergence for this character. The absence of trigonid compression from M/1 to M/3 must be primitive for primates. Eosimias has a relatively broad stylar shelf, primitive; its very large parastyle is also reminiscent of a parastylar lobe, which would make it a primitive character state. On the lower molars, the cristid obliqua does not join the metaconid on M/1, which is primitive in comparison with Donrussellia (a cristid obliqua ascending on a posteriorly shifted metaconid, the "stepped postvallid" of Silcox, 2001, is also convergent in many primates and plesi-adapiforms). The relatively short M/3 talonid of Eosimias is also probably primitive for primates (see suggestion of a possibly short talonid on the M/3 of Altiatlasius in Godinot, 1994). There was again convergence in M/3 third lobe elongation between many plesiadapiforms and primates.

In this context, the interpretation of Altanius, a genus most often considered as a primitive primate, but repeatedly suspected of having plesiadapiform affinities (Rose and Krause, 1984; Rose et al., 1994), remains intriguing. Silcox (2001) places Altanius within the primates, corroborating the view of Gingerich et al. (1991). Altanius clearly does not fit in the radiation of North American plesiadapiforms as understood here. However, it shows a series of similarities with some plesiadapiforms, which must be convergent (and not primitive as in the parsimony analysis of Gingerich et al., 1991), and would be very interesting to understand adaptively (exodaenodonty, high M/1 trigonid). Because it is quite autapomorphic within primates, it appears very difficult to decipher its affinities; and its placement in a subfamily Altaniinae inc. sed. as proposed by Van Valen (1994) seems warranted. Whether this early genus could reveal some characters of the primate morphotype is unknown (P3/ and P4/ triangular, with small protocone?).

Based on dental characters, Silcox (2001) suggests that Toliapinidae, restricted to Toliapina and Avenius, could be closely related to primates. However, these genera are known only from isolated teeth from P4 to M3 and the node is weakly supported; she does not give much credence to this cladistic result. Avenius seems to have a rather typical plesiadapiform-like P/4—a correlative enlarged incisor would prove such an affinity and rule out close primate ties. These tiny low-crowned forms would in any case be rather distant from the well-known primates, which have higher, more pointed cusps, suggestive of a more insectivorous diet. A more convincing close affinity with primates would necessitate more complete dentitions and dental intermediates, or the confirmation of primate affinity through the non-dental characters cited below.

To sum up, not only do derived plesiadapiform families show specializations of their dentitions very divergent from those of primates, evidently irreconcilable with primate ancestry, but also even the most primitive plesi-adapiforms (with or without microsyopids) have derived characters excluding them from possible primate ancestry. A sister group relationship of Purgatorius or a primitive plesiadapoid to primates would reflect a dichotomy going back to the Earliest Paleocene or Late Cretaceous. It is quite hazardous to evaluate such a hypothesis without a better knowledge of the primate dental morphotype and some ideas about primitive tupaiid dentitions (hardly compensated for by very distant insectivore outgroups as in Hooker, 2001). However, a better understanding of the functional meaning of molar character evolution in plesiadapiforms would greatly assist the evaluation of possible morphological changes in primate ancestry.

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  • riikka
    What does the plesiadapiformes?
    8 years ago

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