Lobo et al.—Dental features of Xenorhinotherium
Table 1. Development of each tooth from the specimen MCL 2643; E=eruption stage; R=root development;W=degree of wear; –=missing data.
Right
Side/Tooth dI3
dC
E 4
dP1 –– – 43 5 dP2 dP3 dP4 I1 I2 I3
4 4
4 4
P2 P3 P4
M1 M2 M3
3 3
– 21 – 21 – 21 – 21 – 11 –– – – 11 –– 1 1 1 4 3 2
3 3
5 5 4
4 4 4
3 3 3
5 5 4
1 1 3 2 1
1 1 3 2 1
1 1 4 3 2
1 1 3 2 1
4 –– – 4 –– –
R W E 3
Left R W
1087
evolution, with some cases of adaptive convergence in dental eruption sequences (Smith, 2000). Our results have only a single incongruence in wear
1 1 3 2 1
sequence between juvenile and adult specimens. The wear stage of P4 differs from that expected for adult specimens. Therefore, the wear stage of P4 in adult X. bahiense (MCL 2644, MCL 3546) is greater than that of M3. However, the presence of the dental crypt in P4 and the stage of development of its root establish its placement in the eruption sequence. The incon- gruence observed in adult wear stage of P4 is probably due to the masticatory process, as suggested by Lessa (1992), in which X. bahiense has M3 less subject to wear than other cheek teeth. In addition, the angle of occlusion between the upper and lower jaws could cause this difference in wear rate among teeth that differ in positions (Spinage, 1971). The only data about the permanent eruption sequence for
than in the other specimens, has P4 less worn than M1. This indicates that the wear stages are influenced by other factors besides the sequence eruption of the teeth. MCL 3577 is a fragmented lower jaw with the dental
sequence p2–m3 (Fig. 4.4). Even the p2 is less molarized than the other premolars in this specimen, and it is evident that p2 is less worn than the other premolars. That interpretation is supported by the wear on the protoconid, metaconid, and cristid oblique. Also, p4 is less worn than m1, which is supported by the wear on the metaconid. However, as in the upper dentition of analyzed adult specimens, p4 is more worn than m3, which is supported by the metaconid, and the talonid, which shows no wear on m3. Other specimens that have lower teeth studied are MCL 3559, MCL 3560, MCL 3658, and MCL 3769. All of them exhibit the same pattern as described for MCL 3577, but with less information because these specimens preserve fewer teeth.
Hypsodonty index.—The sample comprises two m3 with no wear (stage 1) and three m3 with little wear (stage 2) (Table 2). The mean of m3 with no wear is 2.24; the mean including the other three m3 (stage 2) is 2.04. Both results place X. bahiense close to the interval of 1.50–3.00, thus characterizing the species as having mesodont crowns.
Discussion
Eruption sequence.—The results indicate that X. bahiense has a sequence of replacement most similar to a rapid-growth mammal, with incisors and premolars erupting after the molars (Smith, 2000). The extant terrestrial ungulates with most similar sequence of replacement are the artiodactyls, specifically spe- cies of families Cervidae and Giraffidae, such as Cervus elaphus Linnaeus, 1758 and Okapia johnstoni (Sclater, 1901), respec- tively (Smith, 2000). On the other hand, the currently accepted phylogenetic hypotheses for placental mammals (Buckley, 2015; Welker et al., 2015) place SANU as more closely related to perissodactyls. This divergence between phylogenetic signal and dental eruption sequence suggests that an ecological pres- sure on this trait was possibly important during ungulate
Litopterna is provided by Bergqvist (2010) using species of the families Protolipternidae and Proterotheriidae: Protolipterna ellipsodontoide Cifelli, 1983 and Paranisolambda prodromus (Paula-Couto, 1952), respectively. Bergqvist’s data provides information about the replacement sequence between the two posterior premolars (P3 and P4) and the molar sequence. Although the data of P. ellipsodontoide and P. prodromus are incomplete, a clear distinction is observed between them and the sequence for X. bahiense: the P3 and P4 erupted before or almost at the same time as M3 (Bergqvist, 2010), whereas in X. bahiense P3 and P4 erupted after M3. Furthermore, in P. ellipsodontoide P4 erupted before P3. This is contrary to that observed for X. bahiense in our study and Phenacodontidae, a family of NorthAmerican “archaic ungulates” closely related to Litopterna (West, 1981). The “delayed dental eruption” has two concepts clarified by
Billet and Martin (2011). Our focus here is on the concept of delayed eruption of permanent dentition relative to skull growth, which is regarded as a synapomorphy of Afrotheria (Asher and Lehmann, 2008). This delay of eruption occurs when one species spends well over half of its lifespan without a completely erupted permanent dentition (e.g., elephants, sea-crows, and hyraxes) (Asher and Lehmann, 2008). Although we do not consider cranial measurements in our sample, they explain, the absence of delayed dental eruption in the juvenile individual (MCL 2643). This is evident because this juvenile individual (MCL 2643) has already erupted most of its permanent dentition. Moreover, the juvenile does not show co-ossified epiphyses on the limb bones as completely ossified, and the fusion between radius and ulna had yet not begun (Cartelle and Lessa, 1988; Lessa, 1992). In summary, our results reinforce the observation that South American native ungulates (SANU) do not show delayed dental eruption, as previously suggested by Billet and Martin (2011) and Kramarz and Bond (2014) for other orders of SANU. This rejects the proposed diagnostic synapomorphy of SANU and Afrotheria by Agnolin and Chimento (2011). The closest related litopterns in which the HI has been
applied are species of the family Proterotheriidae. The sampled species show a pattern of increase of the HI during the diversification of the family in the Cenozoic (Bond et al., 2001). The HI in Proterotheriidae changes from brachyodont teeth in late Oligocene–middle Miocene species to mesodont
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