zalambdalestes - American Museum of Natural History

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20 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 281 Fig. 5. M2 amd m3 of Zalambdalestes lechei ZPAL MgM-I/43 (modified from Crompton and Kielan-Jaworowska, 1978: fig. 9), illustrating the dental terminology employed here. Abbreviations: cec, centrocrista; co, cristid obliqua; efl, ectoflexus; end, entoconid; hyd, hypoconid; hyld, hypoconulid; med, metaconid; met, metacone; metl, metaconule; pad, paraconid; par, paracone; parl, paraconule; pas, parastyle (stylar cusp A); pmc, postmetacrista; pomlc, postmetaconular crista; popc, postprotocrista; poplc, postparaconular crista; prd, protoconid; prmlc, premetaconular crista; pro, protocone; prpc, preprotocrista; prplc, preparaconular crista; st, stylocone (stylar cusp B); tal, talonid; tb, trigon basin; trd, trigonid. specimen’s right side (figs. 7, 8A, 29), a small, single-rooted, styliform tooth is preserved in front of the canine. As predicted from the alveolus on the left side, this tooth is slanted posteriorly, but to a greater degree than in the life position; its tip has been displaced such that it nearly touches the base of the canine. This peglike tooth is within the maxilla. Although we deem it unlikely, the premaxilla may have contributed to the alveolus anteriorly. Based on a facet on the anterolateral surface of the maxilla, the posterolateral part of the palatal process of the premaxilla, which is not preserved on either side, may have sent a narrow, triangular wedge posteriorly that approximated or contributed anteriorly to the alveolus. The small tooth in PSS-MAE 130 could be interpreted as an incisor or as an aberrant, reduced, deciduous or adult canine. Of these possibilities, we think that it most likely is an incisor wholly or nearly completely within the maxilla. A similar arrangement is found in the zalambdalestid Kulbeckia kulbecke, which preserves the roots of three upper incisors (fig. 52): the first is entirely within the premaxilla, the second between the premaxilla and maxilla, and the third entirely within the maxilla (Archibald and Averianov, 2003). The Mongolian Late Cretaceous asioryctitheres also have incisors in the premaxillary-maxillary suture; Ukhaatherium nessovi has the last incisor (I5) in the maxilla, with the premaxilla contributing to the anterior quarter of the alveolar border (Novacek et al., 1997; personal obs.), and Asioryctes nemegtensis and Kennalestes gobiensis have the last incisor (I5 and I4, respectively) in the premaxillary-maxillary suture (fig. 51D, E; Kielan-Jaworowska, 1981). Incisors at least partly within the maxilla may be primitive for Eutheria, although the morphology in question is not known for most of the relevant outgroups. In the basal mammaliaform Morganucodon oehleri, the I4 is in the maxilla (Kermack et al., 1981), and in
2004 WIBLE ET AL.: ZALAMBDALESTES 21 Fig. 6. M2 of Zalambdalestes lechei ZPAL MgM-I/43 (modified from Crompton and Kielan- Jaworowska, 1978: fig. 9), illustrating the dental measurements in table 2. Abbreviations: A, distance between lingualmost point of protocone base to its apex; B, distance between protocone and paraconule; C, distance between paraconule and paracone; D, distance between paracone and labialmost point of crown; L, greatest anteroposterior length; W-A, greatest labiolingual width from anterolabial corner to lingualmost point of protocone base; W-P, greatest labiolingual width from posterolabial corner to lingualmost point of protocone base. the docodontid Haldanodon exspectatus the I5 is in the premaxillary-maxillary suture and the I6 in the maxilla (Lillegraven and Krusat, 1991). In a more proximate outgroup, Dryolestoidea, the I4 is in the maxilla in an undescribed new genus of paurodontid from the Morrison Formation of Fruita, Colorado (Hopson et al., 1999; personal obs.). A second MAE specimen preserves an additional alveolus in front of the canine on one side only. The right side of PSS-MAE 108 has a small alveolus entirely within the maxilla comparable in size to that for the maxillary incisor in PSS-MAE 130. It is, however, situated slightly nearer the canine than is the maxillary incisor in PSS-MAE 130. The left side of PSS-MAE 108 has no alveolus corresponding to that on the right. A third MAE specimen preserves an additional maxillary tooth in front of the canine on one side only. On the right side of cf. Zalambdalestes sp. PSS-MAE 131 is a tooth in front of the canine essentially identical to it in size and morphology, although we cannot evaluate the number of roots on either tooth, as the right side of the specimen is still in the matrix block. As reconstructed from the right premaxilla, which is detached from the skull, this extra tooth is entirely within the maxilla, far from the suture with the premaxilla. Because this extra caniniform is clearly not present on the left side of the specimen, we treat its appearance on the right as an anomaly. Of the last remaining MAE specimen preserving the rostrum, PSS-MAE 129, the occurrence of an extra tooth or alveolus in front of the canine cannot be determined due to damage (fig. 31). The condition in PSS-MAE 131 may reflect a taxonomic difference with previously known Zalambdalestes, but given that the study of this specimen is still in progress, we postpone further consideration of this problem. After discovering these additional teeth in the maxilla in front of the canine, we reexamined the AMNH Zalambdalestes and the stereophotographs of the ZPAL and PIN specimens in Kielan-Jaworowska (1969a, 1984a). The only one that may have borne a maxillary incisor is AMNH 21709, in which there is a narrow gap in the alveolar margin anterior to the right canine in the same place as the alveolus for the maxillary incisor in PSS-MAE 130. The only specimens clearly showing that an alveolus in the maxilla in front of the canine is lacking are AMNH 21708 (fig. 32) and perhaps ZPAL MgM-I/ 13 (see Kielan-Jaworowska, 1984a: plate 17). UPPER CANINE: Gregory and Simpson (1926a) identified the first tooth in the maxilla in AMNH 21708 (fig. 32) and 21704, which is double-rooted, trenchant, and isolated by diastemata, as either a canine or premolar. Simpson (1928a) also questioned the identification of this tooth referring to AMNH 21709, but considered the canine to be the more likely interpretation despite its position well posterior to the premaxillarymaxillary suture. We follow all subsequent authors on the subject (e.g., Kielan-Jaworowska, 1969a, 1984a; Szalay and McKenna,
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