Bardalestes hunco, Goin & Candela & Abello & Oliveira, 2009
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publication ID |
https://doi.org/ 10.1111/j.1096-3642.2008.00471.x |
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persistent identifier |
https://treatment.plazi.org/id/F463DC79-213A-F411-FC43-0CF4FDC2B4BB |
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treatment provided by |
Felipe |
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scientific name |
Bardalestes hunco |
| status |
sp. nov. |
BARDALESTES HUNCO SP. NOV.
( FIG. 2A, B View Figure 2 )
Type: LIEB-PV 1135 ( Fig. 2A, B View Figure 2 ), a fragment of left maxillary with M2–3, and roots and part of the crown of M4.
Hypodigm: The type only.
Locality, horizon and age: La Barda, Paso del Sapo (Chubut Province, Argentina). Andesitas Huancache Formation, Early-Middle Eocene.
Measurements: See Table 1.
Diagnosis: Differs from all other paucituberculatans in that the upper molars lack an enlarged, ‘hypoconelike’, metaconule; the paracone is less reduced; the paracone and the metacone are less twinned to StB and StC + StD, respectively; StB much larger than StC + StD.
Description ( Fig. 2A, B View Figure 2 ): The M2 and M3 of the type specimen are very similar to each other, the M3 being longer and wider than the M2. Although its crown is badly broken, it can be seen that the M4 (not shown in Fig. 2 View Figure 2 ) was very short and narrow, and two-rooted. Its width is approximately half the width of M3. The protocone of M2 is comparatively wider than that of M3. In both molars the anterobasal cingulum is narrow and lingually connected with the preparaconular crest. The trigon is ample and transversely long; in the M2 the metacone is somewhat higher than the paracone, whereas in the M3 both cusps are subequal in height. In both molars the protocone is slightly eccentric, as in the upper molar here referred to Bardalestes sp. (see below), it is not in line with the tips of the StB and the paracone. StA is small and very low, StB is comparatively enormous, suboval in section and connected by means of a crest to the immediately posterior cusp (StC + STD; see below). This latter cusp is much smaller and lower than the StB and is even more reduced and close to StB in the M3 than it is in the M2. The postmetacrista is longer than the preparacrista and is orientated parallel to the postparacrista. The apex of the centrocrista is deep and reaches externally the anterolingual slope of StC + StD. In both the M2 and M3 the metaconule is larger than the paraconule.
Comments: One of the most striking features of the upper molars of Bardalestes hunco is the morphology, position, and relative size of StB. This cusp is very large, high and is clearly facing the paracone, even if it is slightly posteriorly shifted in comparison with other South American marsupials. The cusp immediately posterior to StB is much smaller and occupies an intermediate position between the StC and StD of other more generalized marsupials. Thus, it raises the question of the homology of this cusp in Bardalestes . Derorhynchid ‘didelphimorphians’ show some features that ‘anticipate’ the molar pattern of Paucituberculata : large (although not proportionally huge) StB, a deeply V-shaped centrocrista, winged metaconule that is larger than the paraconule, and molars with a salient hypoconid and, in some cases, a slightly labiolingually compressed entoconid. The morphology of the stylar area of the representatives of this family is worth analysing. In this respect, some indeterminate Derorhynchidae from Itaboraí are suggestive: specimen MNRJ 2506-V exhibits, especially in the M1, some proximity of the paracone to the StB, no reduction of the latter cusp and the reduction of the StC and StD, especially the latter. The same features can be seen in the indeterminate Derorhynchidae specimens MNRJ 2352-V and MNRJ-V 2894-V, DGM 813-M, and MNRJ 2894-V, all from the Itaboraí Basin. A morphological pattern similar to that occurring in the stylar region of Bardalestes can be even more clearly appreciated in the derorhynchid Pauladelphys , from the Middle Eocene of Antarctica and Early Eocene of Paso del Sapo (Goin, Tejedor & Abello, 2000, 2001). In the only known upper molar of Pauladelphys juanjoi (MLP 96-I-5-45), an isolated left M1, a manifest closeness between the reduced StC and StD is observed, so that the latter cusp is relatively distant from the metastylar corner of the tooth. The same condition is seen in upper molars referable to Pauladelphys sp. from Eocene levels of Paso del Sapo (Tejedor et al., in press). In some of these specimens (e.g. LIEB-PV 1116, LIEB-PV 1114), StC and StD are almost subequal and twinned, so that their bases are fused. This pattern suggests that the cusp posterior to the StB of the Bardalestes specimens studied here may have been the result of the fusion of the stylar cusps StC and StD. The subsequent evolution of this cusp in the more derived Paucituberculata may have involved an increase in size and its setting apart from the StD. Interestingly, in caenolestids such as Stilotherium dissimile , the stylar cusp posterior to the StB (which we homologize here with StC + StD) maintains its relative size in the upper molar series: it is larger than the StB in the M1, subequal to StB in the M2, and smaller and closer to the StB in the M3. Even though no M1s are known among the Bardalestes specimens described here, it can be seen that in the M3 of this taxon the StC + StD is smaller and closer to the StB than in the M2.
In brief, from the assemblage of Paleogene South American ‘opossum-like’ marsupials, the upper molar pattern that occurs in the derorhynchid Pauladelphys may be an indication of the process underwent by the earliest paucituberculatans. In the particular case of the stylar area, the small cusp posterior to the StB seems not to be either the StC or the StD but rather the result of the fusion of both cusps.
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