Bardalestes undetermined
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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/F463DC79-2138-F410-FEF5-0B23FAC7B2FE |
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treatment provided by |
Felipe |
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scientific name |
Bardalestes undetermined |
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BARDALESTES SP. ( FIG. 2C–F View Figure 2 )
Referred specimens: MLP 90-II-5-336, an isolated upper molar (M?2; Fig. 2C, D View Figure 2 ) lacking the metastylar area; MLP 90-II-5-335, an isolated left protocone; MLP 90-II-5-337, an isolated right protocone; MLP 90-II-5-300 ( Fig. 2E, F View Figure 2 ), an isolated right talonid.
Locality, horizon and age: Easternmost part of the Southern Cliff of Colhué Huapi lake, Sarmiento Department, Chubut Province, Argentina. Las Flores
Formation, Río Chico Group; Late Palaeocene (Itaboraian Age).
Measurements: See Table 1.
Description ( Fig. 2C, D View Figure 2 ): Even though broken and mostly missing the metastylar area, it is evident that MLP 90-II-5-336 is a wide molar (i.e. its labio-lingual diameter is larger than the mesio-distal one). The trigon basin is labio-lingually wide because of the deep centrocrista. The protocone is moderately developed; its tip is relatively centrally placed, that is, it is not aligned in a transverse axis with StB and the paracone. The preprotocrista is not symmetrical to the postprotocrista as the metaconule, larger than the paraconule, is posteriorly salient. Both the paracone and the metacone are worn down almost to their bases; from their respective sections it can be inferred that the paracone was only slightly smaller than the metacone. The preparacrista is subequal in length to the postparacrista and the premetacrista; it is almost transversally orientated with respect to the dental axis. The postparacrista and the premetacrista are subequal in length; they are not symmetrical to each other but instead the premetacrista is slightly more transverse to the dental axis. The postmetacrista has preserved only its proximal end which is parallel to the postparacrista; its preserved portion suggests that it was moderately developed. The paraconule is half the size of the metaconule; both cusps are ‘winged’, i.e. they have pre- and postconular cristae. The preparaconular crest continues labially in the anterobasal cingulum; the postparaconular crest is reduced or absent; in turn, and even though worn, the premetaconular crest seems to have been longer and more subhorizontal than the postmetaconular one, which descends abruptly downwards and posterolabially. The stylar shelf is moderately developed and is deeply penetrated by the centrocrista at its central portion. The ectoflexus is almost undistinguishable; StB and StC + StD are very close together and have no labial emargination. A tiny cuspule which is very low and subcircular in section can be observed behind the labial apex of the centrocrista. This cuspule is not located near the labial face of the tooth, as are other stylar cusps, but instead it is internally placed on the stylar shelf. The most conspicuous feature of the remaining stylar cusps is the very large size of StB, which occupies most of the anterior half of the stylar shelf. StB is not only large but also very high; it is suboval in section, as it is slightly labiolingually compressed. Its labial and lingual slopes are steep; anteriorly it has a rounded edge, whereas the posterior one is crest-shaped, trenchant in aspect. The metastylar area is broken behind StC + StD. At the lingual slope of StB a minute wrinkle can be observed, just posterior to the labial end of the preparacrista. Compared to other ‘opossum-like’ marsupials, it is evident that the lingual slope of StB and the labial slope of the paracone in Bardalestes are very close to each other relative to the total width of the upper molar (see the reconstruction of the paracone in Fig. 2D View Figure 2 ). The anterolabial corner of the tooth, poorly preserved, is rather small; a moderately developed but very low StA can be inferred from it. The anterior cingulum is poorly developed.
Three additional specimens are also referred to Bardalestes sp. Two of them ( MLP 90-II-5-335 and MLP 90-II-5-337) consist of isolated protocones, one left and the other right; they are roughly similar, in size and morphology, to the protocone of the already described upper molar. The third specimen ( MLP 90-II-5-300; Fig. 2E, F View Figure 2 ) consists of a fragmentary right molar preserving the talonid and part of the posterior wall of the trigonid. Judging from the preserved portion of the metacristid, the trigonid was much higher than the talonid. The latter has a salient hypoconid, a small, laterally compressed entoconid, and a reduced hypoconulid located posterior to the entoconid; the talonid basin is relatively deep; the cristid obliqua ends anteriorly near the labial base of the protoconid; the distal end of the entocristid in m1–3 is relatively high ( Fig. 2F View Figure 2 ). There is a short postcingulum at the posterior face of the talonid; the posthypocristid is almost transverse to the dental axis.
One additional specimen can also be also referred to Bardalestes sp. : MLP 90-II-5-332. This specimen consists of an isolated talonid belonging to a mx. Its morphology resembles that of MLP 90-II-5-300. The specimen was polished in order to examine its enamel structure (specimen KOE 2991; Koenigswald & Goin, 2000). As in the upper and lower molars of the Caenolestidae , the enamel in Bardalestes shows prisms that are orientated more or less straight between the enamel–dentine joint and the outer enamel surface. A thick interprismatic matrix orientated at an angle with the prisms could be observed. This pattern is common among other generalized ‘opossum-like’ marsupials ( Koenigswald & Goin, 2000: 149).
Comments: Among the ‘opposum-like’ marsupials collected from the Las Flores Formation, represented by more than 700 isolated teeth, there are a few taxa of very small size. The specimens here assigned to Bardalestes sp. are among the smallest of them. Specimen MLP 90-II-5-300 has the hypoconulid twinned to the entoconid, a synapomorphy of Late Cretaceous and younger marsupials (Kielan- Jaworowska, Cifelli & Luo, 2004). In addition, the laterally expanded hypoconid, and the labio-lingually compressed entoconid are features diagnostic of the Paucituberculata (see below). Specimens MLP 90-II- 5-335 and MLP 90-II-5-337 are almost identical in size and morphology. In turn, specimen MLP 90-II-5- 300, an isolated lower molar fragment, matches in size and shape the expected pattern for an occlusal antagonist to the type specimen. In short, their size, morphology, and inferred occlusal relationships concur in assigning all specimens to the same taxon.
The size and morphology of specimen LIEB-PV 1135, type of Bardalestes hunco , are very similar to those of specimen MLP 90 View Materials -II-5-336. The greatest differences between the latter and LIEB-PV 1135 are the following: StC + StD larger in the latter, which in addition lacks the cuspule posterior to the centrocrista apex, and, especially in M2, more posteriorly located with respect to the StB. Regarding their size, specimen MLP 90 View Materials -II-5-336 is slightly larger than LIEB-PV 1135. In all other features, the M3 of LIEB-PV 1135 is indistinguishable from specimen MLP 90 View Materials -II-5-336, for which reason we assume the latter to be an M3 as well. The finding of new material could prove that the small differences between them fall within the variability range of a single species, as the greatest difference is the increased size of the cusp posterior to the StB (which we consider homologous to the fusion of cusps StC+ StD) .
| MLP |
Museo de La Plata |
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