CAENOLESTIDAE, Abello & Martin & Cardoso, 2021

Abello, María Alejandra, Martin, Gabriel M. & Cardoso, Yamila, 2021, Review of the extinct ‘ shrew-opossums’ (Marsupialia: Caenolestidae), with descriptions of two new genera and three new species from the Early Miocene of southern South America, Zoological Journal of the Linnean Society 193 : -

CAENOLESTIDAE SP. NOV. AFF. PLIOLESTES

( FIG. 9H–K)

Referred material: MMP 975-M, left mandibular fragment with alveoli for i2, p3-m2 and m4, and complete but worn m3 ( Pascual & Herrera, 1973, Lám. I, fig. 5–6, Lám. II, figs 5–7).

Occurrence: LateMiocene, ChasicoanSALMA ( Marshall et al., 1983; Lower Chasicoan, Tonni et al., 1998). Arroyo Chasicó Formation, Vivero Member. Arroyo Chasicó locality. Buenos Aires province, Argentina.

Measurements: Supporting Information, File S6, Table 1a.

Description: Compared with the remaining caenolestids, specimen MMP 975-M has a mandibular body that is not of uniform height throughout its length, but is higher below p3-m1 and lower below m4 (this character is similar to other non-caenolestoid paucituberculatans). The mandibular portion between the alveoli of i2 and p3, was not preserved. Judging by the size of its alveoli, the p3 was proportionally larger with respect to m1, larger than in species of Pliolestes and similar in its relative size to the p3 of other caenolestids (see Supporting Information, File S6, Table 1b). The alveolar length of m1 is subequal to that of m2, and larger than m3. Posterior to m3 is a single alveolus for an uniradiculate m4. As in P. tripotamicus , m3 has a nearly subequal trigonid and talonid width. The anterobasal cingulum is stout and long, and extends up to the hypoconid base. Despite the molar being heavily worn, several structures have been preserved and are still clearly visible. On the trigonid, the metaconid is anteriorly positioned with respect to the protoconid, and the paraconid is slightly smaller than, and coalescent to, the metaconid. From the bases of the entocristid and cristid obliqua, we infer that the talonid basin was labiolingually narrower than that of the m3 of P. tripotamicus . The entocristid, judging from its base, was lingually curved and contacted the posterior wall of the metaconid. The protocristid is interpreted to have been oblique (i.e. anterolabially to posterolingually oriented). The preserved base of the entoconid shows that it was a relatively small cusp in relation to the total occlusal area of the molar.

Remarks: MMP 975-M was originally referred to as Pliolestes sp. , probably based on the general morphological similarity with specimens of Pliolestes tripotamicus ( Pascual & Herrera, 1973) . Later, the better knowledge of Pliolestes species and the remaining extinct caenolestids led Abello (2007) to consider MMP 975-M as a taxon related to, but not included within, the genus Pliolestes . As a result of the present revision, we agree with Abello (2007) and consider MMP 975-M to be a Caenolestidae , and that the specimen has characters that do not agree with those diagnostic of Pliolestes . The inclusion of MMP 975-M within Caenolestidae is based on the presence of a lingually curved entocristid, an inferred uniradiculate m4 and an oblique postcristid. In addition, we were unable to corroborate if the hypoconulid was anteroposteriorly compressed (a generalized character among Caenolestidae ) due to its heavy wear, but we can infer a small overall size similar to that of other members of this family. Among caenolestids, MMP-M 975 closely resembles Pliolestes and the new genus Gaimanlestes in having paired para- and metaconid, and by having the paraconid in a forward positioned with respect to the protoconid. Among both genera, MMP-M 975 more closely resembles Pliolestes than Gaimanlestes , because both have basally wide molar crowns, whereas Gaimanlestes has more basally compressed molars. Also differing from Gaimanlestes , and thus more similar to Pliolestes , MMP-M 975 has close para- and metaconid. Although MMP-M 975 has greater similarities with Pliolestes , the inferred well-developed p3, the long anterobasal cingulum and the particular mandible morphology prevents us from including it in this genus. These latter features and those shared with Pliolestes and Gaimanlestes , suggest that MMP-M 975 has a particular character combination, which would indicate that it represents a new species, probably more closely related to Pliolestes spp. Further findings of more complete specimens of this taxon should help resolve its closer phylogenetic relationships in the context of the Caenolestidae but, given the current uncertainty, we prefer to leave specimen MMP-M 975 (in open nomenclature) as Caenolestidae sp. nov. aff. Pliolestes .