Caenolestoides miocaenicus, 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 : -

CAENOLESTOIDES MIOCAENICUS SP. NOV.

( FIG. 10F–J)

Zoobank registration: lsid: urn:lsid:zoobank.org:act:42675F71-06BC-49C9-83C0-62689384476C

Etymology: The epithet is composed of the geological era Miocene (in turn from Ancient Greek μείων, less and καινός, new) and the Latin suffix –icus, belonging to, in reference to its chronologic provenance.

Holotype: MPEF-PV 553 , right mandibular fragment with alveoli of the i2, c and p1, fragmentary p2, roots of p3, posterior root and talonid of m1 and complete m2-4 ( Fig. 10F–H).

Referred material: MLP 85-VII-3–9, right mandibular fragment with m2 and alveoli of the p3-m1 ( Fig. 10I, J).

Temporal distribution: Early Miocene, Colhuehuapian SALMA.

Stratigraphic and geographic distribution: Sarmiento Formation, Colhue-Huapi Member. Gran Barranca, Chubut province, Argentina.

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

Diagnosis: Caenolestoides miocaenicus differs from the remaining caenolestids by a (1) long anterobasal cingulum in m2-3, (2) transversally aligned meta- and protoconid in m2-3, (3) close para- and metaconids in m2-3, (4) presence of a shelf lingual to the entocristid of m1-3, (5) entoconid of m2-3 posteriorly situated with respect to the hypoconid, (6) deep protocristid notch in m2-3 and (7) birradiculate m4.

Description: Mandibular body of the type specimen, MPEF-PV 553, is somewhat higher below the molars than in the antemolar region. Labially, it has three mental foramina, with the posterior one smaller and opening below the m1-2 boundary, the intermediate foramen opens below p3, and the anterior largest one, opens below p2. Despite the mandibular body being broken in front of the anterior mental foramen, a well developed, anterior and forwardly directed groove is present. Anterior to the partially preserved p2, there are two alveoli; the anterior one is half the size of the posterior one and both are forwardly inclined, suggesting that they housed somewhat procumbent teeth. The p2 is biradiculate, has the anterior root smaller than the posterior one, is long and narrow, and probably was twice the size of the anterior tooth. The main cusp is missing, but comparing its base with the preserved talonid, it could be inferred that the talonid was as long as the main cusp. The talonid narrows backwards, ending in a small cuspule. Given the dimensions of the roots of p3, this premolar was clearly larger than p2. The m1 is only represented by its posterior root and the lingual half of the talonid, would have been equal or somewhat larger than m2. On the preserved part, the entoconid base shows this cusp might be similar in size to that of m2. The morphology of the talonid in front of, and lingual to, the entoconid suggest that a small shelf was developed lingually to this cusp; however, being incompletely preserved, the shelf cannot be compared in detail with that present in other caenolestids, such as Pliolestes spp. and C. fuliginosus . The m2 is an elongated molar, with an anteroposteriorly shortened trigonid. The anterobasal cingulum is long and well developed, extending posteriorly to the hypoconid base. The protoconid is the largest trigonid cusp and is transversely aligned with the metaconid. The paraconid is the smallest trigonid cusp, located next to the metaconid and separated from this cusp by a narrow valley formed by the posterior wall of the paraconid, and the anterior wall of the metaconid. As in all other caenolestids, the paracristid is divided into a postparacristid and a preprotocristid. The postparacristid is transversely oriented and the preprotocristid is somewhat parallel to the anteroposterior axis of the molar. In contrast to extant cenolestids, the protocristid has a deep notch. The talonid is elongated with respect to the trigonid, and the most notable characteristic of the talonid is the position of the entoconid, which is posteriorly displaced relative to the hypoconid. The entoconid is relatively small and labiolingually compressed. The m2-3 entocristid is represented by a proximal portion that is oriented towards the talonid basin, as in all other caenolestids. The distal portion, which in other caenolestids is extended forward to the posterior wall of the trigonid, is absent or, more probably, worn. Therefore, entocristids do not show the typical curved morphology observed in the caenolestids. The talonid basin of m2 is anteroposteriorly wide and is labially limited by a high cristid obliqua. Towards the lingual border of the basin, a well-developed shelf extends between the metaconid and the entoconid. The hypoconid is labially salient and the postcristid is oblique, and although worn, it can be inferred that the postcristid should have been high. The hypoconulid is anteroposteriorlly compressed and is placed somewhat posterior to the entoconid. The morphology of m3 is close to that of m2, specially the talonid structure. The major differences among both molars are in the trigonid cusp arrangement. In m3, the paraconid is more reduced in relation to the metaconid and is more labially situated with respect to this cusp than in m2. The para- and metaconid are also more closely situated between each other than in m2. The m4 is the smallest molar of the series, it is biradiculate and reduced with respect to m3. As in the anterior molars, the trigonid is anteroposteriorly shortened. The meta- and protoconid are coalescent and the paraconid, although reduced, is well differentiated and placed somewhat labial to the paraconid. The two portions of the paracristid have the same orientation than in the anterior molars. The talonid is quadrangular in occlusal view, somewhat wider than the trigonid, and lacks any recognizable structure.

When compared with the type specimen, MLP 85 -VII-3–9 has a shallower mandibular ramus but similar alveolar dimensions of p3-m1. In the preserved portion of this specimen there is no mental foramina. The only preserved molar, m2, is morphologically similar to that of MPEF-PV 553 , but somewhat larger. It is also less worn so that some molar traits are more clearly visible: the hypoconid, cristid obliqua and postcristid are high, and the cristid oblicua has a notch just behind the contact with the posterior wall of the protoconid .

Remarks: Of all extinct caenolestids, Caenolestoides miocaenicus is the most closely related to the living species, with which it shares three main characters of m2-3: long anterobasal cinguli, entoconid posterior to the hyponconid, and lingual platforms between meta and entoconid. Caenolestoides miocaenicus is similar to C. fuliginosus (Tomes) and L. inca Thomas , sharing with these extant species the proximity between para- and metaconid, and a larger, compared size between m4 and m3, clearly different from the reduced m 4 in R. raphanurus . Caenolestoides miocaenicus differs from extant species L. inca and Caenolestes spp. in some plesiomorphic characters such as a p2 shorter than p3 and deep protocristid notch in m2-3. Compared with C. fuliginosus and L. inca , the para- and metaconid in m2-3 are less twinned, the metaconid is slightly larger than the paraconid and the trigonids are proportionately wider than in the living species. Caenolestoides miocaenicus , G. pascuali and S. parvum are the oldest records for the family.