Thylamyini Hershkovitz, 1992

Stutz, Narla S., Hadler, Patricia, Negri, Francisco R., Marivaux, Laurent, Antoine, Pierre-Olivier, Pujos, François, Jacó, Tiago R., Fontoura, Emmanuelle M., Kerber, Leonardo, Hsiou, Annie S., Santos, Roberto Ventura, Alvim, André M. V. & Ribeiro, Ana Maria, 2023, New records of marsupials from the Miocene of Western Amazonia, Acre, Brazil, Acta Palaeontologica Polonica 68 (3), pp. 457-475 : 464

Tribe Thylamyini Hershkovitz, 1992 Genus Thylamys Gray, 1843

Type species: Didelphis elegans Waterhouse, 1839 , Recent , Chile, by monotypy .

Thylamys? colombianus Goin, 1997

Fig. 6B.

Material.— UFAC-CS 202, left?m4 from the locality PRE 06 , sampling point C, Envira River , State of Acre, Brazil; Solimões Formation , Acre Basin, (?lower) Upper Miocene .

Measurements.—L = 1.91 mm, W = 0.9 mm.

Description.—UFAC-CS 202 is a small lower molar with high, well-developed trigonid cuspids and a talonid much lower than the trigonid, with scarcely developed cuspids. The paraconid is conspicuous, lower than the metaconid, and slightly labial and distant from it, with a sharp anterior and occlusal tip. The protoconid is large and prominent, being the highest and largest cuspid of the whole molar. The metaconid is opposite to and much lower than the protoconid. The talonid is narrower than the trigonid but is well developed compared to the small didelphids’ condition. The entoconid is laterally compressed and inconspicuous, possibly due to the molar wear, and it seems to be situated more anteriorly than the hypoconid. The hypoconid is short and inconspicuous, being positioned lingual to the protoconid. The hypoconulid is well developed, triangular-shaped, and lingually located, connected to the posthypocristid via a subtle postcristid. The mesiolingual vertical crest of the paraconid forms a keel on the anteriormost part of the molar. The postparacristid is well developed and obliquely oriented, meeting a conspicuous preprotocristid. The postprotocristid is almost transversally oriented, meeting a shorter premetacristid and forming a shallow sulcus. The pre- and post-entocristids are poorly developed, the preentocristid having a labial orientation. The postentocristid connects the entoconid to the hypoconulid. The cristid obliqua is small and almost parallel to the anteroposterior dental axis. A narrow anterobasal cingulid is visible from the base of the paraconid to the base of the protoconid. There is a faint posterior cingulid on the posteriormost wall of the molar. Two roots are preserved, an anterior rounded one and a posterior one, which is larger and ovoid.

Remarks.—We interpret UFAC-CS 202 as being a?m4 due to the presence of a slender and long talonid, a more lingual position of the hypoconid than what is commonly observed on m3, and the absence of a wear-facet on the distal side of the tooth. The posterior cingulid present on this specimen is a plesiomorphic characteristic since the absence of this structure is considered to be the single unambiguous synapomorphy of Didelphidae . However, some taxa have already been reported to possess this feature ( Beck et al. 2022). Compared to basal metatherians that possess a posterior cingulid, UFAC-CS 202 differs from Marmosopsis juradoi Paula Couto, 1962 , by having longer talonid and hypoconulid; it differs from Monodelphopsis travassosi Paula Couto, 1952 , by showing high trigonid, which is wider than the talonid and entoconid subequal to hypoconid in size and height ( Marshall 1987). UFAC-CS 202 is smaller than m4 of the species of Chironectes , Didelphis , Caluromys , Lutreolina , and Philander , but larger than those of Chacodelphys and the extinct didelphid Sairadelphys Oliveira , Nova, Goin, and Avilla, 2011. UFAC-CS 202 differs from the species of Monodelphis in having a longer and narrower talonid. UFAC-CS 202 is smaller than m4 of most species of Marmosa . Morphological differences with the species of Marmosa are an anterolabial cingulid and entoconid less developed and the absence of a labial cingulid. The m4 of the species of Gracilinanus have a larger entoconid and no posterior cingulid, and those of Cryptonanus have m4 with a marked labial cingulid. In the species of Marmosops , the hypoconulid is more posterior, the hypoconid has a more labial position, the metaconid and protoconid are closer and the entoconid is basally larger concerning the condition observed on UFAC-CS 202. UFAC-CS 202 has the same size as m4s of the species of Thylamys ; the position of the trigonid cuspids and the entoconid and hypoconulid is also similar. Regarding other fossil species of Thylamys, UFAC-CS 202 is larger than m4 of Thylamys minutus Goin, 1997 , from the Middle Miocene of La Venta, Colombia ( Goin 1997). Nevertheless, it has approximately the same size as Thylamys colombianus , also from La Venta, which has m3 with W = 1.55 mm and m1? with L = 1.9 mm and W = 0.9 mm (only a partially preserved m4 for this taxon was found; Suárez Gómez 2019). Besides, UFAC-CS 202 also resembles lower molars of T. colombianus in having: (i) a paraconid more mesio-labially displaced, being more distant from other cuspids, (ii) a labio-lingually compressed entoconid, (iii) a long talonid, and (iv) a similar relative entoconid/hypoconulid position. Nevertheless, no well-preserved m4 has been described until now for Thylamys colombianus , hampering a more precise comparison. The m1 of Thylamys cf. T. colombianus from TAR-31 (late Middle Miocene, Peru; Stutz et al. 2022) has a paraconid and a hypoconid more anteriorly and more labially placed, respectively, but with a position of the entoconid/hypoconulid similar to that in UFAC-CS 202. Based on living species, recent gene-based phylogenies suggested that Thylamys diverged from other didelphids later than the presumed age of the PRE 06 deposits (not until the Pliocene, and, for the last common ancestor of Thylamys and Gracilinanus + Cryptonanus , not until ~10 Ma) ( Jansa et al. 2014; Beck and Taglioretti 2020). Thus, we chose to consider Thylamys ? as a generic assignment, pending a better assessment of the concerned Miocene specimens, whether they belonged to this genus or a stem representative of Thylamyini .