Didelphidae, Gray, 1821

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 : 466-469

Didelphidae indet.

Fig. 7.

Material.— UFAC-CS 412, right p1; UFAC-CS 414, right?p2; UFAC-CS 416, right?P3; UFAC-CS 427, left dp3 from the locality PRE 06 , sampling point C, Envira River , State of Acre, Brazil; Solimões Formation , Acre Basin, (?lower) Upper Miocene .

Measurements.—UFSC-CS 412 L = 2.27 mm, W = 1.18 mm; UFAC-CS 414 L = 3.36 mm, W = 1.54 mm; UFAC-CS 416 L = 3.24 mm, L = 1.95 mm; UFAC-CS 427 L = 2.3 mm, W tal = 1.42 mm, W tri = 1.32 mm.

Description.—UFAC-CS 412 ( Fig. 7B): small and slender p1 displaying two roots. The crown consists of one single central cuspid, rounded and short (i.e., stocky). It is followed by a subtle cristid running postero-labially. A small cingulid is present posterior to the cuspid, with a minute labially situated accessory cuspid.

UFAC-CS 414 ( Fig. 7C): this specimen is larger than UFAC-CS 412. It is robust, with one single cuspid, which is broken and abraded. Posteriorly to this cuspid, there is a tenuous cristid labially oriented. A posterior cingulid is present, with a marked accessory cuspid.

UFAC-CS 416 ( Fig. 7A): this specimen is a large upper premolar with two roots. A subtle anterior cingulum is present. There is only one cusp, which is high, robust, and posteriorly oriented. The anterior margin of the cusp is rounded and at the posterior one is a marked, sharp, and centrally oriented crest. There is a posterior cingulum, which is particularly developed in its labial margin. A small accessory cusp is present, occupying a posterior labial position.

UFAC-CS 427 ( Fig. 7D): this specimen does not possess any signs of roots in its ventral side, which is open, exposing a broad portion of the tooth’s interior. It presents an oval crown outline and robust molariform pattern, being anteroposteriorly compressed and with a rounded labial margin. The trigonid is much higher than the talonid, especially the proto- and metaconids. Proto-, meta-, hypo-, and entoconid are large and blunt. A small, but individualized paraconid is present, which is labially displaced. The protoconid is a large and high cuspid, connected to the paraconid by a short preprotocristid. The protoconid and metaconid are closely positioned, without an evident postprotocristid. The protoconid is higher than the metaconid, but the latter is also a large cuspid. The talonid is short. The hypoconid appears slightly broken on its dorsal tip and labially displaced, evincing a wide talonid basin. The entoconid is robust and anteroposteriorly long, with marked, but short pre- and postentocristids. A minute hypoconulid can be observed, which is twinned with the entoconid and situated at its posterolabial base. The premetacristid is not visible, but the postmetacristid is strongly developed. The cristid obliqua is marked and obliquely oriented, but short. The posthypocristid is marked, short and slightly oblique in relation to the anteroposterior axis of the tooth. A discrete and short anterolabial cingulid is present.

Remarks.—We identify UFAC-CS 412 as a lower p1 due to its size, and inconspicuous posterior cingulid and accessory cuspid. UFAC-CS 414 is tentatively identified as a p2 because of its size and the development of the posterior cingulid and posterior accessory cuspid. UFAC-CS 416 is considered to likely be a P3 based on its rounded anterior and cutting posterior margins, as observed in didelphids, except for species of Caluromys Allen, 1900 , Caluromysiops Sanborn, 1951 , Glironia Thomas, 1912 , and Hyladelphys Voss, Lunde, and Simmons, 2001 ( Voss and Jansa 2009). According to Voss and Jansa (2009), didelphids, together with other marsupials that display plesiomorphic dental traits (such as microbiotheres), have standard lower premolars with only one dominant cuspid, and a distal cingulid frequently producing a posterior accessory cuspid. The P3 of paucituberculatans has a posterior portion higher than the anterior one and palaeothentoids generally have uniradicular and morphologically indistinct p1 and p2 ( Abello 2007). Members of the clade Argyrolagidae have reduced premolars nearly styliform ( Simpson 1970). Therefore, we preferred to classify the three specimens mentioned above as unidentified Didelphidae due to (i) the lack of diagnostic characteristics on didelphid premolars; (ii) and Didelphidae being the only metatherian family already recognized at the outcrop PRE 06.

UFAC-CS 427 is identified as a left dp3 due to the absence of robust roots and very closely positioned protoconid and metaconid. Compared to other Miocene didelphids, UFAC-CS 427 does not have a compatible size with Thylamys? colombianus and Didelphis cf. D. solimoensis , also found at PRE06 (see above) or with the two Thylamys spp. from the La Venta deposits ( Suárez Gómez 2019). Besides, it is smaller than dp3s of Marmosa laventica Marshall, 1976a , also from La Venta, which possess very different occlusal morphology, with, for instance, well-developed anterolabial cingulid and hypoconulid ( Suárez Gómez 2019). UFAC-CS 427 is also different from Lutreolina materdei Goin and de los Reyes, 2011, from the Upper Miocene of Acre River, in Peru, which has large anterolabial cingulid and paraconid and hypoconid not labially displaced, among other distinctive characteristics. However, UFAC-CS 427 and Lutreolina materdei bear a minute hypoconulid (Goin and de los Reyes 2011). When compared to modern didelphids, UFAC-CS 427 stands out by its molariform morphology, with a complete trigonid, differing from the species of Lestodelphys Tate, 1934 , Marmosa Gray, 1821 , and Thylamys Gray, 1843 , which possess a blade-like trigonid on the dp3, and from those of Caluromys , Caluromysiops , and Tlacuatzin Voss and Jansa, 2003 , which have a very small paraconid or no paraconid at all. Hyladelphys is another didelphid with a peculiar dp3, which is significantly reduced ( Voss and Jansa 2009). Besides, UFAC-CS 427 shows a very discrete anterolabial cingulid, which is a more developed structure in the species of Chironectes Illiger, 1811 , Lutreolina Thomas, 1910 , Metachirus Burmeister, 1854 , Monodelphis Burnett, 1830 , and Philander Brisson, 1762 . UFAC-CS 427 also possesses labially displaced paraconid and hypoconid, differing from the species of Chironectes , Glironia , and Lutreolina . The protoconid and metaconid are very close in UFAC-CS 427, distinguishing it from the species of Hyperdidelphys Ameghino, 1904 , Lutreolina , Metachirus , and Philander . Other distinctive features of UFAC-CS 427 are a short preprotocristid, distinguishing it from the species of Metachirus , which have a long and oblique preprotocristid, a short talonid (long in Didelphis ), and a large entoconid (small in Marmosops Matschie, 1916 ). The most outstanding distinctive feature of UFAC-CS 427 is its extremely reduced hypoconulid compared to the species of Chironectes , Didelphis , Glironia , Lutreolina , Marmosops , Monodelphis , Philander , and Thylophorops Reig, 1952 . The species of Cryptonanus Voss, Lunde, and Jansa, 2005 , Gracilinanus Gardner and Creighton, 1989 , and Chacodelphys Voss, Gardner, and Jansa, 2004 , have much smaller teeth than UFAC-CS 427. The corresponding didelphid was approximately Marmosa -sized. Thus, the scarcity of the material and the set of characteristics found in UFAC-CS 427 do not allow its attribution to any of the described didelphid genera, pending the discovery of more specimens.

Order Paucituberculata Ameghino, 1894

Superfamily Palaeothentoidea Sinclair, 1906

Family Palaeothentidae Sinclair, 1906

Subfamily Palaeothentinae Sinclair, 1906 Palaeothentinae indet.

Fig. 8A, B.

Material.— UFAC-CS 65, left M2; UFAC-CS 418 right?M2 from the locality PRJ 33 ’ (UFAC-CS 65); and PRJ 25 (UFAC-CS 418), Juruá River , State of Acre, Brazil; Solimões Formation , Acre Basin,? Middle Miocene .

Measurements.—UFAC-CS 65 L = 1.91 mm, W = 2.2 mm; UFAC-CS 418 L = 1.98 mm, W = 1.91 mm.

Description.—UFAC-CS 65 ( Fig. 8A) is an upper molar, quadrangular in occlusal outline, with a much broader base compared to the crown in lateral perspective. Anterolingually to the StB, at its base, there is a smaller cusp, the paracone, that is poorly developed. The protocone is wide but short and is placed opposite to StB. The metacone is a well-developed cusp, slightly smaller than StC+D, and placed on the anterior portion of the latter. The trigon basin is deep. The metaconule is well developed and wide but not much higher than the talon basin, having approximately the same height as the protocone. It is located opposite to StC+D. At the base of the metacone, there is a sulcus separating it from the metaconule. The postprotocrista is subequal in length to the preprotocrista ( Fig. 8A 3); dorsal to it there is a shallow entoflexus. The postmetaconular crest is moderately wide and labially oriented, the molar having a shorter outline, thereby differing from the condition that characterizes M1s of other palaeothentids. A narrow anterior cingulum extends from a point immediately anterior to the StB, converging toward the preprotocrista, but oblique and dorsally to the latter. The StA is vestigial or absent. The StB is conical-shaped and represents the highest cusp. The StC+D is well developed but smaller than the StB, from which it is separated by a deeply excavated ectoflexus, with a small style at its base.

UFAC-CS 418 ( Fig. 8B) is also a quadrangular-shaped upper molar, appearing worn and abraded due to transport, the labial and lingual sides of the molar having a thicker enamel layer compared to the structures on the occlusal surface. The paracone is small and twinned with the StB, being placed at the antero-lingual slope of the latter. The protocone is well developed, but short. The metacone is twinned with the StC+D, but shorter than the latter and located on its anterior portion. The trigon basin is deep, small, and narrow. Opposite to StC+D there is a well-developed metaconule, preceded by a short premetaconular crest, with a small premetaconular cuspule, and followed posteriorly by a longer postmetaconular crest that runs parallel to the lingual side of the tooth. Both the preprotocrista and postprotocrista are short. A broad but short anterior cingulum is present, running from a well developed StA to almost the dorsolingual base of the protocone. Beneath the metaconule and protocone, there is a deep entoflexus on the lingual margin of the tooth. There is a deep U-shaped ectoflexus on the labial margin of the tooth. The StB is larger and longer than the StC+D. The StC+D is well-developed and the highest cusp of the molar.

Remarks.— UFAC-CS 65: the presence of a premetaconular cusp and the metacone situated in the first half of the StC+D allow us to assign this specimen to the family Palaeothentidae according to Abello (2007). Also, according to this same author, the protocone and metaconule directly opposite to StB and StC+D, regarding an axis perpendicular to the anteroposterior one, unambiguously point to Palaeothentinae affinities for UFAC-CS 65. Conversely, decastine palaeothentids are characterized by a protocone posterior to the StB on M1–2, differing from UFAC-CS 65 ( Abello 2007). Among Palaeothentinae , Carlothentes chubutensis Bown and Fleagle, 1993 , is solely known through lower molars, precluding further comparison with UFAC-CS 65. The upper molars of Palaeopanorthus primus Ameghino, 1902 , although having conspicuous paracone and metacone, have a sizeable premetaconular cusp and a moderately developed to large metaconule, differing from UFAC-CS 65 ( Abello 2007). In the species of Palaeothentes Ameghino, 1887 , a paracone on M2 is only registered in Palaeothentes marshalli Bown and Fleagle, 1993 , Palaeothentes migueli Bown and Fleagle, 1993 , and Palaeothentes serratus Engelman, Anaya, and Croft, 2017 ( Abello 2007; Engelman et al. 2017). Palaeothentes serratus from Quebrada Honda (Middle Miocene of Bolivia), differs from UFAC-CS 65 in possessing a highly reduced metacone. The other species of Palaeothentes from Quebrada Honda, Palaeothentes relictus Engelman, Anaya, and Croft, 2017 , is not represented by upper molars ( Engelman et al. 2017), hampering comparison with UFAC-CS 65. As for P. migueli and P. marshalli , the molars of P. migueli are smaller than UFAC-CS 65, while those of P. marshalli are closer in size to UFAC-CS 65. Besides, P. marshalli has well-developed metacone and teeth that are more triangular-shaped in occlusal view, as UFAC-CS 65. Nevertheless, the M1–M2 of the species of Palaeothentes do not present many significant differences with the ones of Palaeopanorthus primus (i.e., differences in the proportional size of the paracone, metacone and premetaconular crest; Bown and Fleagle 1993; Abello 2007). Therefore, considering that UFAC-CS 65 is an isolated specimen and that the M2 is not a very distinctive tooth among some Palaeothentinae taxa, we chose an open taxonomic identification, highlighting the closer affinities of UFAC-CS 65 especially with P. marshalli , but also to P. migueli and Palaeopanorthus primus .

UFAC-CS 418 can be considered as belonging to Palaeothentoidea due to the differences in the enamel thickness observed between the lateral and occlusal sides of the molar, and as a representative of Palaeothentidae due to (i) the metacone located on the anterior half of the StC+D, and (ii) the presence of a premetaconular cusp on upper molars ( Abello 2007). As mentioned above, for UFAC-CS 65, the M2 of palaeothentines is not very diagnostic; therefore, an open taxonomic identification is again favored. As with UFAC-CS 65, UFAC-CS 418 also has closer affinities with P. marshalli , P. migueli , and Palaeopanorthus primus .

Compared to UFAC-CS 418, UFAC-CS 65 has a less developed anterior cingulum, StA and paracone; a more developed metacone; a more defined triangular outline; and it is larger. Because both are here considered as M2s, these differences would not be related to distinct dental loci but possibly to different taxa. Thus, both UFAC-CS 65 and UFAC-CS 418 are considered to belong to palaeothentines. Yet, a more precise taxonomic assignment would require more associated specimens (especially of other dental loci) and a better knowledge of the morphology of the already described Palaeothentinae .