Gaimanlestes, NANUS, 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 : -

GAIMANLESTES ? NANUS ( AMEGHINO, 1891)

Halmarhiphus nanus Ameghino, 1891, p. 308 ; 1894, p. 358; 1898, p. 186.

Referred material: MACN-A 5720 , lectotype of Halmarhiphus nanus Ameghino, 1891 . Fragmentary right mandible with basal part of the alveolus of i2, alveoli of m1, m2-3 present but worn, and root of m4 .

Occurrence: Early Miocene, Santacrucian SALMA. Santa Cruz Formation. Cerro Observatorio, Santa Cruz province, Argentina.

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

F–H, type 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 m 2-4 in occlusal (F), labial (G) and lingual (H) views; I, J, MLP 85-VII-3–9, right mandibular fragment with m2 and alveoli of the p3-m1, detail of m 2 in occlusal (I) and lingual (J) views. Scale bar, 1 mm.

Description: The mandibular body is relatively deep and uniform in depth below the tooth row and has a small retromolar foramen. Labially, there is a small mental foramen that opens below m1. Judging by the alveolar dimensions, m1 was subequal in length to m2, while m2-3 are short and wide. The m2 has a more salient hypoconid than in m3, so the relative trigonid/talonid width is smaller in m2. The anterobasal cingulum is relatively short, reaching the trigonid-talonid boundary in m2 and somewhat longer in m3. Despite m2-3 being worn in both molars (but more clearly in m2), the metaconid is larger than the paraconid and is anteriorly positioned in relation to the protoconid. In m2 the entocristid is partially preserved and appears to have been curved; this character is not comparable in m3. The hypoconulid is reduced and anteroposteriorly compressed in both molars. Given that there is a single alveolus posterior to m3, the missing m4 is inferred to have been uniradiculate.

Remarks: MACN-A 5720 was selected by Ameghino (1891) as the type of Halmarhiphus nanus . Later, Marshall (1980, 1982) considered this species to be a synonym of Stilotherium dissimile . From the present study of MACN-A 5720, we conclude that it can be confidently referred to the Caenolestidae , but its generic determination is uncertain. Several diagnostic characters of the lower dentition of caenolestids can be observed in MACN-A 5720: uniradiculate m4, small and anteroposteriorlly compressed hypoconulid, and lingually curved entocristids. However, because molars are badly worn, other important traits cannot be evaluated and morphological comparisons within the family could not be made. Some of the molar characters in MACN-A 5720 prevent its inclusion in the genera Stilotherium or Caenolestoides , and approximate it to Pliolestes and Gaimanlestes : paraconid smaller than the metaconid, metaconid anteriorly situated in relation to the protoconid in m2-3 and short anterobasal cingulum in m2. Among both genera, MACN-A 5720 appears to be closer to Gaimanlestes due to its molars being basally narrow.

The characters we were able to compare show closer similarities between MACN-A 5720 and G. pascuali , the only known species of Gaimanlestes . However, due to the fragmentary nature of the specimen, some diagnostic characters of Pliolestes (e.g. small p3, well developed lingual shelf in m1-2) could not be evaluated and the relationship of MACN-A 5720 to this genus cannot be ruled out. The evaluated characters show that MACN-A 5720 probably corresponds to a new species, morphologically closer to Gaimanlestes but of uncertain affinities, so it is here referred as Gaimanlestes ? nanus .