Argyrohyrax proavus Ameghino, 1897

Pino, Santiago Hernández Del, Seoane, Federico D. & Cerdeño, Esperanza, 2022, New craniodental material of the typotherian notoungulates from the upper Oligocene of Mendoza, central-western Argentina and their taxonomical importance, Acta Palaeontologica Polonica 67 (4), pp. 983-997 : 993-995

Argyrohyrax proavus Ameghino, 1897

Fig. 5A–K ; SOM 1: fig. S5.

Holotype: MACN A 52-472 , palate with left I1–M3 and isolated right I1–I3, C, P1, P3–M3.

Type locality: Cabeza Blanca, Chubut Province, Argentina.

Type horizon: Upper Oligocene, Deseadan SALMA.

Material.— 26 specimens including maxillary and mandibular fragments, and isolated teeth (detailed list in SOM 2 ) from Mendoza Province, Argentina, upper Oligocene , Deseadan SALMA .

Description.—All assigned teeth (SOM 2–3) are comparable in size and morphology to the sample of Argyrohyrax proavus from Quebrada Fiera previously studied ( Vera et al. 2017). The new material includes a juvenile individual, MCNAM-PV 4962 ( Fig. 5A ), preserving DP1–DP4–M1, with more advanced wear than the previously published MCNAM-PV 4059 and 3968 ( Vera et al. 2017: fig. 5d, f). These authors interpreted the first premolar as a P1, while Fernández et al. (2021) supported that there is a DP1 without replacement in the Miocene genera Interatherium and Protypotherium . In both MCNAM-PV 4962 and 4059, the first premolar is rooted (a little piece of bone has been removed in both cases for better observation; SOM 1: fig. S5A), like DP2–DP4, but with a higher crown and embedded deeper in the alveolus, which is evidence that it erupted later; this eruption pattern is also observed in the younger MCNAM-PV 3968 ( Vera et al. 2017: fig. 5f) and the posterior to anterior eruption pattern has been previously established for interatheriines ( Fernández et al. 2021, and references therein). The DP1 (following Fernández et al. 2021) of MCNAM-PV 4962 is simpler than DP2–DP4, subtriangular, longer than wide, with a main labial cusp (paracone) and a short (in height and length) parastyle. DP2–DP3 are characterized by the long parastyle directed anteriorly, less individualized from the paracone fold in DP4; the low crowns of MCNAM-PV 4962 contrast with the observed high crown of P 3 in MCNAM-PV 4961 ( Fig. 5B ). This specimen preserves the left erupting canine (SOM 1: fig. S5A, B) together with the DP1–P2–M2 series in use. The canine has a short parastyle and a low lingual cingulum, as described for DI–DC of MCNAM-PV 3968 ( Vera et al. 2017: 616); its paracone fold is more posteriorly placed than in the following premolars; the parastyle projects labially in P3 and to a lesser extent in P4, while it does not project in DP1–P2 or M1–M2. Dental dimensions of MCNAM-PV 4961 fall in the variation range of Argyrohyrax proavus as provided by Vera et al. (2017), although the premolars are slightly smaller (SOM 3: table 5). MCNAM-PV 4961 also preserves the anterior border of the orbit and a large, circular infraorbital foramen at the level of P4, similar to MCNAM-PV 4233 and MACN A 52- 475 ( Vera et al. 2017: 615, fig. 5a, b). In MCNAM-PV 4986 ( Fig. 5C ), the parastylar groove is fully visible on M1, reaching the base of the tooth; the paracone fold is more labially extended in P4.

Concerning the lower dentition, MCNAM-PV 4245 ( Fig. 5D ) preserves fragments of i1–i3, i2 being the largest incisor. MCNAM-PV 4646 ( Fig. 5E ) bears dp1–p2–p3, p2 clearly larger than dp1 (following the same interpretation as for the upper first premolar); the posterior border of the mandibular symphysis reaches the level of p2 (as in MCNAM-PV 5082, Fig. 5F ), broadly coinciding with MACN A 52-474 ( Vera et al. 2017: fig. 3a). MCNAM-PV 4676–4678 are p2s (SOM 1: fig. S5C, D) of different individuals, rooted, with a V-shaped trigonid with a convex labial face, and a short talonid; they are similar in morphology and size (SOM 3: table 2) to the p2 of MCNAM-PV 4697 ( Vera et al. 2017: fig. 6d, erroneously identified as a p 1 in the figure caption). MCNAM-PV 4963 ( Fig. 5G ) shows dp3 (with the start of roots) and p4–m 2 in situ, and m3 separated; both m2 and m3 show a high crown and part of the roots. In MCNAM-PV 4970, the preserved p4 is high, little worn (narrow union between trigonid and talonid), and the open base of the tooth shows a curiously wrinkled enamel in the labial wall of the trigonid (SOM 1: fig. S5E). In MCNAM-PV 4607 (SOM 1: fig. S5F), trigonid and talonid are still unfused in the m3; its U-shaped trigonid is similar to that of the young individual MACN A 52-473 ( Vera et al. 2017: fig. 3l, holotype of “ Notohyrax conicus ”). MCNAM-PV 4308 ( Fig. 5H ) is a barely worn molar (m3?); occlusally, each lobe shows a centered irregular depression that would be obliterated soon with wear; the length of the trigonid increases to the crown base and the metaconid area extends a little backward, delimited by an anterior groove; the talonid presents two lingual grooves. Similarly, the m3s MCNAM-PV 4622 and 4623, and the m1 or m2 MCNAM-PV 5081 ( Fig. 5I–K ) are little worn, with a narrow contact between the trigonid and talonid.

Stratigraphic and geographic range.— Upper Oligocene, Deseadan SALMA; Sarmiento Formation, Santa Cruz and Chubut provinces; base of the Agua de la Piedra Formation ,

Mendoza Province; and Fray Bentos Formation, Corrientes Province, Argentina .

Figures

Fig. 5. Dental remains (anterior to the left, except for D) of interatheriid notoungulates from Quebrada Fiera, Mendoza, Argentina; upper Oligocene, Deseadan SALMA. A–K. Argyrohyrax proavus Ameghino 1897. A. MCNAM-PV 4962, right maxillary fragment with DP1–DP4–M1, occlusal view reversed). B. MCNAM-PV 4961, maxillary fragments with left C–M2 in occlusal view (B1), see erupting C in SOM 1: fig. S5A, B and right P2–M2 in occlusal view (B2). C. MCNAM-PV 4986, left maxillary fragment with M1–M2 in occlusal view. D. MCNAM-PV 4245, mandibular symphysis with broken teeth in rostral view. E. MCNAM-PV 4646, right mandibular fragment with broken dp1–p2–p3 in occlusal view. F. MCNAM-PV 5082, right mandibular fragment with p2–m1 in occlusal view. G. MCNAM-PV 4963, right mandibular fragment with dp3–p4–m2 and separated m3 in occlusal G1, G2) and lingual (G3, G4) views. H. MCNAM-PV 4308, right lower molar (unerupted m3?) in occlusal view. I. MCNAM-PV 4622, left m3 (reversed) in occlusal view. J. MCNAM-PV 4623, right m3 in occlusal view. K. MCNAM-PV 5081, left m1 or m2 (reversed) in occlusal view. L. Interatheriidae indet. MCNAM-PV 4724, right upper molar in occlusal (reversed) (L1) and opposite (L2) views.