Protypotherium compressidens Ameghino, 1891b

(Figs 5, 6, 8E–K; Supporting Information, Table S2) 1891b Protypotherium compressidens Ameghino, p. 292 (original description).

Holotype: MACN-A 4029 (Fig. 8E), less maxillary fragment with P3–M3, and MACN-A 4030 (Fig. 8F), less mandibular fragment with p4 (talonid)–m3, of the same individual (see Fernández et al. 2018), Early–Middle Miocene, SCF, Santa Cruz Province (Argentina).

Referred materials: MACN-A 41-55 lot b.f, less mandibular fragment with poorly preserved p3–4 (trigonid); MACN-A 3867 (Fig. 8G), right mandibular fragment with dp1–2–p3–4 (trigonid); MACN-A 9716–9732?g (Fig. 8H), right mandibular fragment with m1 (talonid)–3; MPM-PV 19761 (Fig. 8I), right maxillary fragment with M1–2; MPM-PV 19781 (Fig. 8J), right mandibular fragment with p2–m1; and MPM-PV 19782 (Fig. 8K), less maxillary fragment with P2–M3.

Extended diagnosis: Protypotherium compressidens differs from Pr. antiquum, Pr. australe, Pr. praerutilum, Pr. columnifer, Pr. claudum, Pr. minutum, and Pr. colloncurensis by narrower cheek teeth, even more noticeable in P2–M3; from Pr. antiquum by the talonid of m3 without a labial groove; from Pr. colloncurensis and Pr. columnifer by the talonid of p2–4 sub-triangular in outline; from Pr. claudum by having p2–3 with a distinctive entoflexid; from Pr. colloncurensis by a descending process of the maxilla with a moderate development; from Pr. praerutilum by having a larger size; from Pr. minutum and Pr. distinctum by less cement covering the cheek teeth, the P2–4 with an entoflexus located in a more distal position that is slightly less expanded, M1–3 with straighter ectolophs, and infraorbital foramen above premolars; from Pr. columnifer by the talonid of p4 narrower than the trigonid; and from Pr. antiquum by the talonid of m3 without its labial groove.

Geographic and stratigraphic provenance: Santa Cruz Province (Argentina), SCF, Early–Middle Miocene, Santacrucian SALMA.

Measurements: Supporting Information, Table S2.

Remarks: The ratio between the width and length of each upper molar (R W/L) of Pr. compressidens (holotype plus referred materials) for M1 and M2 is <0.5, whereas it is ≥ 0.55 in Pr. australe and Pr. praerutilum, and for M3 (with enlarged distal lobe), R W/L ≤ 0.42, whereas it is ≥ 0.47 in Pr. australe and Pr. praerutilum . The scaưer diagrams for M1 (Fig. 5A) and M2 (Fig. 6A) show the separation of these species in terms of the length and width of M1–2. A similar condition is observed in m1 and m2: R L/W ≤ 0.41 in Pr. compressidens, whereas it is ≥ 0.47 in Pr. australe and Pr. praerutilum, and in m3 (with enlarged talonid), R W/L ≤ 0.33 for Pr. compressidens, whereas it is ≥ 0.35 for Pr. australe and Pr. praerutilum .