Archaeopithecidae (Mammalia, Notoungulata) from the early Eoceno of Patagonia, Argentina

LOPEZ G. M.; BAUZA, NICOLAS; GELFO J. N.; BOND M.

Congreso; VI Congreso Latinoamericano de Paleontología de Vertebrados; 2018

The Archaeopithecidae join together small notoungulates from the Eocene with ageneralized dentition, brachydont molars but with an incipient trend towards the increasein crown height. Although this family has a very low taxonomic diversity, the tworecognized genera, Archaeopithecus and Acropithecus are very common in EocenePatagonian outcrops. Recently, a formal revision of the family recognized Archaeopithecusrogeri as the only valid species, while the rest of the taxa were regarded as juniorsynonyms. The stratigraphic range of the group was also restricted to the middle Eocene(i.e. Vacan and Barrancan subages) and several older and younger records were consideredas uncertain. Here we preliminarily describe the Archaeopithecidae from Laguna Fríalocality in the Paso del Sapo area (Northwestern Chubut province, Argentina). Theremains come from outcrops at the Volcanic-Pyroclastic Complex, and their early Eoceneage is inferred from geochronological data taken from nearby volcanic deposits thatunderlie and overlie the mammal-bearing deposits. At least two different archaeopithecidmorphotypes can be identified from several well-preserved maxillary fragments plusisolated upper and lower molars. The first type, represented by a left maxillary fragmentwith P4-M3 (LIEB-PV 1625) and a right one with P4-M3 (LIEB-PV 4937), resembles A.rogeri but with some clear differences that justify the erection of a new taxon: 1) premolarsand molars with a lower crown height; 2) anterolabial cingulum of premolars more clearlymarked and robust; 3) P3-4 without lingual furrow in the protocone; 4) M1-2 withprotocone and hypocone subequal; 5) M1-2 posterolabial fossette more persistent than themesiolabial one. These characters are here considered as more generalized or primitivethan those of A. rogeri, something congruent with the older age of the Paso del Sapospecimens. The other morphotype, only represented by a right maxillary fragment withP3-M2 (LIEB-PV 1627) differs from known archaeopithecids by the presence of a labialpillar or column in the face of P3-4. This structure is independent from the ectoloph,reaches halfway up the crown in P3 but up to the occlusal surface in P4. This labial pillarwas also mentioned by Simpson for a P4 in a specimen referred to Acropithecus rigidusfrom Cañadón Vaca (Vacan Subage) of Chubut province. Even though Simpson suggestedthe possibility of a teratological tooth, the repeated presence of this pillar in otherspecimens could be interpreted as a derived structure, reinforcing the presence of a newtaxon. These new records increase the diversity of Archaeopithecidae and expand theirbiochron to the early Eocene.