New remains of kollpaniine “condylarths” (Panameriungulata) from the early Palaeocene of Bolivia shed light on hypocone origins and molar proportions among ungulate-like placentals

Abstract

The description of new specimens of kollpaniines “condylarths” from Tiupampa (early Palaeocene of Bolivia) represents a significant addition to the knowledge of the earliest fauna of South American ungulates. Several partial mandibles and maxillae of Molinodus suarezi and Simoclaenus sylvaticus are described. The morphology of the lower premolars of Molinodus, being associated to lower molars, is established and a previous referral of an isolated p4 is rejected. A maxilla of Simoclaenus reveals the morphology of the so far unknown P1-4 of this taxon and allows a discussion on the development of the protocone in Palaeocene “condylarths”. The subvertical maxilla-premaxilla suture and the vertical implantation of the P1/p1 confirm the shortness of the snout of Simoclaenus, whereas the procumbency of the p1 of Molinodus indicates a longer rostrum. The upper molars of Molinodus confirm the presence of a tendency to duplication of the protocone, which is regarded as the incipient development of a pseudohypocone. The various patterns of formation of a hypocone (or pseudohypocone) are considered and, among other South American Native Ungulates, a protocone-derived pseudohypocone (i.e. Molinodus-like) is hypothesized in Lamegoia, Raulvaccia, and notoungulates, whereas a postcingulum-derived, hypocone is present in didolodontids and litopterns.The new specimens confirm the conspicuous small size of the M1/m1 of Molinodus and Simoclaenus as compared to the M2/m2. Consequently, we examined the relative proportions of molars in these taxa as compared to a variety of extant and extinct euungulates. Their proportions were plotted into the ‘developmental’ morphospace based on the predictive mathematical model of Kavanagh et al. (2007) (Inhibitory Cascade Model, or IC model), which might explain a large part of the mammalian diversity in molar proportions. Based on the upper molars, the Tiupampa kollpaniines were retrieved in a separate area of the predicted morphospace with other North American “condylarths” with large M2; this departure is also consistent with previous results concerning the lower molars (large m2). These peculiar molar proportions were found distinct from many other mammals, and might represent clade-specific differences: the large size of both the upper and lower second molars relative to other molars thus possibly representing a derived character state shared by some “condylarths” and kollpaniines.