Within the abundant mammal fauna of Tiupampa (Early Paleocene, Bolivia), the pantodont Alcidedorbignya inopinata Muizon & Marshall, 1987 is the eutherian represented by the largest number of specimens. The sample recovered in this locality includes numerous isolated teeth and jaws as well as six skulls, most of them being associated with complete or partial skeletons. It is composed of a similar number (excluding isolated teeth) of adults (71) and juveniles (75), which likely belong to the same population. The present study focusses on the dental eruption sequence of juveniles, which spans ontogenetically from perinatal individuals to sub-adults. Eight ontogenetic stages have been defined for juveniles (1, 2, 2+, 3, 3+, 4, 5, 6) and criteria for their definition are proposed. Two more stages (7 & 8) are defined for adults. Stage 7 includes specimens with fully erupted and little worn dentition, and Stage 8 is composed of a small number of specimens with worn dentition but not totally erasing the inner profile of the tooth, even on M1/m1. Our stages have been correlated to the six “Individual Dental Age Stages” (IDAS) proposed by others for fossil and extant placental mammals. Description of the dental ontogenetic stages of Alcidedorbignya establishes the dental eruption sequence for this taxon. Excluding incisors (for which we have almost no data), the upper eruption sequence is as follows: dC-dP2-4 (probably erupted at birth, order unknown), M1, dP1, M2, I3, C, P4, P3, P2-3, M3. The lower eruption sequence (excluding i1-2 for which we have no data) is as follows: di1-3-dc-dp3-4 (probably erupted at birth, order unknown), dp2, m1, dp1, m2, i3, c, p2, p4, p3, m3. This sequence is identical to that of Coryphodon, the most derived pantodont genus. In Alcidedorbignya, as in Coryphodon Owen, 1845, the M3/m3 erupt last, which may constitute a derived, although very convergent, character within placentals. The use of computed tomography provided further information regarding tooth mineralisation. It indicates that on lower cheek teeth the trigonid mineralises before the talonid and on the former the order of mineralisation is protoconid, metaconid and paraconid. On the upper cheek teeth the paracone mineralises first. This mineralisation sequence for cusps fits well with the general pattern described for placentals. The scrutiny of wear facets and eruption sequence of cheek teeth then enabled us to propose hypotheses regarding the timing of life history events such as weaning and sexual maturity, using comparisons with extant placentals. Several measurements of the skull and dentary also indicate that the juveniles of Alcidedorbignya with no more than 20% of erupted permanent cheek teeth (p3 to m3, P3 to M3) are rather small compared to adults, which may represent a plesiomorphic pattern. Finally, the study of the mortality profile of this pantodont assemblage shows that the number of juveniles remains is similar to that of adults. Among juveniles, the greater abundance of specimens is observed for unweaned individuals. Alcidedorbignya inopinata is, by far, the Paleocene taxon represented by the greatest number of juvenile individuals, which suggests that Tiupampa may have been a breeding locality for this gregarious Paleocene placental. The remarkably well-documented collection of juvenile and adult fossils of Alcidedorbignya offers an unprecedented glimpse into the craniodental ontogeny of an early diverging placental mammal.
Placentalia, Pantodonta, Alcidedorbignya, Paleocene, Tiupampa, Bolivia, dental ontogeny, eruption stages, mortality profile