Developmental insights and dental formula of the Coquerel's Sifaka Propithecus coquereli (Indriidae, primates)

The question of dental reduction in primates has been a longtime debate, especially in Indriidae, where the tooth type of the dental comb has been extensively investigated. While the presence of two incisors without the canine is largely accepted today, some questions remain concerning the homology of teeth preserved in the lower postincisor dentition. Although the presence of the dc₁, dp₂, and dp₃ is clearly distinguishable during development, two teeth disappear at the adult stage. Consequently, some authors identify the remaining postincisor replacement tooth as a P₃, while some studies indicate the identity as the P₂. Here, the observation of several specimens of Propithecus coquereli at different developmental stages (late fetal, newborn, infant, and adult) using 3D reconstructions and histological sections sheds light on the characterization of this tooth as P₂, with a loss of the lower canine and P₃. Among the deciduous teeth, dc₁ and dp₃ are smaller, at an advanced stage of eruption, and lack adjacent replacement teeth. In contrast, the first deciduous premolar (dp₂) has a replacement tooth (P₂) clearly adjacent to it at the late fetal stage. The development of the permanent dentition in P. coquereli reflects its precociality, with the replacement tooth buds already developed and the M3 showing an advanced stage of mineralization at birth. In addition, the morphological variability of P₂ and P₄ may have implications on diet adaptations, being involved in the two phases of oral food processing (ingestion and mastication). Thus, this work brings new insights concerning the issue of dental reduction in primates, clarifying the homology of teeth at the adult stage in this species. Addressing this question provides new perspectives on the history of dental reduction in the genus Propithecus and on the functional role of premolars in relation to dietary adaptations.