How the youngsters teach the “old timers”: Terminology of turbinals in adult primates inferred from ontogenetic stages

Abstract

Comparative studies rely on the identification of homologous traits, which is challenging especially when adult stages alone are available. Inferring homology from developmental series represents the most reliable approach to recognize similar phenotypes. The primate nasal cavity exhibits a plastic morphology (shape) and topology (structure) which challenge the terminology of turbinals. Turbinal development largely corresponds to the therian template: turbinals emerge from the cartilaginous nasal capsule, ossify endochondrally, and increase their size through appositional bone growth. We studied histological serial sections and µCT data of eleven primate species in six genera representing four to five age stages (fetal to adult), and the neonate and adult stage of another primate species. We reconstructed cartilaginous precursors and followed their growth patterns until adulthood to inform the identification of structures. The developmental stages were transformed to character states for better comparison across the sample. Strepsirrhines conserved the plesiomorphic condition, with turbinal morphology similar to other placentals. In contrast, haplorhines showed a reduced turbinal number. Most strikingly, some cartilaginous turbinals are absent in the ossified nasal cavity (Saguinus); others seem to emerge as appositional bone without a cartilaginous precursor (Aotus, Pithecia). Our observation that successive developmental sequences differ from the established placental template emphasizes the significance of ontogenetic series for comparative anatomy. Structures which exhibit analogous growth patterns might be falsely considered as being homologous in adults, resulting in biased phenotypic data that strongly affects comparative analyses (e.g., phylogenetic reconstructions).