The internal nasal morphology of Rooneyia viejaensis and euarchontans : using [mu] CT scan data to understand and predict patterns of nasal fossa evolution in primates

Primates have historically been viewed as having a reduced sense of smell, or olfaction, relative to other mammals. This is often inferred through the complexity of the bony structures within the nasal fossa, or turbinals. These turbinals are often covered in olfactory epithelium, which contains olfactory receptor neurons that detect odors. These turbinal numbers and complexity can then be used as a rough proxy for olfactory sensitivity and the reliance on olfactory cues in the ecology of an extinct animal. Unfortunately, turbinals are delicate and rarely preserved in fossil specimens, making it difficult to understand the timing of the reduction in olfactory sensitivity in primates. Recent high-resolution computed tomography scans reveal the presence of turbinals in the nasal fossa of a fossil primate from West Texas, Rooneyia viejaensis. This late Eocene species has not been phylogenetically well-resolved and has been classified as a crown-primate, stem-strepsirrhine, stem-haplorhine, stem-tarsiiform, and stem-anthropoid. Here we describe nasal fossa anatomy in Rooneyia and extant euarchontans to understand and calibrate the evolutionary timing of the reduction in turbinal complexity in primates. Rooneyia was found to have 1 nasoturbinal, 4 bullar ethmoturbinals, 1 frontoturbinal, 1 interturbinal, and a scrolled maxilloturbinal. This is broadly similar to the condition seen in strepsirrhine primates but differs substantially from the condition seen in anthropoids - 1 nasoturbinal, 1-2 non-bullar ethmoturbinals, and a scrolled maxilloturbinal. This finding suggests that Rooneyia is likely not a stemtarsiiform or stem-anthropoid but may be a crown-primate, stem-strepsirrhine, or early stem haplorhine. This study also revealed greater variation in turbinal and nasolacrimal canal morphology potentially reflecting either greater variability or frequent homoplasy in the nasal fossa