Evolution of olfaction in non-avian theropod dinosaurs and birds

Abstract

Little is known about the olfactory capabilities of extinct basal (non-neornithine) birds or the evolutionary changes in olfaction that occurred from non-avian theropods through modern birds. Although modern birds are known to have diverse olfactory capabilities, olfaction is generally considered to have declined during avian evolution as visual and vestibular sensory enhancements occurred in association with flight. To test the hypothesis that olfaction diminished through avian evolution, we assessed relative olfactory bulb size, here used as a neuroanatomical proxy for olfactory capabilities, in 157 species of nonavian theropods, fossil birds and living birds. We show that relative olfactory bulb size increased during non-avian maniraptoriform evolution, remained stable across the non-avian theropod/bird transition, and increased during basal bird and early neornithine evolution. From early neornithines through a major part of neornithine evolution, the relative size of the olfactory bulbs remained stable before decreasing in derived neoavian clades. Our results show that, rather than decreasing, the importance of olfaction actually increased during early bird evolution, representing a previously unrecognized sensory enhancement. The relatively larger olfactory bulbs of earliest neornithines, compared with those of basal birds, may have endowed neornithines with improved olfaction for more effective foraging or navigation skills, which in turn may have been a factor allowing them to survive the end-Cretaceous mass extinction.

From sauropsids to mammals and back: New approaches to comparative cortical development

“…However, the proportion of abventricular mitoses in the hyperpallium (proposed to be homologous to the dorsal cortex of mammals (Molnár and Butler, 2002)) is negligible compared to the proportions observed in the meso and nido‐pallium (Cheung et al, 2007). Even more relevant is that chickens and quails belong to a basal avian order (Galliformes) and zebra finch and parakeets to evolutionarily distant avian groups, the songbirds (order Passeriformes and Psittaciformes, respectively) (Zelenitsky et al, 2011). Thus, abventricular mitoses that occur in a well‐defined zone would be a derived evolutionary change occurring in these late avian orders.…”

From chemotaxis to the cognitive map: The function of olfaction

“…Moreover an analysis of phylogenetic trends showed that the direct ancestors of modern birds did not show the modern bird's reduction in relative OB size, which must therefore be a secondary adaptation (55). This implies that carnivorous predators, whether diurnal theropods or nocturnal terrestrial mammals (40), are olfactory predictors, and require an enhanced OS system to track mobile, dispersed prey.…”

Sensory Evolution and Ecology of Early Turtles Revealed by Digital Endocranial Reconstructions

“…In a series of studies the olfactory ratio (ratio between olfactory bulb and cerebral hemisphere maximum diameters; OR values) were used as a proxy to study olfactory acuity and capacity in theropod dinosaurs (including birds) and crocodilians (Zelenitsky et al, 2009(Zelenitsky et al, , 2011. More recently this has also been applied to turtles (Paulina-Carabajal et al, 2017), showing that tortoises and meiolaniids (both terrestrial taxa) have the highest OR values (36-62 and 20-45%, respectively).…”