Abstract
Extinct animal behavior has often been inferred from qualitative assessments of relative brain region size in fossil endocranial casts. For instance, flight capability in pterosaurs and early birds has been inferred from the relative size of the cerebellar flocculus, which in life protrudes from the lateral surface of the cerebellum. A primary role of the flocculus is to integrate sensory information about head rotation and translation to stabilize visual gaze via the vestibulo-occular reflex (VOR). Because gaze stabilization is a critical aspect of flight, some authors have suggested that the flocculus is enlarged in flying species. Whether this can be further extended to a floccular expansion in highly maneuverable flying species or floccular reduction in flightless species is unknown. Here, we used micro computed-tomography to reconstruct “virtual” endocranial casts of 60 extant bird species, to extract the same level of anatomical information offered by fossils. Volumes of the floccular fossa and entire brain cavity were measured and these values correlated with four indices of flying behavior. Although a weak positive relationship was found between floccular fossa size and brachial index, no significant relationship was found between floccular fossa size and any other flight mode classification. These findings could be the result of the bony endocranium inaccurately reflecting the size of the neural flocculus, but might also reflect the importance of the flocculus for all modes of locomotion in birds. We therefore conclude that the relative size of the flocculus of endocranial casts is an unreliable predictor of locomotor behavior in extinct birds, and probably also pterosaurs and non-avian dinosaurs.
Highlights
Paleoneurology investigates the evolution of the vertebrate brain through time and makes inferences about the behavior of extinct vertebrates using two main sources of information
There appears to be a relationship between relative FFV and brachial index (BI), the need to remove the outliers Apteryx and Struthio to achieve significance even in this small selection of taxa indicates that relative floccular fossa size is not a reliable indicator of flying ability in extant birds
It is possible that a significant proportion of the neural flocculus is not detected using this approach because it occurs within the vermis of the cerebellum and cannot be estimated from the endocranial surface
Summary
Paleoneurology investigates the evolution of the vertebrate brain through time and makes inferences about the behavior of extinct vertebrates using two main sources of information. The second comes from advances in our understanding of behavior-related neural function in extant animals This information is only useful if differences in neural function are causally related to changes in brain region volume that are expressed in the external morphology of the brain, and if the impression of the brain on the internal surface of the brain cavity is reasonably accurate [1]. While comparative neurology has advanced greatly over the last century [2], arguably the most important improvement in paleoneurological investigation has been the advent of non-invasive X-ray tube and synchrotron source micro computed-tomography (mCT) imaging [3] These techniques are affected by similar problems to those inherent in older serial sectioning methods [4], they have greatly increased the total number of fossil taxa for which endocranial anatomy is known [3]
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