Abstract
The growing availability of virtual cranial endocasts of extinct and extant vertebrates has fueled the quest for endocranial characters that discriminate between phylogenetic groups and resolve their neural significances. We used geometric morphometrics to compare a phylogenetically and ecologically comprehensive data set of archosaurian endocasts along the deep evolutionary history of modern birds and found that this lineage experienced progressive elevation of encephalisation through several chapters of increased endocranial doming that we demonstrate to result from progenetic developments. Elevated encephalisation associated with progressive size reduction within Maniraptoriformes was secondarily exapted for flight by stem avialans. Within Mesozoic Avialae, endocranial doming increased in at least some Ornithurae, yet remained relatively modest in early Neornithes. During the Paleogene, volant non-neoavian birds retained ancestral levels of endocast doming where a broad neoavian niche diversification experienced heterochronic brain shape radiation, as did non-volant Palaeognathae. We infer comparable developments underlying the establishment of pterosaurian brain shapes.
Highlights
Detailed comparative investigations into brain shape and size at the very onset of dinosaurian flight have been incapable of identifying specific endocranial conditions unequivocally linked with dinosaurian volancy
Conventional computed tomography (CT) setups typically allow for reliable endocranial visualisation of modern material, fossils encased in and filled with a sedimentary matrix may require the application of more elaborate tomographical techniques, especially when the fossil is preserved in a plate-like slab composed of a lithic substrate with a density comparable to that of the sample of interest
Because brain tissue itself rarely fossilises[24,25], palaeoneurology resorts to studying the cranial endocast that reflects the surface geometry of the endocranial cavity in which the brain was housed during life[12,17]
Summary
Detailed comparative investigations into brain shape and size at the very onset of dinosaurian flight have been incapable of identifying specific endocranial conditions unequivocally linked with dinosaurian volancy. Because the variation in relative endocranial and cerebral volumes of modern birds and their non-avian ancestors was found to not primarily reflect the presence or absence of powered flight[16], other geometrical parameters should be explored to resolve the potential influence of volancy on the evolution of the archosaurian brain. We plotted endocast doming (C/D) against log D for all adult archosaurs subjected to PCA, and included a broad selection of lepidosaurs, as an outgroup to archosaurs, and an ontogenetic growth series of Crocodylus niloticus This resolved a partially overlapping distribution of archosaurian endocranial shapes and sizes that can be directly compared against the complete ontogenetic variability of C. niloticus
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