preprints.org > biology and life sciences > ecology, evolution, behavior and systematics > doi: 10.20944/preprints202311.0235.v1

Preprint Article Version 1 Preserved in Portico This version is not peer-reviewed

Version 1 : Received: 2 November 2023 / Approved: 3 November 2023 / Online: 3 November 2023 (10:12:41 CET)

That birds are maniraptoran theropod dinosaurs is now considered an evolutionary consensus. An “open” (i.e., completely, or substantially perforate) acetabulum is considered an important synapomorphy verifying the bird-dinosaur nexus. Here I present anatomical evidence from morphological aspects of the acetabulum and its important appurtenances, the supracetabular crest and the antitrochanter, that the hip anatomy differs substantially between dinosaurs and birds and for the current hypothesis of origins to be correct unlikely anatomical reversals would have been required. Given the thin bone of the acetabular walls and the varied tissue, both hard and soft, in the acetabular region and especially the lower part of the basin, it is apparent that many avian skeletons exhibit some anatomical loss of soft tissue and thin bone, some perhaps related to changes in gait, but also in part related to the dramatic trend in bone reduction associated with flight, especially in more advanced crown taxa. Many basal birds and early diverging neornithines tend to have a nearly closed or various levels of partially closed acetabula, thus rendering the current terms “open” or “closed” acetabula inaccurate; they should be modified or replaced.

acetabulum; antitrochanter; archosaur; clade; convergence; dinosauriform; epigenesis, foramen acetabuli; gallinaceous; gallo-anseres; paedomorphosis; pelvis; supracetabular crest; synapomorphy

Biology and Life Sciences, Ecology, Evolution, Behavior and Systematics

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