Egawa et al. 2018
bring us “a morphogenetic mechanism of the acquisition of the open dinosaur-type acetabulum.” Using embryos, they found, “the avian perforated acetabulum develops via a secondary loss of cartilaginous tissue in the acetabular region.”
the open acetabulum develops as a semi-perforation (slight erosion of the inner wall) in PVL 4597 (Fig. 1), close to the last common ancestor of all archosaurs in the large reptile tree (LRT, 1308 taxa). It closes in all crocs (except Terrestrisuchus and Trialestes). It opens more (but not completely) in basal dinos. It opens completely in basal phytodinosaurs and theropods. It closes slightly in the Procompsognathus–Marasuchus clade of theropods. It also closes slightly in the paleognath (basal flightless bird theropod, Fig. 1) clade…
… and many other birds (Fig. 2).
The authors conclude,
“We hypothesize that during the emergence of dinosaurs, the pelvic anlagen became susceptible to the Wnt ligand, which led to the loss of the cartilaginous tissue and to the perforation in the acetabular region.”
Not sure why
the authors did not consider a comparison with phylogeny. It’s more interesting and visual.
On the same note…
certain aquatic taxa, like derived ichthyosaurs also have an open acetabulum due to the embryonic development of small, almost useless pelvic bones that fail to suture and close at the acetabulum.
Egawa S, Saito D, Abe G ande Tamura K 2018. Morphogenetic mechanism of the acquisition of the dinosaur-type acetabulum. Royal Society Open Science 5(10): 180604 DOI: 10.1098/rsos.180604.Â http://rsos.royalsocietypublishing.org/content/5/10/180604