The origin of the open acetabulum in dinosaurs: Egawa et al. 2018

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.” 

Figure 4. The genesis of the Archosauria embodied in PVL 4597 to scale with a modern archosaur, Cyanocitta, the blue jay.

Figure 4. The genesis of the Archosauria embodied in PVL 4597 to scale with a modern archosaur, Cyanocitta, the blue jay.

Phylogenetically
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 ProcompsognathusMarasuchus clade of theropods. It also closes slightly in the paleognath (basal flightless bird theropod, Fig. 1) clade…

Figure 1. Acetabulum of Struthio.

Figure 1. Acetabulum of Struthio.

… and many other birds (Fig. 2).

Figure 2. Unidentified bird pelvis. Note the semi-closed acetabulum.

Figure 2. Unidentified bird pelvis. Note the semi-closed acetabulum.

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.

References
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. http://rsos.royalsocietypublishing.org/content/5/10/180604

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.