Comments on ‘comments on the morphology of the paravian shoulder girdle’

Novas et al. 2021 update Ostrom’s 1976 study
of the motion of the humerus on the shoulder girdle of Archaeopteryx, Cathartes (a New World vulture) and other dinosaurs based on the many more pertinent taxa described since 1976.

This is a laudable effort, but does not go far enough.
Novas et al. do not mention the convergent appearance of bird-like shoulder girdles in bats and pterosaurs, both flapping vertebrates. Nor do they shed any new light on the evolution of the coracoid (see below) with regard to the flapping ability of birds. Without flapping, there is no flight in tetrapods.

Novas et al. report,
“The aim of the present paper is to emphasize the anatomical similarities of some key features of the pectoral girdle in basal paravians, basal avialans and extant flightless paleognaths. Specifically, comparisons are detailed with the Greater Rhea (Rhea americana), and we provide a brief description of its musculature with the goal of comparing it with the inferred musculature of basal paravians. With all this information at hand we briefly re-analyze here the “hypothetical stages” in the acquisition of bird flight as originally proposed by Ostrom.”

The authors cite Ostrom 1976 throughout the paper,
but they mistakenly provide a reference for Vincent Ostrom 1976, who wrote about “The American Experiment in Constitutional Choice.” rather than John H Ostrom who wrote about “Archaeopteryx and the origin of birds.” The work was edited by JK O’Connor and reviewed by AWA Kellner and D Marjanovic´ after submission by the five co-authors.

Ostrom 1976 reported,
“The question of the origin of birds can be equated with the origin of Archaeopteryx, the oldest known bird. The skeletal anatomy of Archaeopteryx is extraordinarily similar to that of contemporaneous and succeeding coelurosaurian dinosaurs. Rejection of these similarities as adaptive structures only (parallel or convergent similarities), and therefore of no phylogenetic importance, is here considered invalid.”

The coracoids (in pink)
Figure 1. The coracoids (in pink) slide along the sternum behind the interclavicle in Sphenodon and Tyrannosaurus.

Earlier we looked at the importance of an elongate, locked-down coracoid
in the acquisition of flapping in birds, pre-pterosaur fenestrasaurs (Cosesaurus) and bats (Fig. 2; lacking a coracoid bats substitute a structurally similar clavicle) here in 2012. Novas et al. mention ‘flapping flight‘ in their abstract, then only once more in their text as they discussed the “main adductor/ depressor of the humerus.”

Bat clavicles
Figure 2. Bat clavicles acting as coracoid substitutes anchoring the scapulae and providing muscle anchors for the humeri. Clavicles highlighted in green. Image courtesy of Barry Roades at Wesleyan College.
Figure 1. Partial skeleton of Velociraptor compared to similar bones in the living bird, Rhea americana, photographed from a skeleton I processed several decades ago. This illustration was originally published in Don Lessem's "Raptors, the Nastiest Dinosaurs."
Figure 3. Partial skeleton of Velociraptor compared to similar bones in the living bird, Rhea americana, photographed from a skeleton I processed several decades ago. This illustration was originally published in Don Lessem’s “Raptors, the Nastiest Dinosaurs.” Note the locked-down elongate coracoid in both flapping tetrapods.
Tritosaur pectoral girdles demonstrating the evolution and migration of the sternal elements to produce a sternal complex.
Figure 4. Tritosaur pectoral girdles demonstrating the evolution and migration of the sternal elements to produce a sternal complex. Note the appearance of an elongate, locked down coracoid.

Sometimes a focused study
can be too focused. If you’re going to make a vigorous and valid hypothesis, don’t leave some of your horses in the barn.

References
Novas FE, Motta MJ, Agnolín FL, Rozadilla S, Lo Coco GE and Brissón Egli F 2021. Comments on the Morphology of Basal Paravian Shoulder Girdle: New Data Based on Unenlagiid Theropods and Paleognath Birds. Front. Earth Sci. 9:662167.
doi: 10.3389/feart.2021.662167
Ostrom JH 1969. Osteology of Deinonychus antirrhopus, an Unusual Theropod from the Lower Cretaceous of Montana. Peabody Mus. Bull. 30, 1–165. doi:10. 2307/j.ctvqc6gzx
Ostrom JH 1974. The Pectoral Girdle and Forelimb Function of Deinonychus (Reptilia: Saurischia): a Correction. Postilla 165, 1–11.
Ostrom JH 1976. Archaeopteryx and the origin of birds. Journal of the Linnean Society8(2):91–182,

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