Short one today
as Heers et al. 2021 miss the one key trait in the evolution of flapping flight in bird origins: a locked-down elongate coracoid necessary for left-right simultaneous flapping. This morphology is derived from the primitive disc-like coracoid sliding on the sternal rim, originally used during quadrupedal locomotion to extend each stride (Fig. 1). Disc-like coracoids are retained in bipedal theropods like Tyrannosaurus, Khaan, and Haplocheirus.
in Velociraptor, Archaeopteryx and living birds the coracoid is no longer mobile and disc-like, but narrows and becomes immobile. These are flapping bipeds. This fact was overlooked by Heers et al. who looked a baby birds.
From the Heers et al. abstract:
“Although extinct theropods are most often compared to adult birds, studies show that developing birds can uniquely address certain challenges and provide powerful insights into the evolution of avian flight: unlike adults, immature birds have rudimentary, somewhat “dinosaur-like” flight apparatuses and can reveal relationships between form, function, performance, and behavior during flightless to flight-capable transitions. Here, we focus on the musculoskeletal apparatus and use CT scans coupled with a three-dimensional musculoskeletal modeling approach to analyze how ontogenetic changes in skeletal anatomy influence muscle size, leverage, orientation, and corresponding function during the development of flight in a precocial ground bird (Alectoris chukar).”
Unfortunately, even baby birds have a locked-down elongate coracoid. So the transition to flapping must be found in the fossil record, not the ontogeny of chicks. Heers et al. needed an outgroup and a convergent set of taxa.
starting with Cosesaurus (Figs. 3, 4) had bird-like immobile coracoids.
(Fig. 5) don’t have coracoids. but elongate locked-down clavicles are analogous.
Heers AM, Varghese SL, Hatier LK and Cabrera JJ 2021. Multiple Functional Solutions During Flightless to Flight-Capable Transitions