Which theropods were capable of flapping flight?

Both Pei et al. 2020 and Pittman and Xu et al. 2020 looked into
the origin of flight in birds and bird mimics. They calculated maximum and minimum estimates of wing loading and specific lift. These results confirm powered flight potential in early birds and its rarity among the ancestors of closest avialan relatives.

wing loading  (= the total weight of an aircraft divided by the area of its wing).

specific lift (not defined, even when googled, but Pei et al. report, “In powered flyers, specific lift is critical to weight support and generation of thrust (thrust is primarily a component of lift in vertebrate flapping flyers”) If you find that confusing, so do I. Thrust and lift are typically considered separately, not as a component of each other.

In both papers there was no mention
of elongate, locked-down coracoids. When you find such coracoids, that’s how we know pterosaur ancestors, like Cosesaurus (Fig. 1), started flapping. Here the former disc-like sliding coracoids are reduced by posterior erosion to slender immobile still curved stems. The scapula is likewise a narrow immobile strap, as in flapping birds.

Figure 1. Cosesaurus flapping - fast. There should be a difference in the two speeds. If not, apologies. Also, there should be some bounce in the tail and neck, but that would involve more effort and physics.

Figure 1. Click to enlarge and animate. Cosesaurus flapping – fast. There should be a difference in the two speeds. If not, apologies. Also, there should be some bounce in the tail and neck, but that would involve more effort and physics.

All extant birds have elongate coracoids.
All extant birds can flap. All have flying ancestors, even those that no longer fly. So when do birds begin to have locked down coracoids and strap-like scapulae in the large reptile tree (LRT, 1738+ taxa)? Let’s look, clade by clade.

All Solnhofen ‘birds’ have elongate coracoids and strap-like scapulae.

Prior to that Xiaotingia and kin have the same.

Prior to that Daliansaurus does not preserve a pectoral girdle, but descendant taxa have elongate coracoids and strap-like scapulae. Call that x1 for flapping.

Prior to that Jinfengopteryx and kin have elongate coracoids and strap-like scapulae. Call that x2.

Prior to that Bambiraptor and Haplocheirus have a short disc-like coracoid, but Velociraptor and Balaur have elongate pectoral elements. Call that x3.

Prior to that Ornitholestes had a short, round coracoid, but Changyuraptor and descendants like Microraptor and Sinornithosaurus had elongate pectoral elements. Call that x4.

Prior to that all theropods in the LRT have a short, round coracoid that slid along the left and right sternae. So, they were not flapping according to this hypothesis.

Highlights of Pei et al. 2020:
One: Support Deinonychosauria as sister taxon to birds and Anchiornithinae as early birds

Supported by the LRT

Two: Powered flight potential evolved ≥3 times: once in birds and twice in dromaeosaurids

Supported by the LRT

Three: Many ancestors of bird relatives neared thresholds of powered flight potential

Supported by the LRT

Four: Broad experimentation with wing-assisted locomotion before theropod flight evolved

Supported by the LRT

Figure 2. Subset of the LRT focusing on Pennaraptora 2014 = Tyrannoraptora 1999. Here Khaan and Velociraptor substitute for Oviraptor and Deinonychus.

Figure 2. Subset of the LRT focusing on Pennaraptora 2014 = Tyrannoraptora 1999. Here Khaan and Velociraptor substitute for Oviraptor and Deinonychus.

The authors note:
Scansoriopterygians (Figs. 3, 4) are included in the phylogenetic analysis, but are excluded from the flight parameters because Yi’s wing (Fig. 3) is skin-based rather than feather-based like the other winged taxa in this dataset, while Epidexipteryx (Fig. 4) does not possess pennaceous feathers.”

Both are incorrect. We looked at the Yi and Ambopteryx issues here. Both are descendants of Solnhofen bird . So they had feathers, not bat-like skin membranes.

Figure 4. Yi qi tracing of the in situ specimen using DGS method and bones rearranged, also using the DGS method, to form a standing and flying Yi qi specimen. Note the lack of a styliform element, here identified as a displaced radius and ulna.

Figure 4. Yi qi tracing of the in situ specimen using DGS method and bones rearranged, also using the DGS method, to form a standing and flying Yi qi specimen. Note the lack of a styliform element, here identified as a displaced radius and ulna.

Figure 3. Epidexipteryx, another scansoriopterygid with a bird-like pelvis.

Figure 3. Epidexipteryx, another scansoriopterygid bird.

The authors note:
“For Rahonavis (Fig. 4), given only the radius and ulna are known, we reconstructed its wing with similar intralimb proportions to Microraptor where the ulna is 37% of the forelimb length.”

This is guessing, inappropriate for science. In the LRT, Rahonavis (Fig. 4) and Microraptor are not related. We don’t have a hand/manus or a coracoid for Rahonavis. In the LRT Rahonavis is a small therizinosaur, close to Jianchangosaurus, not related to taxa with a long, locked-down coracoid.

Figure 2. Rahonavis nests in the LRT as a tiny derived therizinosaur based on the few bones currently known.

Figure 4. Rahonavis nests in the LRT as a tiny derived therizinosaur based on the few bones currently known. The unknown coracoid is restored as a disc here.

From the Pei et al. Summary:
“Uncertainties in the phylogeny of birds (Avialae) and their closest relatives have impeded deeper understanding of early theropod flight. To help address this, we produced an updated evolutionary hypothesis through an automated analysis of the Theropod Working Group (TWiG) coelurosaurian phylogenetic data matrix. Our larger, more resolved, and better-evaluated TWiG-based hypothesis supports the grouping of dromaeosaurids + troodontids (Deinonychosauria) as the sister taxon to birds (Paraves) and the recovery of Anchiornithinae as the earliest diverging birds.”

With exceptions, Pei et al. confirm the origin of flapping topology
found in the large reptile tree (LRT, 1738+ taxa, subset Fig. 1), except in the LRT large ‘troodontids’ nest with dromaeosaurids. Small ‘troodontids’ nest with Anchiornis basal to birds. Some near birds (see list above) developed, by convergence, the elongate locked-down coracoids seen in Solhnhofen birds and their descendants.

There are two ways to get slender locked-down coracoids in vertebrates,
by erosion of the disc to a remaining stem (as in pterosaur ancestors) or by elongation of the entire disc to produce a stem (as in birds and crocs).

Lacking coracoids, bats 
have elongated and locked down clavicles for symmetrical forelimb flapping. Bats are inverted bipeds.

Wing loading issue
Since birds/theropods depend on feathers for wing chord and span it would seem necessary to use only those theropods in which feathers were well known and to show in graphic form the extent of those wing feathers. I don’s see that in this study.

The rapidity of flapping
permits some certain taxa (ducks, hummingbirds, etc.) to have relatively short and small wings while flying. Gliding is not a primitive trait in birds, pterosaurs or bats.

More from the Pei et al. summary:
“Although the phylogeny will continue developing, our current results provide a pertinent opportunity to evaluate what we know about early theropod flight. With our results and available data for vaned feathered pennaraptorans, we estimate the potential for powered flight among early birds and their closest relatives. We did this by using an ancestral state reconstruction analysis calculating maximum and minimum estimates of two proxies of powered flight potential—wing loading and specific lift. These results confirm powered flight potential in early birds but its rarity among the ancestors of the closest avialan relatives (select unenlagiine and microraptorine dromaeosaurids). For the first time, we find a broad range of these ancestors neared the wing loading and specific lift thresholds indicative of powered flight potential. This suggests there was greater experimentation with wing-assisted locomotion before theropod flight evolved than previously appreciated. This study adds invaluable support for multiple origins of powered flight potential in theropods (≥3 times), which we now know was from ancestors already nearing associated thresholds, and provides a framework for its further study.”

Figue 1. A new reconstruction of the basal bipedal croc, Pseudhesperosuchus based on fossil tracings. Some original drawings pepper this image. Note the interclavicle, missing in dinosaurs and the very small ilium, only wide enough for two sacrals. The posterior dorsals are deeper than the anterior ones.

Figue 5. A new reconstruction of the basal bipedal croc, Pseudhesperosuchus based on fossil tracings. Some original drawings pepper this image. Note the interclavicle, missing in dinosaurs and the very small ilium, only wide enough for two sacrals. The coracoids are elongate and immobile, but does that mean this taxon flapped. Maybe.

Not going to leave this topic without discussing 
the elongate coracoids in bipedal crocodylomorphs and their living, quadrupedal, non-flapping descendants, which retain long coracoids (Fig. 6) and mobile pectoral girdles. Experiments by Baier et al. 2018 documented the rotation of the elongate coracoids was less than expected, but the unossified sternum itself rotated left and right. They wrote, “To our knowledge, this is the first evidence of sternal movement relative to the vertebral column (presumably via rib joints) contributing to stride length in tetrapods.” 

All crocodylomorphs lack clavicles, 
and this likely contributes to pectoral girdle mobility. The basalmost archosaur, PVL 4597 does not preserve any element of the pectoral girdle or forelimb, so it does not shed light on the loss or retention of the clavicles. Among more distantly related proximal outgroup taxa, only Poposaurus and Lotosaurus appear to retain clavicles. Appearances vary in more primitive rauisuchids and erythrosuchids. Euparkeria has clavicles.

So, were basal crocodylomorphs flapping in the Triassic?
Pseudhesperosuchus (Fig. 5) would have been flapping without membranes, elongate fingers and feathers. But look at the clearance between the dangling forelimbs and sprinting hind limbs (Fig. 5). Perhaps Pseudhesperosuchus evolved elongate pectoral elements to lift the forelimbs laterally and keep them elevated while running, giving the narrow-gauge hind limbs room to extend anteriorly during the running cycle. Animators: take note!

Also worthwhile noting,
this is also when the proximal carpals became elongated, a crocodylomorph hallmark. As a biped, Pseudhesperosuchus had less use for its forelimbs. They could have evolved to become something else. Based on the elongation of the proximal carpals, the small size of the manus and the rather long forelimbs, the best guess I’ve seen is that the forelimbs occasionally acted much like those of similar forelimbs on much larger hadrosaurs (duckbill dinosaurs), providing more stability with a quadrupedal pose, without giving up its bipedal abilities. More aquatic short-legged, quadrupedal crocs evolved later. Long coracoids and long proximal carpals were retained in extant crocs from earlier Triassic ancestors.

Exceptions and reversals.
A few small basal bipedal crocodylomorphs, like Scleromochlus and Litargosuchus, re-evolved disc-like coracoids.

Figure 6. At the lower right hand corner is a pectoral girdle typical of crocs.

Figure 6. At the lower right hand corner is a pectoral girdle typical of crocs.

Flapping requires an immobile pectoral girdle
in order that both limbs move symmetrically, the opposite of basal tetrapods with mobile pectoral girdles. Flapping is the first step toward flying in pterosaur and bird ancestors.


References
Baier DB, Garrity BM, Moritz S and Carney RM 2018. Alligator mississippiensis sternal and shoulder girdle mobility increase stride length during high walks. Journal of Experimental Biology 2018 221: jeb186791 doi: 10.1242/jeb.186791
Kruyt JW, et al. 2014. Hummingbird wing efficacy depends on aspect ratio and compares with helicopter rotors. Royal Society Interface. nterface. 2014;11(99):20140585. doi:10.1098/rsif.2014.0585
Pei R et al. 2020.
Potential for Powered Flight Neared by Most Close Avialan Relatives, but Few Crossed Its Thresholds. Current Biology online here.
Pittman M, O’Connor J, Field DJ, Turner AH, Ma W, Makovicky P and Xu X 2020. Pennaraptoran Systematics. Chapter 1 from Pittman M and Xu X eds. 2020. Pennaraptoran theropod dinosaurs. Past progress and new Frontiers. Bulletin of the American Museum of Natural History 440; 353pp. 58 figures, 46 tables.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4233735/#idm140660310181296title

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