The origins of flight in theropods: Serikornis, a flightless bird in the LRT, not a pre-bird

A two-part post today
united by the topic of basal birds and short coracoids.

Nebreda, Fernandez and Marugán-Lobón 2021
discussed locomotor modules and the origins of flight in theropods. “Here, we review the most relevant aspects of limb morphological transformation during the so-called ‘dinosaur-bird’ transition to stress the importance of assessing the role of modularity and morphological integration in such macroevolutionary transition, which ultimately involves the origins of flight in dinosaurs.”

Oddly, the keyword “coracoid” is not found in the text of this paper.
Elongate, locked-down coracoids are the key to the origin of flapping and flight in birds, pterosaurs and bats, which lack corarcoids and substitute long clavicles. The authors include only one Solnhofen bird (Archaeopteryx) and thus miss the initial radiation due to taxon exclusion. For instance, the authors mistakenly nest Scansoriopterygids (e.g. Yi qi) with oviraptors. They also mistakenly nest chickens and geese in a single invalid traditional clade, Galloanserae. Regarding the origin of flight, the authors conclude, “much is still unknown about the proximate and ultimate causes of these macroevolutionary trends.”

So enough with recent papers… Let’s meander back to the past now.

We’ll start with a phylogenetic analysis.
The feathered theropod, Serikornis, nests with another feathered theropod, the Solnhofen bird, Jurapteryx (originally Archaeopteryx, the Eichstätt specimen) in the large reptile tree (LRT, 1915+ taxa). Many workers consider Serikornis a richly-feathered but non-flapping archaeornithid, a theropod more primitive than the Solnhofen birds.

Figure 1. Serikornis is one of the first flightless birds, one which has reversed to a short coracoid, indicating a reduction or absence of flapping, yet is derived from the Solnhofen bird, Jurapteryx.

‘Consider’ is the key word here.
Opinions and cherry-picking traits do not a cladogram make. Always run the characters to see where your taxon nests. We looked at this Serikornis paper (Lefevre et al. 2017) earlier here.

Figure 2. Serikornis and Jurapteryx (Archaeopteryx) recurva to scale. These two nest as sisters in the LRT.

Archaeopteryx (Jurapteryx) recurva (JM2257); the Eichstätt specimen; Howgate 1985; Late Jurassic) is one of the smaller Solnhofen birds. Here it nests as the last common ancestor of all extant birds. A gap spanning the entire Cretaceous separates this taxon from extant taxa and their kin. As in other bird lines, the loss of tail length, the fusion of the pygostyle and the fusion of manus elements are convergent.

Serikornis sungei (Lefèvre et al. 2017; Late Jurassic; 50cm long) was originally considered a pre-bird sister to Eosinopteryx. Here it nests with Jurapteryx, the Eichstätt specimen first attributed to Archaeopteryx. Serikornis is larger than its sister, had larger teeth and was flightless. These two sisters nest with other flightless birds in the LRT, including Early Cretaceous Sapeornis.

A blogpost from 2017,
“Everything Dinosaur,” features some pertinent quotes and professional insight.

Lead author Ulysse Lefèvre stated:
“The feathering of Serikornis shows for the first time a complete absence of barbules—that is, the microstructures that allow feathers to resist air pressure during wing beats.  The plumage is composed of four wings, as with many Theropod dinosaurs from China, but it did not allow “Silky” to take off from the ground or from a tree.”

Quoting from the Everything Dinosaur blogpost,
“Professor Benton and many other leading academics support the idea that the anatomical arrangement of four wings is a good contender as a transitional stage between gliding and the evolution of powered flight.  The professor added that in his opinion the body plan of Serikornis was: “a model for the origin of flight, in which little dinosaurs such as Serikornis clambered into trees, perhaps chasing insects and other small tree-dwellers for food.  To escape predators or to get around, they would glide from bough to bough.”

This, of course, ignores
the key trait that made terrestrial theropods into flying birds: the ability to flap with sufficient thrust to enable bipedal vertical tree-climbing leading to flight. Not sure why gliding keeps entering any ‘origin of flght’ conversation because gliders never turn into flappers.

The phylogenetic analysis by Lefèvre et al.
placed Serikornis outside troodontids + velociraptors + anchiornithids + birds, close to another feathered non-flapper, Eosinopteryx, which nests in the LRT just primitive to birds.

Figure 2. Sapeornis specimen STM-1515 reconstructed from DGS tracing. Note the locked-down elongate coracoids (bright green chest area) on this flapping bird.

References
Lefèvre U, Cau A, Cincotta A, Hu D-Y, Chinsamy A, Escuillié F and Godefroit P 2017. A new Jurassic theropod from China documents a transitional step in the macrostructure of feathers. Sci Nat 104:74. DOI 10.1007/s00114-017-1496-yebreda SM, Fernandez MH and Marugán-Lobón J 2021. Dinosaur‑bird’ macroevolution, locomotor modules and the origins of flight. Journal of Iberian Geology

https://doi.org/10.1007/s41513-021-00170-3

https://blog.everythingdinosaur.co.uk/blog/_archives/2017/09/01/silky-dinosaur-ruffles-feathers.html

Serikornis: Pre-bird or flightless bird?

https://www.nationalgeographic.com/science/article/feathered-dinosaur-four-wings-species-serikornis-science