Archaeorhynchus and ornithuromorph flight apparatus evolution (free pdf)

[Ben Creisler]

Ben Creisler

bcre...@gmail.com

A new paper:

Free pdf:

Qian Wu, Thomas A. Stidham, Jingmai K. O’Connor, Alida M. Bailleul, Zhonghe Zhou & Zhiheng Li (2026)
Critical innovations in the assembly of the modern flight apparatus in Early Cretaceous birds
iScience (advance online publication)
DOI: https://doi.org/10.1016/j.isci.2026.115506
https://www.cell.com/iscience/fulltext/S2589-0042(26)00881-3

Free pdf:
https://www.cell.com/action/showPdf?pii=S2589-0042%2826%2900881-3


Highlights

Ornithothoracines possess a paedomorphic cartilaginous coracoscapular joint.
Enantiornithines lacked an acrocoracoclavicular joint and closed triosseal canal.
Ornithuromorphs evolved a closed triosseal canal via the acrocoracoclavicular joint.
Canal closure enhanced flight performance and drove ornithuromorph diversification.

SUMMARY

A pivotal innovation in the evolution of powered flight in dinosaurs was the triosseal canal—a specialized passage formed by the scapula, coracoid, and furcula that guides the wing-elevation tendon. However, the origins of this structure remained obscure. Here, we applied integrated histological analysis and micro-CT scanning of a new enantiornithine specimen and the basal ornithuromorph Archaeorhynchus. Results indicate that the triosseal canal evolved first through paedomorphosis of the coracoscapular joint into a synchondrosis in ornithothoracines and subsequent acquisition of the acrocoracoclavicular joint in ornithuromorphs. The complete lack of a connection between the furcula and coracoid represents one of the crucial skeletal disparities between enantiornithines and ornithuromorphs. We propose that the closure of the triosseal canal in ornithuromorphs markedly improved tendon stability, facilitating a greater range of wing motion and more efficient flight compared to enantiornithines, serving as one of the critical functional triggers responsible for their ecological diversification.

[Tim Williams]

I'm confused by this statement: "Ground take-off, the most energetically demanding flight maneuver 75,76, would have been particularly challenging for enantiornithines, possibly requiring a running start to launch from the ground as inferred for Archaeopteryx 77."

Birds typically initiate flight by leaping into the air from a stationary position - regardless of whether the bird is on the ground or on a tree branch.  The launch kinematics are the same.  In both cases, the hindlimbs are the primary take-off accelerator (Earls, 2000 doi: 10.1242/jeb.203.4.725.).  This was almost certainly true for the first flying theropods, including the earliest birds, which appear to have been capable of launching from a standing position, with the initial impulse provided by the hindlimbs (Dececchi et al., 2016; DOI 10.7717/peerj.2159). 

There seems to be a misconception that weak or nascent fliers need to run in order to take-off from the ground.  But among modern birds, a running take-off is typically used for launches from compliant surfaces (especially water) - as both Earls and Dececchi &c make clear.

In asserting that enantiornithines were limited to an arboreal ecology because of their poor flight abilities ("inability to perform continuous flight") this study by Wu &c might have it the wrong way round.  If most enantiornithines were adapted for a life within the canopy, they might only have needed to fly short distances.  

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