Avian jaw joint and middle ear morphologies evolution + avian neck flexibility + acrocoracoid process in flapping flight evolution

[Ben Creisler]

Ben Creisler

bcre...@gmail.com

New papers:

Free pdf:

Takumi Watanabe, Fernando E. Novas, Tatsuya Hirasawa (2026)
Evolution of the Jaw Joint and Middle Ear Morphologies in the Lineage Towards Birds
Zoological Science 43(1):  
doi: https://doi.org/10.2108/zs250108
https://bioone.org/journals/zoological-science/volume-43/issue-1/zs250108/Evolution-of-the-Jaw-Joint-and-Middle-Ear-Morphologies-in/10.2108/zs250108.full


In tetrapod evolution, the middle ear evolved independently at least four times and represents key innovations for airborne hearing. As the tympanic membrane developed at the interface between the mandibular and hyoid arches, the evolution of middle ears likely involved modifications of jaw joint morphologies. The caudal end of the lower jaw in extant birds accompanies the medial and caudal processes, while the other extant diapsids possess a single process, namely the retroarticular process (RAP). The evolutionary process of these skeletal morphologies, however, remained unclear. Here we examined embryonic developments of the chicken, American alligator, and Chinese soft-shelled turtle, as well as the fossil record, to show that the medial process of the birds is homologous with the retroarticular process. During embryonic development of the chicken, the caudal end of the developing lower jaw became shifted medially, and subsequently the caudal process was secondarily formed. This fact together with the fossil data of Mesozoic theropods indicates that the medial process of the avian lower jaw would be homologous with the RAP. Based on our observations, the invagination of the external auditory meatus likely involves the infolding of the hyoid arch elements including the RAP. Considering the medially oriented RAP as an osteological correlate of the deep external auditory meatus seen in the extant birds, the deep ear canal can be traced back to the common ancestor of coelurosaurian dinosaurs.

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Yohei Utsuki & Yoshitsugu Kobayashi (2026)
Functional Morphology of the Archosaur Neck Provides Evolutionary Insights Into the Avian Neck Flexibility
Journal of Morphology 287(2): e70114
doi: https://doi.org/10.1002/jmor.70114
https://onlinelibrary.wiley.com/doi/10.1002/jmor.70114


The amniote neck has evolved extensive mobility and this has contributed to functional and ecological diversification across the clade. Among amniotes, extant birds exhibit an S-shaped neck with multiple flexion points and displays significant variation in vertebral counts and morphology, enabling diverse foraging and feeding behaviors. While avian neck flexibility has increasingly attracted attention in ethology and functional morphology in recent years, its evolution within the archosaur lineage leading to birds remains unclear. Here, we quantified the ratio of the lever lengths (RLL) of selected muscles in the presacral vertebrae of extant crocodilians and birds as an index of joint rotation efficiency. Both crocodilians and birds display high RLL values at vertebral joints near the skull and the cervico-thoracic transition, suggesting that the functional cervical region lies anterior to the latter joint in both clades. In crocodilians, the position of this posterior high RLL joint varies among major lineages, implying that differences in cervical vertebral counts may be linked to lineage specific feeding ecologies. In contrast, birds possess an additional cervical region with high RLL values between the two major high RLL joints. This mid-posterior cervical subregion appears to be an avian evolutionary novelty that enhances rotational efficiency and likely facilitated the evolution of the highly flexible, multi-point bending neck unique to birds.

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Free pdf:

Yawara Takeda, Taira Kuramoto-Ahuja, Sayuri Yonei-Tamura, Hirotoshi Shibuya, Masaru Tamura, Masahiro Uesaka & Koji Tamura (2026)
Ontogenic Development of the Acrocoracoid Process Responsible for the Evolution of Avian Flapping Flight
Zoological Science 43(1):
doi: https://doi.org/10.2108/zs250113
https://bioone.org/journals/zoological-science/volume-43/issue-1/zs250113/Ontogenic-Development-of-the-Acrocoracoid-Process-Responsible-for-the-Evolution/10.2108/zs250113.full


When modern birds engage in powered flight by flapping their wings, the upstroke movement is produced mainly by a ventrally located muscle called the musculus (m.) supracoracoideus. To achieve this unique mechanism for elevating the humerus, birds have evolved the triosseal canal, a tunnel-like structure which deflects the tendon of the m. supracoracoideus. This canal comprises bony projections of three different skeletal elements of the pectoral girdle (i.e., scapula, coracoid, and clavicle). Importantly, the acrocoracoid process of the coracoid structurally constitutes an essential component for the realization of the triosseal canal's function. Consequently, it has been identified as a key morphological feature for flapping in modern birds. Although limb and girdle development has been extensively studied using avian embryos, the morphogenesis of the avian coracoid itself remains poorly described. To address this knowledge gap, we performed morphological and histological analyses of the shoulder musculoskeletal system of the avian embryo, focusing on the development of the acrocoracoid process. We found that the acrocoracoid process emerges during embryonic development as a lateral protrusion of the dorsal coracoid. Meanwhile, the tendon of the m. supracoracoideus elongates laterally. Using histological analysis, we demonstrate that several shoulder and limb muscles, including the mm. biceps brachii, coracobrachialis, and deltoideus minor, attach to the acrocoracoid process during its development. Our results provide the first detailed description of the ontogenic development of the acrocoracoid process.

[Mickey Mortimer]

" Takumi Watanabe, Fernando E. Novas, Tatsuya Hirasawa (2026)

Evolution of the Jaw Joint and Middle Ear Morphologies in the Lineage Towards Birds

...  Considering the medially oriented RAP as an osteological correlate of the deep external auditory meatus seen in the extant birds, the deep ear canal can be traced back to the common ancestor of coelurosaurian dinosaurs"

The paper itself merely says "These observations of embryonic developments demonstrate that the medial process of the avian lower jaw is comparable with the RAPs (Fig. 7). The fossil record of the Mesozoic theropods is consistent with this relationship, as the gradual establishment of the medial process from the RAP is traceable in the phylogeny...", ...

... but is anyone else underwhelmed by the demonstration of this? Their Figure 6 and description in the supp info doesn't show any difference between e.g. abelisaurids and allosaurids on one hand and e.g. deinocheirids (Garudimimus) and therizinosaurids (Erlikosaurus) on the other, they just changed the way the red arrow goes (supposed to "indicate the orientation of the retroarticular process"). Similarly, the condition in Plateosaurus is similar to the 'coelurosaur condition' in having a medial process with dorsal projection, which is visible in their Figure S1A-B, but they incorrectly colored the whole medial/dorsal process blue in S1B as if it's the glenoid, even though it is convex and dorsally projecting as seen in S1A and e.g. Prieto-Marquez et al. (2011: Fig. 32E-H). So if we're going by the oldest dorsomedial process and assuming the more crownward absences are losses, it would take the development back to at least Saurischia. And using basic parsimony and state distribution from their own cladogram and descriptions, the dorsomedial process would be most likely to converge in tyrannosaurids, parvicursorines and paravians  (absent in Bicentenaria, Proceratosaurus, Ornitholestes, ornithomimosaurs, therizinosaurids and oviraptorosaurs) .

The latter is more likely in my opinion, with no necessary association with the deep ear canal since hesperornithids lack the medial process as the authors state (though note contra their citation, it's demonstrated in Figure 11 of Witmer 1990, not his nonexistent Figure 18) but had a typical bird ear. The galloanserine retroarticular process is generally seen as pretty unique and unusual among birds, so since they used Gallus as their developmental exemplar, it wouldn't surprise me if its development was also different from the rest of the post-glenoid area including the dorsomedial process and retroarticular processes of other theropods.

Mickey Mortimer

[Jaime Headden]

I faced an interest conundrum when looking an basal parasuchid ankles, and the orientation of the calcaneal tuber. Yes, it was an odd diversion, but it turns out the two issues are comparable. Why? Because the calcaneal orients along the axis of rotation, with deflection to the lateral resulting in a reduced vertical inlever during mandibular depression (opening), which relates to a more sprawling attitude and rotation of the pes in locomotion. Something similar happens in the retroarticular process of the mandible, with medial deflection this time relating to angularity across the transverse articular joint. The mroe in-line the RAP becomes to the mandible, the more transverse and perpendicular the quadrate-articular joint becomes, leading to a "clean" in-lever/rotational motion. More deflected, the more likely one or the other halves of the articular joint become dominant, and we see that in birds, pterosaurs, and most coelurosaurian theropods. Thus, it's no surprise that taxa like caenagnathoids among oviraptorosaurs have more or less in-line RAPs with the mandible, slightly inflected medially but following the mandibular shaft, which relates to a slight lateral-dominant quadrate-articular joint. The comparison might not be 1:1, but its close enough to warrant further study.

The other thing that came up when considering the comparison between RAPs and calcaneal tubers is the insertion of their corresponding muscles. In some taxa, the distal end alone receives the muscle, whereas in others the enter process does. For taxa like oviraptorids, where the distal RAP is a distinct knob with a crunkly surface ripe for insertion iof the mDM (m. depressor mandibulae), it doesn't compare well to birds in which the entire medial process bears a set of muscles on the dorsal and internal surfaces, while the ventral will also bear portions of the ventral mPt (m. pterygoideus) and some accessory muscles that correspond (roughly) to ancestral sternal-hyoid musculature now present only in other sauropsids and extant mammals (such as m. sternocleidomastoideus). These muscles interact with the RAP in birds, get displaced to the posterior process, or remain in place behind and associated with the jaw joint itself, stabilizing it or exerting rotational forces. (Some birds have more processes, and the medial process in others lacks these muscular attachments, owing to the articulation with the basicranium, in which it simply cannot function as an RAP.)

Anyways, the orientation of the RAP owes a lot to the mandibular function and forces generated by the associated musculature, and there is a strong need to consider those muscles and biomechanics to discriminate when, or if, the RAP orients itself at all. In the case of the diagrams and data suggested, the RAP in some theropods is a broad knob crossing, and forming the entire back half of the articular fossa. Here, only the acuminate corner of it is called out as an RAP. This seems like an area worth further scrutiny before developing an elegant model of medial deflection for accommodation of the external auditory meatus, which is the actual conclusion of the paper. The later argument is inline with comments from other researchers, who have variously posited that the general homology and placement of various cranial orifices doesn't vary that heavily among sauropsids, with the EAM occurring near the jaw joint, and not high up under the paroccipital processes, as is commonly reconstructed (save that it might occupy the entire "gap" between the two, which is something some people like to draw to have their cake and eat it, too). As dinosaurs likely did not have an externally-exposed tympanum, there is no reason not to move the EAM down where it should belong, and also leaving the relatively short stapes up near the paroccipital process.

In sum:

There are mechanical reasons for the deflection of the RAP that have little to do with the EAM position, given the absence of detailed embryological data [presented] on the development of the RAP in non-avian theropods (or any dinosaur), whereas other constraints and permissions to the RAP orientation equating to the medial process in birds have their own special issues, and also do not relate to the position of the EAM. In the narrower consideration of gradual medial deflection over Coelurosauria, it's probably safe to say, "it varies."

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Jaime A. Headden

The Bite Stuff: http://qilong.wordpress.com/

"Innocent, unbiased observation is a myth" - P. B. Medawar (1969)

[Ronald ORENSTEIN]

A point about the avian RAP:  the size of this process varies markedly in birds. It provides the insertion point for the depressor muscle, and can be very large (as can the muscle) in birds that open their bills against force (gaping, eg to pry open stems or dead leaves, or to spread clumps of grass to look for food on the ground as some starlings and icterids do). It is also large in flamingos. 

One of the largest RAPs relative to jaw size is in the Scarlet-headed Blackbird (Amblyramphus holosericeus) of southern South America.  This bird has a remarkable chisel-shaped bill that it uses to split and pry apart, by gaping, the stems of bulrushes, an action that must require considerable force. 

 I’m not aware of any studies on this bird’s anatomy, though, but I well remember being astonished by its RAP in skeletons at the UMMZ during my graduate days.  

Ronald Orenstein 1825 Shady Creek Court Mississauga, ON L5L 3W2 Canada ronorenstein.blogspot.com

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