Origin of the avian predentary and evidence of a unique form of cranial kinesis in Cretaceous ornithuromorphs

Abstract

The avian predentary is a small skeletal structure located rostral to the paired dentaries found only in Mesozoic ornithuromorphs. The evolution and function of this enigmatic element is unknown. Skeletal tissues forming the predentary and the lower jaws in the basal ornithuromorph Yanornis martini are identified using computed-tomography, scanning electron microscopy, and histology. On the basis of these data, we propose hypotheses for the development, structure, and function of this element. The predentary is composed of trabecular bone. The convex caudal surface articulates with rostromedial concavities on the dentaries. These articular surfaces are covered by cartilage, which on the dentaries is divided into 3 discrete patches: 1 rostral articular cartilage and 2 symphyseal cartilages. The mechanobiology of avian cartilage suggests both compression and kinesis were present at the predentary–dentary joint, therefore suggesting a yet unknown form of avian cranial kinesis. Ontogenetic processes of skeletal formation occurring within extant taxa do not suggest the predentary originates within the dentaries, nor Meckel’s cartilage. We hypothesize that the predentary is a biomechanically induced sesamoid that arose within the soft connective tissues located rostral to the dentaries. The mandibular canal hosting the alveolar nerve suggests that the dentary teeth and predentary of Yanornis were proprioceptive. This whole system may have increased foraging efficiency. The Mesozoic avian predentary apparently coevolved with an edentulous portion of the premaxilla, representing a unique kinetic morphotype that combined teeth with a small functional beak and persisted successfully for ∼60 million years.

DiceCT applied to fossilized hard tissues: A preliminary case study using a miocene bird

“…Calcified cartilage shows an extracellular matrix (ECM) but with some irregular, islands of globular darker material representing chondrocyte lacunae (Figure 3e). This is similar to what was observed in the nano‐CT scans of bone and calcified cartilage of the fossil bird Yanornis martini (Bailleul et al, 2019). In some areas however it is more difficult to identify calcified cartilage only based on CT scans (e.g., Figure 3g).…”

“…Based on extant species and the principals of X‐ray absorption, the ECM of calcified cartilage in this fossil pheasant should appear whiter than the bone if the original chemistries (i.e., calcium contents) had been preserved during fossilization. This was observed in the nano CT‐scans of the predentary of Yanornis (voxel size 1 micron; see Figure 2e‐h in Bailleul et al, 2019); and in high resolution microradiographs of calcified cartilage of young hadrosaur skull bones (see Fig. 7 in Bailleul et al, 2016).…”

“…see Figure 2e-h in Bailleul et al, 2019); and in high resolution microradiographs of calcified cartilage of young hadrosaur skull bones (see Fig. 7 in Bailleul et al, 2016).…”

New Information on the Keratinous Beak of Confuciusornis (Aves: Pygostylia) From Two New Specimens

“…Only a premaxillary nail is inferred to be present in this taxon based on the presence of numerous neurovascular foramina on the rostral portion of the edentulous premaxilla, presumably accentuating the hooked morphology of this element (Field et al, 2018). A mandibular nail was absent but a small keratinous sheath likely covered the outer surface of the predentary bone present in these non-neornithine ornithurines (Zhou and Martin, 2011;Bailleul et al, 2019), thus the condition in Hesperornis and Ichthyornis represents a primitive rhamphothecal morphology not present in neornithines (the predentary being a feature restricted to non-neornithine ornithuromorphs).…”

“…In addition, some ornithuromorphs (e.g., Yanornis, Yixianornis) possess a rostrum that is only edentulous at the tip of the premaxillae, caudally followed by teeth (Zhou and Zhang, 2001). This edentulous tip, which ventrally articulates with the edentulous avian predentary, is inferred to have been covered by a small rhamphotheca (Bailleul et al, 2019). The entire premaxilla is edentulous in the Early Cretaceous Iteravis (Zhou et al, 2014) and in the Late Cretaceous ornithurines Ichthyornis (Field et al, 2018) and the Hesperornithiformes (Gingerich, 1973).…”