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Origin of the Avian Predentary and Evidence of a Unique Form of Cranial Kinesis in Cretaceous Ornithuromorphs

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Origin of the Avian Predentary and Evidence of a Unique Form of Cranial Kinesis in Cretaceous Ornithuromorphs Origin of the avian predentary and evidence of a unique form of cranial kinesis in Cretaceous ornithuromorphs Alida M. Bailleula,b,1, Zhiheng Lia,b, Jingmai O’Connora,b, and Zhonghe Zhoua,b,1 aKey Laboratory of Vertebrate Evolution and Human Origins, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, 100044 Beijing, China; and bCenter for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences, 100044 Beijing, China Contributed by Zhonghe Zhou, October 14, 2019 (sent for review July 11, 2019; reviewed by Anthony Herrel and Adam K. Huttenlocker) The avian predentary is a small skeletal structure located rostral to in 5 species of nonornithurine ornithuromorphs from the Early the paired dentaries found only in Mesozoic ornithuromorphs. The Cretaceous Jehol Biota of northeastern China (SI Appendix, Fig. evolution and function of this enigmatic element is unknown. S2) (5, 7). Because all extant birds lack this bone, it was hypoth- Skeletal tissues forming the predentary and the lower jaws in the esized that the APD was a common feature of Cretaceous orni- basal ornithuromorph Yanornis martini are identified using computed- thuromorphs that was subsequently lost (6). Additionally, the tomography,scanningelectronmicroscopy,andhistology.Onthe APD is apparently only present in toothed birds, being absent in basis of these data, we propose hypotheses for the development, all known specimens of the edentulous Archaeorhynchus (12–14). structure, and function of this element. The predentary is composed This study aims to shed light on the origin and the function of of trabecular bone. The convex caudal surface articulates with ros- the APD. No developmental hypothesis for the APD has ever tromedial concavities on the dentaries. These articular surfaces are been proposed. Based on its anatomical location, we see 3 pos- covered by cartilage, which on the dentaries is divided into 3 discrete sibilities: It may be that it originated within Meckel’s cartilage, patches: 1 rostral articular cartilage and 2 symphyseal cartilages. The which belongs to the splanchnocranium (developmental hy- mechanobiology of avian cartilage suggests both compression and pothesis 1), or it may have arisen from the DEs (developmental kinesis were present at the predentary–dentary joint, therefore sug- hypothesis 2), in which case the APD would be of dermatocranial gesting a yet unknown form of avian cranial kinesis. Ontogenetic – EVOLUTION processes of skeletal formation occurring within extant taxa do not origin (15 17). A third possibility is that the APD is a sesamoid suggest the predentary originates within the dentaries, nor Meckel’s (e.g., ref, 18) that arose ectocranially within the connective tis- cartilage. We hypothesize that the predentary is a biomechanically sues located rostral to the DEs (developmental hypothesis 3). ’ induced sesamoid that arose within the soft connective tissues Meckel s cartilage first arises within the developing skull as a located rostral to the dentaries. The mandibular canal hosting primary cartilage model, which then gets partially replaced by the alveolar nerve suggests that the dentary teeth and predentary bone through the process of endochondral ossification, whereas of Yanornis were proprioceptive. This whole system may have in- the DEs are membrane (dermal) bones that arise directly from creased foraging efficiency. The Mesozoic avian predentary appar- ently coevolved with an edentulous portion of the premaxilla, Significance representing a unique kinetic morphotype that combined teeth ∼ with a small functional beak and persisted successfully for 60 The avian predentary is a small, rarely preserved skeletal million years. structure found only in toothed Mesozoic ornithuromorph birds. The origin and function of this enigmatic bone is poorly predentary | birds | sesamoid | kinesis | proprioception understood. Based on skeletal tissue identification in a speci- men of the Early Cretaceous Yanornis martini, we propose he predentary (PD) is a bone located just rostral to the developmental, structural, and functional hypotheses for this Tpaired, unfused dentaries (DEs). This element is rare, only element. It consists of trabecular bone with a caudal articular found in 3 groups of extinct vertebrates: Actinopterygian fishes surface for the dentary covered by cartilage. Tissue mecha- (1–3), ornithischian dinosaurs (4), and Cretaceous nonneornithine nobiology suggests this contact was kinetic, representing a ornithuromorph birds (5–8). Among extant taxa, a structure re- previously unknown type of avian cranial kinesis. We hypoth- ferred to as the PD is known in some (but not all) sailfishes and esize that the avian predentary was a biomechanically induced marlins of the Istiophorid family (actinopterygians) (3, 9), al- sesamoid that coevolved with an edentulous portion of the though the development of the PD in this lineage has yet to be premaxilla, together representing a successful morphotype investigated. The disparate distribution of this element among that combined a small beak with a proprioceptive system. vertebrates does not favor any hypothesis of homology and par- simony strongly suggests that the PD in actinopterygians, ornith- Author contributions: A.M.B., Z.L., J.O., and Z.Z. designed research; A.M.B., Z.L., J.O., and Z.Z. performed research; Z.Z. contributed new reagents/analytic tools; A.M.B. and Z.L. ischians, and Cretaceous nonneornithine ornithuromorph birds all collected the data; A.M.B., Z.L., J.O., and Z.Z. analyzed data; and A.M.B. and J.O. wrote evolved independently, resulting in the proposal that the avian PD the paper. be alternatively referred to as the intersymphyseal ossification to Reviewers: A.H., Muséum National d’Histoire Naturelle; and A.K.H., University of Southern avoid mistaken inferences regarding homology with the ornithis- California. chian PD (SI Appendix,Fig.S1) (10). However, because the os- The authors declare no competing interest. sification is not strictly intersymphyseal, in fact located rostral to Published under the PNAS license. the DEs and the mandibular symphysis, we will refer to the PD in Data deposition: We have deposited large computed tomography data in the Open nonneornithine ornithuromorphs as the “avian PD” (APD). Science Framework, OSF, https://osf.io/q9xsp/. The APD was first described in the Late Cretaceous orni- 1To whom correspondence may be addressed. Email: [email protected] or thurines Hesperornis and Parahesperornis (8). Although no PD [email protected]. has ever been recovered in the ornithurine Ichthyornis (11), the This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. morphology of its dentaries strongly suggests a PD was also 1073/pnas.1911820116/-/DCSupplemental. present in this species (7, 8). More recently, it has been identified www.pnas.org/cgi/doi/10.1073/pnas.1911820116 PNAS Latest Articles | 1of11 Downloaded by guest on September 29, 2021 the cephalic mesenchyme via ossification centers without going scanning, scanning electron microscopy [SEM], standard ground- through a primary cartilage model (15, 17). The distribution and sectioning techniques, and paraffin histology) on the extracted PD identification of skeletal tissues, such as bone and cartilage, are key and on an extracted fragment of the right DE in order to under- to differentiating between these competing hypotheses, as these stand the origin and function of the APD. tissues reflect different embryological origins within the skull. The function of the APD is also mysterious. It has been noted Results that it is always associated with an edentulous rostral portion of Gross Morphology and Nondestructive μCT-Scanning. The edentu- the premaxillae (PM) dorsally overlapping the PD (7). This may lous PD in Yanornis IVPP V13358 is ovoid in dorsal view. In suggest that these features coevolved, possibly representing an lateral view it is semicircular, such that the ventral margin is early stage in the evolution of the beak in the Ornithuromorpha, straight and the dorsal margin is strongly convex (Fig. 1). The a clade in which a beak evolved multiple times [including that of caudal half of this convex surface articulates with the DEs (Fig. the Neornithes but also in the Archaeorhynchus and Eogranivora 1A). The right lateral half of the PD in IVPP V13358 is damaged, lineages and others (14, 19)]. The DE–PD junction has been revealing an internal trabecular architecture (Fig. 1B). The left interpreted as a synovial joint linked to a piscivorous diet (7), but lateral surface was covered by sediments. CT scans indicate this details beyond gross morphology are necessary to truly understand surface was concave and perforated by 3 small foramina (Fig. the function of this skeletal feature and the tissues linking it to the rest 2B). The DEs of Yanornis bear numerous hypertrophied teeth of the lower jaws. In addition to providing clues reflecting different (also present in the caudal portion of the PM and rostral portion embryological origins (e.g., dermatocranial, splanchnocranial, or of the maxilla) (26, 27). The rostral tips are unfused at the ectocranial), bone and cartilage are also a direct reflection of the symphysis (Fig. 1). The rostral articular surface of the DEs is biomechanical stimuli (e.g., tension, compression) in which they relatively straight and angled rostrodorsal-caudoventrally, with a are deposited and maintained (20–23). slight protuberance at the rostrodorsal margin visible in
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