An ankylosaur larynx provides insights for bird-like vocalization in non-avian dinosaurs

The larynx, a crucial structure in tetrapods involved in respiration, airway protection, and vocalization, has a sparse fossil record, particularly in birds and other reptiles. Understanding its evolutionary trajectory is challenging due to the typically cartilaginous nature of the larynx, hindering fossilization. This study focuses on the *Pinacosaurus grangeri* specimen (IGM 100/3186), which exceptionally preserves a hyolaryngeal apparatus, including the larynx. The research question revolves around characterizing the *Pinacosaurus* larynx and comparing its morphology to both modern birds and non-avian reptiles to infer its functional role and implications for vocalization in non-avian dinosaurs. The study's importance lies in its potential to significantly advance our understanding of the evolution of vocal communication in archosaurs, bridging the gap between non-avian dinosaurs and birds. The discovery of a well-preserved larynx in a non-avian dinosaur provides crucial anatomical data previously unavailable, potentially resolving long-standing questions about the origins and evolution of avian-like vocalizations.

Previous research has extensively studied the hyoid apparatus in various tetrapods, highlighting its functional diversity in feeding, respiration, and vocalization. However, the larynx itself has remained largely elusive in the fossil record, especially in non-avian reptiles. While hyoid elements have been documented in some non-avian dinosaurs, a complete and well-preserved larynx had not been previously reported. This scarcity of data has limited our understanding of laryngeal evolution in archosaurs and the emergence of bird-like vocalization. The authors review existing literature on the anatomy and function of the larynx in modern archosaurs (crocodilians and birds), highlighting the differences in their vocalization mechanisms and laryngeal structure. This sets the stage for comparing the fossil *Pinacosaurus* larynx to these extant groups to infer its functional morphology and evolutionary significance. This review also includes work on the hyolaryngeal apparatus of other extinct tetrapods, illustrating the limited data available to understand the evolution of this complex anatomical system and the need for discoveries like this one to provide new insights.

The study focuses on the *Pinacosaurus grangeri* specimen (IGM 100/3186), which preserves a well-preserved hyolaryngeal apparatus. The researchers used a combination of techniques to analyze the specimen. This includes detailed anatomical description, morphometric analysis, and comparison with extant reptile and bird larynges. Computed tomography (CT) scanning and laser scanning were employed to create three-dimensional reconstructions of the larynx, enabling detailed visualization and measurement of its components. Morphometric data, including measurements of the cricoid and arytenoid cartilages, were collected and normalized using the mandible width as a scaling factor. Statistical analyses, such as linear regression, Student's t-tests, and Tukey HSD tests, were performed to compare the size and proportions of the *Pinacosaurus* larynx to those of a wide range of modern reptiles and birds. The researchers compared this data against a dataset compiled from various sources, including museum collections (American Museum of Natural History, National Museum of Nature and Science, Tokyo), anatomical models, and published literature. This comparative approach allows the authors to assess the uniqueness of the *Pinacosaurus* larynx within the broader context of archosaur laryngeal evolution. The use of Morphobank (http://www.morphobank.org; ID: 7201) for data sharing facilitated the broader scientific community to access and verify the research findings.

The *Pinacosaurus* larynx consists of ossified cricoid and arytenoid cartilages. The cricoid is characterized by its large size, an arrowhead shape in dorsal view, and a prominent anterior projection. The arytenoids are elongated and possess a prominent process. The cricoid-arytenoid joint is firm and kinetic, facilitating horizontal rotation of the laryngeal framework. Morphometric analyses reveal that the arytenoid length and cricoid width in *Pinacosaurus* are significantly larger relative to mandible width compared to those of most non-avian reptiles. These features are more similar to birds than to other reptiles. The large size and kinetic nature of the larynx in *Pinacosaurus* suggest a role beyond simple airway protection, indicating involvement in vocalization. The morphology of the *Pinacosaurus* larynx, especially the large cricoid, firm cricoid-arytenoid joint, and long arytenoids, are consistent with a vocal modifier function, similar to the role of the larynx in birds. This indicates the potential for sophisticated airflow regulation and sound modification, suggesting complex vocalizations in this dinosaur. Comparative analyses show a clear distinction between the laryngeal morphology of *Pinacosaurus*, birds, and other reptiles, supporting the hypothesis of a convergent evolution of bird-like vocal mechanisms in non-avian dinosaurs. The discovery of a larynx in *Pinacosaurus* pushes back the known presence of ossified larynges in the fossil record to the Late Cretaceous.

The findings suggest that bird-like vocalizations likely evolved in non-avian dinosaurs before the advent of Aves. The *Pinacosaurus* larynx exhibits several key features associated with vocal modification in birds, including a large and kinetic larynx with a firm cricoid-arytenoid joint. This contrasts sharply with the laryngeal morphology of most non-avian reptiles, where the larynx serves primarily as a vocal source. The presence of a large cricoid, consistent with vocalization in extant archosaurs, further supports this interpretation. The combination of a large, kinetic larynx and the likely presence of an air sac system in *Pinacosaurus* points to the capability of sophisticated sound production and communication. The implication is that the evolution of avian-like vocalizations may have occurred earlier and involved different evolutionary pathways than previously thought. Future research should focus on exploring the presence of similar laryngeal structures in other non-avian dinosaur groups to further refine our understanding of the evolution of vocalization in archosaurs.

This study presents the first confirmed fossilized larynx from a non-avian dinosaur, *Pinacosaurus grangeri*. The unique morphological features of this larynx, particularly its large size, kinetic joint, and elongated arytenoids, suggest a role in sound modification, similar to the avian larynx. This finding provides strong evidence for bird-like vocalizations in non-avian dinosaurs and significantly advances our understanding of archosaur vocal evolution. Future work should focus on searching for similar laryngeal structures in other dinosaur groups and investigating the possible presence of additional vocal mechanisms, such as the syrinx, in non-avian dinosaurs.

The study is based on a single well-preserved specimen of *Pinacosaurus grangeri*. While the findings are compelling, further discoveries are needed to confirm whether these laryngeal features are widespread among non-avian dinosaurs or specific to certain lineages. The absence of soft tissues prevents a complete understanding of the musculature and the exact mechanism of sound production. However, the strong morphological evidence presented here provides a robust starting point for future research.