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[BPT04-05] Morphological Evolution of Ankylosaurian Beaks Revealed Through a Geometric Morphometric Approach
Keywords:Cretaceous, Dinosaurs, Beak, Geometric Morphometrics, Feeding Adaptation
The Beak serves as a primary feeding apparatus in vertebrates, including birds. Its morphological variation is widely considered to reflect dietary differences. Therefore, beak morphology is a key feature for reconstructing the feeding ecology of extinct vertebrates. Ankylosaurs, a group of quadrupedal herbivorous dinosaurs, also possessed beaks, which likely played a significant role in their feeding adaptations. Throughout the Cretaceous, ankylosaurs exhibited remarkable diversification and have traditionally been classified into two major groups: ankylosaurids and nodosaurids. While representatives of both clades are abundant in North America, only ankylosaurids are commonly found in Asia.
Previous studies have demonstrated that the beaks of North American ankylosaurids tend to be broad and quadrangular, whereas those of nodosaurids are relatively rounded and tapering. Additionally, Mongolian ankylosaurids exhibit a range of beak morphologies, from broad and wide to relatively rounded and pointed. However, these prior studies were limited in scope due to the small sample size, often relying on only two genera and the qualitative properties of beak shape assessments. As a result, discussions regarding the morphological diversity and evolutionary trends of ankylosaurian beaks have remained constrained.
In this study, a series of analyses using three-dimensional geometric morphometrics was conducted to address these limitations and comprehensively investigate the evolutionary trends in ankylosaurian beak morphology. First, photogrammetry was employed to generate 3D models of the skulls of 18 genera and 19 individuals. Subsequently, using the geomorph package in the statistical software R, beak morphologies were digitized through landmark-based placement, aligned via Generalized Procrustes Analysis (GPA), and subjected to Principal Component Analysis (PCA) for dimensionality reduction.
The PCA results showed that PC1 accounted for 43.03% of the contribution ratio, PC2 for 24.25%, and PC3 for 13.93%, with the cumulative contribution ratio exceeding 80%. PC1 was interpreted as representing variations in the anteroposterior length of the beak, PC2 as indicating the degree of anterolateral projection of the beak margins, and PC3 as reflecting the sharpness of the beak tip. The data distribution for both ankylosaurids and nodosaurids was similar along PC1 and PC3. However, along PC2, nodosaurids exhibited a greater range of variation compared to ankylosaurids. Additionally, no clear distinction was observed between the data distributions of North American and Asian ankylosaurids.
These results indicate that, although the trends of variation differ between the two groups, both ankylosaurids and nodosaurids possessed a greater diversity of beak morphologies than previously recognized. This suggests that both clades had the potential for a more flexible dietary range than assumed before. Furthermore, the morphological variation in ankylosaurid beaks remained consistent regardless of geographic distribution, implying that this trait played a significant role in their adaptation to different habitats and environmental conditions.
Previous studies have demonstrated that the beaks of North American ankylosaurids tend to be broad and quadrangular, whereas those of nodosaurids are relatively rounded and tapering. Additionally, Mongolian ankylosaurids exhibit a range of beak morphologies, from broad and wide to relatively rounded and pointed. However, these prior studies were limited in scope due to the small sample size, often relying on only two genera and the qualitative properties of beak shape assessments. As a result, discussions regarding the morphological diversity and evolutionary trends of ankylosaurian beaks have remained constrained.
In this study, a series of analyses using three-dimensional geometric morphometrics was conducted to address these limitations and comprehensively investigate the evolutionary trends in ankylosaurian beak morphology. First, photogrammetry was employed to generate 3D models of the skulls of 18 genera and 19 individuals. Subsequently, using the geomorph package in the statistical software R, beak morphologies were digitized through landmark-based placement, aligned via Generalized Procrustes Analysis (GPA), and subjected to Principal Component Analysis (PCA) for dimensionality reduction.
The PCA results showed that PC1 accounted for 43.03% of the contribution ratio, PC2 for 24.25%, and PC3 for 13.93%, with the cumulative contribution ratio exceeding 80%. PC1 was interpreted as representing variations in the anteroposterior length of the beak, PC2 as indicating the degree of anterolateral projection of the beak margins, and PC3 as reflecting the sharpness of the beak tip. The data distribution for both ankylosaurids and nodosaurids was similar along PC1 and PC3. However, along PC2, nodosaurids exhibited a greater range of variation compared to ankylosaurids. Additionally, no clear distinction was observed between the data distributions of North American and Asian ankylosaurids.
These results indicate that, although the trends of variation differ between the two groups, both ankylosaurids and nodosaurids possessed a greater diversity of beak morphologies than previously recognized. This suggests that both clades had the potential for a more flexible dietary range than assumed before. Furthermore, the morphological variation in ankylosaurid beaks remained consistent regardless of geographic distribution, implying that this trait played a significant role in their adaptation to different habitats and environmental conditions.