Disentangling biological variability and taphonomy: shape analysis of the limb long bones of the sauropodomorph dinosaur Plateosaurus

Abstract

Sauropodomorph dinosaurs constitute a well-studied clade of dinosaurs, notably because of the acquisition of gigantism within this group. The genus Plateosaurus is one of the best-known sauropodomorphs, with numerous remains from various localities. Its tumultuous taxonomic history suggests the relevance of addressing its intrageneric shape variability, mixed with taphonomic modifications of the original bone shape. Here we investigate quantitatively the morphological variation of Plateosaurus occurring at the genus level by studying the shape variation of a sample of limb long bones. By means of 3D geometric morphometrics, the analysis of the uncorrelated variation permits separation of the variation estimated as obviously taphonomically influenced from the more biologically plausible variation. Beyond the dominant taphonomic signal, our approach permits interpretation of the most biologically plausible features, even on anatomical parts influenced by taphonomic deformations. Those features are thus found on a quantitative basis from the variation of samples containing fossil specimens, by taking the impact of taphonomy into account, which is paramount in order to avoid making biologically ambiguous interpretations.

Keywords: 3D Geometric morphometrics; Comparative anatomy; Deformation; Dinosauria; Paleobiology; Saurischia; Sauropodomorpha.

Figure 1. Results of the PCA on the PC1 and PC2 of the humerus analysis (right side illustrated).

On the PCA plot (A), the green cluster represents the morphospace occupied by the genus Plateosaurus, the orange dots correspond to the Efraasia specimens. Extrema of shape changes along PC1 (B) and PC2 (C) are represented in anterior and lateral views. Abbreviations: adp, apex of deltopectoral crest; dpc, deltopectoral crest; hh, humeral head; hlt, humeral lateral tubercle; hmt, humeral medial tuberosity; hrc, humeral radial condyle; huc, humeral ulnar condyle.

Figure 2. Results of the PCA on the PC5 and PC6 of the humerus analysis (right side illustrated).

On the PCA plot (A), the green cluster represents the morphospace occupied by the genus Plateosaurus, the orange dots correspond to the Efraasia specimens. Extrema of shape changes along PC5 (B) and PC6 (C) are represented in anterior and lateral views.

Figure 3. Results of the PCA on the PC1 and PC2 of the radius analysis (right side illustrated).

On the PCA plot (A), the green cluster represents the morphospace occupied by the genus Plateosaurus, the orange dot corresponds to the Efraasia specimen, the blue dot corresponds to the Ruehleia specimen. Extrema of shape changes along PC1 (B) and PC2 (C) are represented in medial and posterior views. Abbreviations: rhc, radial humeral cotyle.

Figure 4. Results of the PCA on the PC5 and PC6 of the radius analysis (right side illustrated).

On the PCA plot (A), the green cluster represents the morphospace occupied by the genus Plateosaurus, the orange dot corresponds to the Efraasia specimen, the blue dot corresponds to the Ruehleia specimen. Extrema of shape changes along PC5 (B) and PC6 (C) are represented in medial and posterior views.

Figure 5. Results of the PCA on the PC1 and PC2 of the ulna analysis (right side illustrated).

On the PCA plot (A), the green cluster represents the morphospace occupied by the genus Plateosaurus, the orange dots correspond to the Efraasia specimens, the blue dot corresponds to the Ruehleia specimen. Extrema of shape changes along PC1 (B) and PC2 (C) are represented in medial and posterior views. Abbreviations: ap, anterior process; lp, lateral process; ol, olecranon; uhc, ulnar humeral cotyle.

Figure 6. Results of the PCA on the PC5 and PC6 of the ulna analysis (right side illustrated).

On the PCA plot (A), the green cluster represents the morphospace occupied by the genus Plateosaurus, the orange dots correspond to the Efraasia specimens, the blue dot corresponds to the Ruehleia specimen. Extrema of shape changes along PC5 (B) and PC6 (C) are represented in medial and posterior views.

Figure 7. Results of the PCA on the PC1 and PC2 of the femur analysis (right side illustrated).

On the PCA plot (A), the green cluster represents the morphospace occupied by the genus Plateosaurus, the orange dot corresponds to the Efraasia specimens, the blue dots correspond to the Ruehleia specimen and the brown dot correspond to SMNS 12220. Extrema of shape changes along PC1 (B) and PC2 (C) are represented in anterior and lateral views. Abbreviations: 4t, fourth trochanter; fh, femoral head; flc, femoral lateral condyle; fmc, femoral medial condyle; gt, greater trochanter; lt, lesser trochanter.

Figure 8. Results of the PCA on the PC3 and PC4 of the femur analysis (right side illustrated).

On the PCA plot (A), the green cluster represents the morphospace occupied by the genus Plateosaurus, the orange dot corresponds to the Efraasia specimens, the blue dots correspond to the Ruehleia specimen and the brown dot correspond to SMNS 12220. Extrema of shape changes along PC3 (B) and PC4 (C) are represented in anterior and lateral views.

Figure 9. Results of the PCA on the PC1 and PC2 of the tibia analysis (right side illustrated).

On the PCA plot (A), the green dots correspond to the specimens of the genus Plateosaurus. Extrema of shape changes along PC1 (B) and PC2 (C) are represented in lateral and posterior views. Abbreviations: cn, cnemial crest; tap, tibial ascending process; tdp, tibial descending process; tfc, tibial fibular condyle; tic, tibial internal condyle.

Figure 10. Results of the PCA on the PC5 and PC6 of the tibia analysis (right side illustrated).

On the PCA plot (A), the green dots correspond to the specimens of the genus Plateosaurus. Extrema of shape changes along PC5 (B) and PC6 (C) are represented in lateral and posterior views.

Figure 11. Results of the PCA on the PC1 and PC2 of the fibula analysis (right side illustrated).

On the PCA plot (A), the green dots correspond to the specimens of the genus Plateosaurus. Extrema of shape changes along PC1 (B) and PC2 (C) are represented in lateral and posterior views. Abbreviations: amp, anteromedial projection (following Otero et al., 2015); ifi, iliofibularis insertion.

Figure 12. Results of the PCA on the PC5 and PC6 of the fibula analysis (right side illustrated).

On the PCA plot (A), the green dots correspond to the specimens of the genus Plateosaurus. Extrema of shape changes along PC5 (B) and PC6 (C) are represented in lateral and posterior views.

Figure 13. Examples of obviously taphonomically influenced variations not taken into account in the biological interpretations.

Observations on theoretical shapes representing variations on the humerus in anterior (A), proximal (B) and distal (C) views; the femur in anterior (D), proximal (E) and distal (F) views; the radius in posterior (top) and proximal (bottom) views (G); the fibula (H) in anterior view; the tibia in anterior view (I); the ulna in anterior (top) and proximal (bottom) views (J); the fibula in proximal view (K); the tibia in proximal view (L); the ulna in distal view (M) and the tibia in distal view (N). The letter T denotes the areas affected by taphonomic deformations. Associated double-headed arrows correspond to a flattening variation, associated curves correspond to a bending or a modification of torsion variation. Associated single headed arrows or absence of associated signs correspond to particular patterns of variation (see text).

Figure 14. Deltopectoral crest morphological variation on the biologically plausible PCs (i.e., PCs 4.5 & 6).

On the PC4 (A), PC5 (B) and PC6 (C), the theoretical shapes corresponding to the minimum of variation along the axis are represented on the left, whereas the shapes corresponding to the maximum of variation are represented on the right. Abbreviations: adp, apex of deltopectoral crest; dedp, distal edge of deltopectoral crest; dpdp, distal process of deltopectoral crest; pedp, proximal edge of deltopectoral crest.

Figure 15. Selected close-ups of biologically plausible humeral variation (i.e., PCs 4, 5 & 6).

Variation on PC5, humeral shaft in medial view (A), on PC6, proximal end in proximal view (B), distal end on PC5 in anterior view (C) and on PC4 in distal view (D). The theoretical shapes corresponding to the minimum of variation along the axis are represented on the left, whereas the shapes corresponding to the maximum of variation are represented on the right.

Figure 16. Selected close-ups of biologically plausible radial and ulnar variation (i.e., PCs 4, 5, 6 & 7).

For radii, variation on PC4 of radial shaft in medial view (A) and proximal end in proximal view (C), on PC5, distal end in distal view (E); for ulnae, variation on PC4 of ulnar shaft approximatively in medial view (B) and of proximal end in proximal view (D), on PC5, distal end in PC5 (F). The theoretical shapes corresponding to the minimum of variation along the axis are represented on the left, whereas the shapes corresponding to the maximum of variation are represented on the right.

Figure 17. Selected close-ups of biologically plausible femoral variation (i.e., PCs 3, 4 & 6).

Variation on PC3 of femoral shaft in medial view (A), on PC4, proximal end in proximal view (B), on PC3, fourth trochanter in medial view (C), and in PC4, distal end in distal view (D). The theoretical shapes corresponding to the minimum of variation along the axis are represented on the left, whereas the shapes corresponding to the maximum of variation are represented on the right.

Figure 18. Selected close-ups of biologically plausible tibial (i.e., PCs 5 & 6) and fibular variation (i.e., PCs 2, 6 & 7).

For tibiae, variation on PC5 of tibial shaft in anterior view (A) and proximal end in proximal view (C), on PC6, distal end in distal view (E); for fibulae, variation on PC6 of fibular shaft in medial view (B), on PC2, variation of proximal end in proximal view (D), and of distal end in medial view (F). The theoretical shapes corresponding to the minimum of variation along the axis are represented on the left, whereas the shapes corresponding to the maximum of variation are represented on the right.

Figure 19. Most biologically compelling variation observed on sampled specimens.

Observations of sampled bones for humeral deltopectoral crest outline in lateral view (A), femoral fourth trochanter outline in medial view (B) & head shape in proximal view (C). Not to scale.

Figure 20. Less biologically compelling variation observed on sampled specimens.

Observations of sampled bones for humeral shaft shape in medial view (A), radial shaft shape in posterior view (B), ulnar shaft shape in posterior view (C), fibular proximal end in proximal view (D) & distal end (E) in medial (top) and posterior view (bottom). Not to scale.

Figure 21. NJ clustering analysis on the biologically plausible variation of the ulna (PCs 4-7).

The displayed theoretical shapes correspond to the two subgroups discriminated by the analysis, showing the mean variation between each groups.

Figure 22. Intergeneric variation occurring between Plateosaurus and Ruehleia.

Observations of sampled specimens of the variation on femoral shaft curvature between SMNS 13200 (Plateosaurus) and the two sampled specimens of Ruehleia in medial view (A), and variation occurring between the two sampled of Ruehleia femora on femoral head in anterior view (B). Not to scale.