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Temporal and Phylogenetic Evolution of the Sauropod Dinosaur Body Plan

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Temporal and Phylogenetic Evolution of the Sauropod Dinosaur Body Plan

Karl T Bates et al. R Soc Open Sci.

Abstract

The colossal size and body plan of sauropod dinosaurs are unparalleled in terrestrial vertebrates. However, to date, there have been only limited attempts to examine temporal and phylogenetic patterns in the sauropod bauplan. Here, we combine three-dimensional computational models with phylogenetic reconstructions to quantify the evolution of whole-body shape and body segment properties across the sauropod radiation. Limitations associated with the absence of soft tissue preservation in fossils result in large error bars about mean absolute body shape predictions. However, applying any consistent skeleton : body volume ratio to all taxa does yield changes in body shape that appear concurrent with major macroevolutionary events in sauropod history. A caudad shift in centre-of-mass (CoM) in Middle Triassic Saurischia, associated with the evolution of bipedalism in various dinosaur lineages, was reversed in Late Triassic sauropodomorphs. A craniad CoM shift coincided with the evolution of quadrupedalism in the Late Triassic, followed by a more striking craniad shift in Late Jurassic-Cretaceous titanosauriforms, which included the largest sauropods. These craniad CoM shifts are strongly correlated with neck enlargement, a key innovation in sauropod evolution and pivotal to their gigantism. By creating a much larger feeding envelope, neck elongation is thought to have increased feeding efficiency and opened up trophic niches that were inaccessible to other herbivores. However, we find that relative neck size and CoM position are not strongly correlated with inferred feeding habits. Instead the craniad CoM positions of titanosauriforms appear closely linked with locomotion and environmental distributions, potentially contributing to the continued success of this group until the end-Cretaceous, with all other sauropods having gone extinct by the early Late Cretaceous.

Keywords: biomechanics; body shape; centre-of-mass; computer modelling; gigantism; phylogeny.

Figures

Figure 1.
Figure 1.
Time-calibrated phylogeny showing taxa included in this study (partly generated using [8]), with silhouettes of the convex hull volumetric models in left lateral view. Silhouettes not to scale.
Figure 2.
Figure 2.
Reconstructed sauropod dinosaur (Dicraeosaurus) body volumes. We used an automated algorithm to produce an initial minimum convex hull volume (bottom model, green) around digitized fossil skeletons to minimize subjectivity [9,10]. Two geometrically similar expansions of this minimal volume were produced (‘Plus21%’ middle, grey (in accordance with [9]); ‘maximal’ top, red) from which we selected combinations of body segments that produced the most caudal (left) and cranial (right) CoM positions.
Figure 3.
Figure 3.
Examples of neck orientations used in the sensitivity analyses. Giraffatitan model in right lateral view with neck inclined to (a) 45° and (b) in the osteologically straight, undeflected state. In (b), the neck rises at a slope of between 18 and 27° above the horizontal (depending upon the reconstruction of the pectoral girdles upon the ribcage; fig. 4 in [17]). The pose in (a), on the other hand, corresponds to the familiar giraffe-like interpretation of macronarian neck posture, wherein the neck rises steeply either by reconstructing the vertebrae as if wedge-shaped at the base (as in the Berlin reconstruction) or by suggesting they habitually bent their necks to the limit of dorsiflexion at the base [19,20].
Figure 4.
Figure 4.
Raw CoM predictions for all taxa with normalization conducted using (a) distance cranial to the hip divided by body mass0.33 and (b) as a fraction of gleno-acetabular distance. Data plotted come from the Plus21% model iteration with densities in the neck and thoracic segments of sauropodomorph models varied to represent the effects of differential levels of pneumatic air space or ‘air space proportion’ (ASP, 50%, 70% and 90%) within the vertebral column in these regions. Error bars represent the CoM position of the maximum caudad and craniad models.
Figure 5.
Figure 5.
Reduced major axis regression of CoM against mean body mass using raw data for all taxa modelled in this study with CoM normalized by (a) distance in front of the hip divided by body mass0.33 and (b) as a fraction of gleno-acetabular distance. Regression statistics for (a) distance in front of the hip divided by body mass0.33 are: all taxa RMA regression slope = 2.52 × 10−6, intercept = 0.034, r2 = 0.157, p = 0.068; Sauropodomorpha RMA regression slope = 2.65 × 10−6, intercept = 0.276, r2 = 0.172, p = 0.098; Sauropoda RMA regression slope = 2.68 × 10−6, intercept = 0.027, r2 = 0.088, p = 0.282. Regression statistics for (b) as a fraction of gleno-acetabular distance are: all taxa RMA regression slope = 1.83 × 10−5, intercept = 0.258, r2 = 0.327, p = 0.005; Sauropodomorpha RMA regression slope = 1.85 × 10−5, intercept = 0.244, r2 = 0.243, p = 0.045; Sauropoda RMA regression slope = 1.80 × 10−5, intercept = 0.253, r2 = 0.138, p = 0.172.
Figure 6.
Figure 6.
Estimated evolutionary patterns in whole-body CoM position along the craniocaudal axis of the body with data normalized by (a) distance in front of the hip divided by body mass0.33 and (b) as a fraction of gleno-acetabular distance.
Figure 7.
Figure 7.
Estimated evolutionary patterns in individual body segment properties, expressed as (a) segment length normalized by body mass0.33, (b) segment mass as a proportion of body mass, (c) distance of segment CoM position from the hip normalized by body mass0.33 and (d) segment first mass moment normalized by body mass1.33.
Figure 8.
Figure 8.
Comparison of our original estimated evolutionary patterns in whole-body CoM position (figure 6) to alternative reconstructions with inclined necks in macronarian taxa and increased/decreased neck lengths in Sauroposeidon, Dreadnoughtus and Neuquensaurus.

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