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A Buoyancy, Balance and Stability Challenge to the Hypothesis of a Semi-Aquatic Spinosaurus Stromer, 1915 (Dinosauria: Theropoda)


A Buoyancy, Balance and Stability Challenge to the Hypothesis of a Semi-Aquatic Spinosaurus Stromer, 1915 (Dinosauria: Theropoda)

Donald M Henderson. PeerJ.


A recent interpretation of the fossil remains of the enigmatic, large predatory dinosaur Spinosaurus aegyptiacus Stromer 1915 proposed that it was specially adapted for a semi-aquatic mode of life-a first for any predatory dinosaur. To test some aspects of this suggestion, a three-dimensional, digital model of the animal that incorporates regional density variations, lungs and air sacs was generated, and the flotation potential of the model was investigated using specially written software. It was found that Spinosaurus would have been able to float with its head clear of the water surface, although it was laterally unstable and would tend to roll onto its side. Similarly detailed models of another spinosaurid Baryonyx (Suchomimus) tenerensis Sereno et al. 1998, along with models of the more distantly related Tyrannosaurus rex Osborn 1905, Allosaurus fragilis Marsh 1877, Struthiomimus altus Lambe 1902, and Coelophysis bauri Cope 1887 were also able to float in positions that enabled the animals to breathe freely, showing that there is nothing exceptional about a floating Spinosaurus. Validation of the modelling methods was done with floated models of an alligator and an emperor penguin. The software also showed that the center of mass of Spinosaurus was much closer to the hips than previously estimated, similar to that observed in other theropods, implying that this dinosaur would still have been a competent walker on land. With its pneumatised skeleton and a system of air sacs (modelled after birds), the Spinosaurus model was found to be unsinkable, even with its lungs deflated by 75%, and this would greatly hinder a semi-aquatic, pursuit predator. The conclusion is that Spinosaurus may have been specialized for a shoreline or shallow water mode of life, but would still have been a competent terrestrial animal.

Keywords: Body mass; Buoyancy; Computer modelling; Dinosaurs; Functional morphology; Pneumaticity; Spinosaurids; Stability; Theropods.

Conflict of interest statement

The author declares that he has no competing interests.


Figure 1
Figure 1. Dorsal and lateral views of the theropod models used for flotation tests.
(A) Coelophysis bauri; (B) Struthiomimus altus; (C) Allosaurus fragilis; (D) Baryonyx (Suchomimus) tenerensis; (E) Spinosaurus aegyptiacus; (F) Tyrannosaurus rex. Animals in order of increasing mass. Lung volumes and positions are represented by the dark grey cylinders in the chest regions. Black ‘+’ denotes the computed center of mass. See Tables 1 and 2 for model image sources and model details, respectively.
Figure 2
Figure 2. Detailed view of the Spinosaurus ‘sail’ and its associated neural spines (after Ibrahim et al. (2014)).
These details were used to determine the relative fractions of the bony and soft tissue components of the sail which were then used to compute the mass and center of mass of the sail. These latter two values were components in the final calculations of the mass, center of mass, and buoyant characteristics of the complete Spinosaurus model. Small white ‘+’s are the centroids of the individual spines. Large black ‘+’ is the centroid of the entire sail. See ‘Methods’ for details of the calculations.
Figure 3
Figure 3. Three-dimensional alligator (Alligator mississippiensis) model as a validation of the methods.
(A) Basic model; (B) floating model that has attained buoyant equilibrium with a fully inflated lung. Thin, horizontal black line is the water surface. Light coloured dorsal regions are ‘dry’ and exposed to the air. Black ‘+’ denotes the center of mass, while the white ‘◊’ indicates the center of buoyancy. These figures are derived and updated from Henderson (2003b). See Tables 1 and 2 for details of the model and its floating state.
Figure 4
Figure 4. Dorsal, lateral and anterior views of the floating model of the emperor penguin (Aptenodytes forsteri).
This example of an extant, aquatic, predatory theropod was done as another test of the validity of the methods employed with the extinct theropods. The model is in its final, equilibrium flotation state with a full lung, and replicates the situation seen in living emperor penguins floating at the water surface. Unlike all the other flotation tests, this one is done with seawater of density 1,026 gm/l. Colours and symbols as per Fig. 3. See Table 2 for details of the model and its floating state.
Figure 5
Figure 5. Floating spinosaurids in lateral and dorsal views.
(A) Spinosaurus aegyptiacus; (B) Baryonyx (Suchomimus) tenerensis. Determination of the buoyant state required knowing the masses and centers of mass of the axial body (accounting the presence of a lung), all four limbs, and in both cases, the dorsal ‘sail.’ See Table 2 for model details.
Figure 6
Figure 6. Floating theropods with masses ranging from 10.3 to 9,360 kg.
(A) C. bauri; (B) S. altus; (C) A. fragilis; (D) T. rex. See Fig. 3 explanation of symbols. All models floated with full lungs. See Table 2 for model details.
Figure 7
Figure 7. Graphical views of the metacentric heights (MC ‘□’), centers of buoyancy (CB ‘◊’), and centers of mass (CM ‘+’) computed from the three-dimensional models.
(A) Alligator mississippiensis; (B) Spinosaurus aegyptiacus. A center of mass above the metacentric height indicates an unstable situation, which is clearly the case for the Spinosaurus. Stated measurements are relative to the water line and are in meters. See ‘Methods and Results’ sections for more details. Green indicates the ‘dry’ area above the waterline, while the blue is the ‘wet,’ immersed portion.
Figure 8
Figure 8. A test of the lateral stability of the floating Alligator model using a disk representing the transverse section of the immersed axial body at the position of the CM from the floating model of Fig. 3B.
The disk was given a 20 sideways tip, but over the course of 42 simulation cycles it slowly returned to an upright orientation by passive self-righting. Symbols and colors as per Fig. 7.
Figure 9
Figure 9. A test of lateral stability of the floating Spinosaurus model using a disk representing the cross-sectional area of the axial body at the position of the CM from the floating model of Fig. 5A.
The disk was given a 20 sideways tip, but over the course of 10 simulation cycles it quickly rolled onto its side to a new position of stable equilibrium. Symbols and colors as per Fig. 7.
Figure 10
Figure 10. Isometric views of hindlimb model of A. fragilis using the right limb from Fig. 1C and three-dimensional models of the large limb bones based on illustrations in Madsen (1976).
The volumes of these shapes, combined with the appropriate densities, were used to investigate the effects of higher than normal bone densities on the mass and density of the host animal. See ‘Results.’
Figure 11
Figure 11. Centres of mass determinations for the axial body of Spinosaurus using two different methods.
(A) Two-dimensional silhouette with constant areal density; (B) three-dimensional mesh without lung cavity or air sacs. In neither case does the CM reside at the midpoint of the trunk region as claimed by Ibrahim et al. (2014). See ‘Discussion’ section.
Figure 12
Figure 12. Relative mass fractions of the hindlimbs of the theropods in the present study highlighting the small size of the restored Spinosaurus hindlimbs.
Dashed line represents the mean value of 12.6%. Grey band spans plus and minus one standard deviation about the mean. The Spinosaurus limb mass was not used in the calculation of the mean and standard deviation. A.f, Allosaurus fragilis; B.t, Baryonyx (Suchomimus) tenerensis; C.b, Coelophysis bauri; S.a, Struthiomimus altus;, Spinosaurus aegyptiacus; T.r, Tyrannosaurus rex.

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