A buoyancy, balance and stability challenge to the hypothesis of a semi-aquaticSpinosaurusStromer, 1915 (Dinosauria: Theropoda)

Abstract

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.

Tail-propelled aquatic locomotion in a theropod dinosaur

“…Recently, potential semi-aquatic lifestyles have been hypothesised for a small number of dinosaurs 18,19 . However, the only group of dinosaurs for which multiple plausible lines of evidence indicate aquatic adaptations are the spinosaurids, large-bodied theropods interpreted as near shore waders that fed on fish along the margins of, rather than within, water bodies 10,15,20 .…”

“…A recent reappraisal of the largest known spinosaurid, Spinosaurus aegyptiacus, identified a series of adaptations consistent with a semi-aquatic lifestyle, including reduced hindlimbs, wide feet with large, flat unguals, long bones with a highly reduced medullary cavity, and a suite of cranial features such as retracted nares, interlocking conical teeth, and a rostromandibular integumentary sensory system 7 . This interpretation has been challenged on the basis of taphonomy 8 , biomechanical modeling 10 , and anatomical concerns 8 . Locomotion in water is a major point of contention 10,11 , because no unambiguous evidence for a plausible mode of propulsion has been presented.…”

“…This interpretation has been challenged on the basis of taphonomy 8 , biomechanical modeling 10 , and anatomical concerns 8 . Locomotion in water is a major point of contention 10,11 , because no unambiguous evidence for a plausible mode of propulsion has been presented. Furthermore, our understanding of the anatomy and ecology of this highly derived theropod has been hampered by the fact that only one associated Spinosaurus skeleton exists, with all other associated remains having been destroyed in World War II 7 .…”

Histology of spinosaurid dinosaur teeth from the Albian-Cenomanian of Morocco: Implications for tooth replacement and ecology

“…Further, we performed a linear regression on the replacement vs. formation rate plots in order to predict the tooth replacement rate of the spinosaurine based on the maximum tooth formation rate. The body mass for the spino-saurid was taken from Henderson (2018), all other data were taken from Erickson (1996a) Table 2). The regression was done on the total data set and on a data set including only theropods (Table 2, Appendix 1).…”

“…Isotopic data suggested a semi-aquatic lifestyle for spinosaurs (Amiot et al, 2010), although some spinosaur teeth were found with a similar isotopic signal as other terrestrial theropods. Ibrahim et al (2014;2020) suggested anatomical features, such as the retraction of the fleshy nostrils, modifications in the pelvic girdle and hind limbs, and a propulsive tail to also support a semi-aquatic life style and associated piscivory, although their reconstruction of Spinosaurus as a semi-aquatic animal has been criticized on anatomical (Evers et al, 2015) and biomechanical reasons (Henderson, 2018). Similarly, Gimsa et al (2016) put forward a new hydrodynamic hypothesis concerning the function of the back sail of Spinosaurus, but this hypothesis was also made rather unlikely by the biomechanical considerations of Henderson (2018).…”