The Spinosaurus tail

Reconstructed skeleton and caudal series of Spinosaurus aegyptiacus. From Ibrahim et al., 2020.

Spinosaurus aegyptiacus is one of the most famous dinosaur of all time. It was discovered by German paleontologist and aristocrat Ernst Freiherr Stromer von Reichenbach in 1911. This gigantic theropod possessed highly derived cranial and vertebral features sufficiently distinct for it to be designated as the nominal genus of the clade Spinosauridae. Unfortunatelly, the holotype of Spinosaurus aegyptiacus was destroyed after a British Royal Air Force raid bombed the museum and incinerated its collections. Only two photographs of the holotype of Spinosaurus aegyptiacus were recovered in the archives of the Paläontologische Museum in June 2000, after they were donated to the museum by Ernst Stromer’s son, Wolfgang Stromer, in 1995. These photographs provide additional insight into the anatomy of the holotype specimen of the animal.

Almost a century later, a partial skeleton of a subadult individual of S. aegyptiacus was discovered in the Cretaceous Kem Kem beds of south-eastern Morocco. At the time of deposition, this part of Morocco was located on the southern margin of the Tethys Ocean and it was characterized by an extensive fluvial plain dominated by northward flowing rivers and terminating in broad deltaic systems on Tethys’ southern shores. The neotype of S. aegyptiacus preserves portions of the skull, axial column, pelvic girdle, and limbs. An international team led by Nizar Ibrahim published the first description of the fossil in 2014 and suggested that Spinosaurus may have been specialised to spend a considerable portion of their lives in water.

 

Selected caudal vertebrae and chevrons of Spinosaurus. From Ibrahim et al., 2020.

Spinosaurus clearly show some adaptations to a partially or predominantly piscivorous diet (because of their morphological convergence with those of crocodilians and other fish-eating reptiles, isolated spinosaurid teeth have frequently been misinterpreted). Furthermore, the presence of a short, robust femur with hypertrophied flexor attachment and the low, flat-bottomed pedal claws are consistent with aquatic foot-propelled locomotion. Now, the description of a nearly complete and partially articulated tail of S. aegyptiacus reinforces the hypothesis that this giant theropod spent plenty of time underwater.

Proximal and distal elements of the tail are complete and preserved in three dimensions, indicating a minimal taphonomic distortion. The preserved tail is approximately 400 cm long. The zygapophyses are significantly less developed than in most tetanurans, hinting at a different functional capacity for the tail in this taxon. The neural arches are also distinctive elements of the Spinosaurus tail, while the morphology of the neural spines shows considerable variation. The elongate neural and haemal arches result in a tail shape that is markedly vertically expanded and has an extensive lateral surface area. The highly specialized morphology of the Spinosaurus tail allowed it to function as a propulsive structure for aquatic locomotion. The anterior positioning of the center of mass within the ribcage may have also enhanced balance during aquatic movement. The model proposed by Ibrahim indicates that Spinosaurus tail shape was capable of generating more than 8 times the thrust of the tail shapes of other theropods, and achieved 2.6 times the efficiency.

 

 

References:

Ibrahim, N., Maganuco, S., Dal Sasso, C. et al. Tail-propelled aquatic locomotion in a theropod dinosaur. Nature (2020). https://doi.org/10.1038/s41586-020-2190-3

Ibrahim, N., Sereno, P. C., Dal Sasso, C., Maganuco, S., Fabbri, M., Martill, D. M., Zouhri, S., Myhrvold, N., Iurino, D. A. (2014). Semiaquatic adaptations in a giant predatory dinosaur. Science, 345(6204), 1613–1616. doi:10.1126/science.1258750 

HONE, D. W. E. and HOLTZ, T. R. (2017), A Century of Spinosaurs – A Review and Revision of the Spinosauridae with Comments on Their Ecology. Acta Geologica Sinica, 91: 1120–1132. doi: 10.1111/1755-6724.13328

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  1. Pingback: Neuroanatomy of Irritator challengeri | Letters from Gondwana.

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