Spinosaurus is not an aquatic dinosaur
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Evaluation Summary:
The Cretaceous dinosaur Spinosaurus has recently drawn significant attention as it was hypothesized to be the first aquatic dinosaur, using tail-powered swimming in surface waters and at depth. In a reappraisal of the "aquatic hypothesis", new lines of evidence – including the CT-based skeletal restoration of Spinosaurus and biomechanical tests – support the alternative "semi-aquatic hypothesis". This article will be of interest to vertebrate paleontologists and functional morphologists, as well as wider academic and non-academic audiences.
(This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private feedback with suggested changes to the manuscript. Reviewer #1, Reviewer #2 and Reviewer #3 agreed to share their name with the authors.)
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Abstract
A predominantly fish-eating diet was envisioned for the sail-backed theropod dinosaur Spinosaurus aegyptiacus when its elongate jaws with subconical teeth were unearthed a century ago in Egypt. Recent discovery of the high-spined tail of that skeleton, however, led to a bolder conjecture that S. aegyptiacus was the first fully aquatic dinosaur. The ‘aquatic hypothesis’ posits that S. aegyptiacus was a slow quadruped on land but a capable pursuit predator in coastal waters, powered by an expanded tail. We test these functional claims with skeletal and flesh models of S. aegyptiacus . We assembled a CT-based skeletal reconstruction based on the fossils, to which we added internal air and muscle to create a posable flesh model. That model shows that on land S. aegyptiacus was bipedal and in deep water was an unstable, slow-surface swimmer (<1 m/s) too buoyant to dive. Living reptiles with similar spine-supported sails over trunk and tail are used for display rather than aquatic propulsion, and nearly all extant secondary swimmers have reduced limbs and fleshy tail flukes. New fossils also show that Spinosaurus ranged far inland. Two stages are clarified in the evolution of Spinosaurus , which is best understood as a semiaquatic bipedal ambush piscivore that frequented the margins of coastal and inland waterways.
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Evaluation Summary:
The Cretaceous dinosaur Spinosaurus has recently drawn significant attention as it was hypothesized to be the first aquatic dinosaur, using tail-powered swimming in surface waters and at depth. In a reappraisal of the "aquatic hypothesis", new lines of evidence – including the CT-based skeletal restoration of Spinosaurus and biomechanical tests – support the alternative "semi-aquatic hypothesis". This article will be of interest to vertebrate paleontologists and functional morphologists, as well as wider academic and non-academic audiences.
(This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private feedback with suggested changes to the manuscript. Reviewer #1, Reviewer #2 and Reviewer #3 agreed to share their name with the authors.)
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Reviewer #1 (Public Review):
The Cretaceous dinosaur Spinosaurus has recently drawn significant attention as it was hypothesized as the first aquatic dinosaur. The authors provide additional lines of evidence including the CT-based skeletal restoration of Spinosaurus and biomechanical tests to challenge the 'aquatic hypothesis'. The key claims of the manuscript are supported by the new data and the new analyses are important for the further clarification of the Spinosaurus lifestyle.
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Reviewer #2 (Public Review):
The authors provide an important discussion of the often-murky issue of what makes an "aquatic" animal and clarifies various grades of aquaphily in terms of both lifestyle and adaptation. One aspect they might consider adding is an appropriate discussion of shoreline/riparian animals (such as wading birds, fishing cats, and the like) which lack specific adaptations for aquatic propulsion but which nevertheless are a part of the aquatic food web. This is relevant for issues such as the potential aquatic adaptation on p. 4, lines 122-126: namely, the posterior retraction of the external naris. As discussed in Hone and Holtz (2021), and specifically illustrated in Figs. 6 and 7 of that paper, the particular position in derived spinosaurines is not comparable to the dorsally-elevated position found in aquatic …
Reviewer #2 (Public Review):
The authors provide an important discussion of the often-murky issue of what makes an "aquatic" animal and clarifies various grades of aquaphily in terms of both lifestyle and adaptation. One aspect they might consider adding is an appropriate discussion of shoreline/riparian animals (such as wading birds, fishing cats, and the like) which lack specific adaptations for aquatic propulsion but which nevertheless are a part of the aquatic food web. This is relevant for issues such as the potential aquatic adaptation on p. 4, lines 122-126: namely, the posterior retraction of the external naris. As discussed in Hone and Holtz (2021), and specifically illustrated in Figs. 6 and 7 of that paper, the particular position in derived spinosaurines is not comparable to the dorsally-elevated position found in aquatic and semiaquatic (in terms of locomotory repertoire), but rather to that in wading feeders which are part of the aquatic ecosystem but not aquatic in terms of locomotion.
The most important contribution of this paper is the development and analysis of a new, more up-to-date skeletal restoration and 3D flesh model of Spinosaurus as a means of examining various aspects of its hydrodynamic properties. This new model is found (using reasonable approximations of body density parameters, including the distribution of air sacs) to have numerous attributes inconsistent with the hypothesis of a primarily submerged swimming lifestyle. For instance, aspects of drag prevent speeds much greater than 1 m/s whether on the surface or submerged. Additionally, the stability of the dinosaur would have been problematic, as the cross-sectional and mass distribution properties tend towards lateral tipping and an inability to easily right itself. Both the ability to maneuver and to dive were limited at best; the body proportions of this animal are considerably different than the often fusiform configuration of other large-bodied swimmers.
As previously discussed by Hone and Holtz (2021), the large dorsal sails of the torso and tail are characteristic of animals that use them in visual display, and unlike the propulsive organs of swimming forms. The authors here provide an interesting look at the typical pattern of limb reduction in large swimming animals, and how, even though the appendages of Spinosaurus are reduced relative to other theropods, they do not scale with aquatic animals.
Additionally, the authors provide an examination of the known paleogeographic distribution of Spinosaurus and related taxa and show that they include both coastal and continental fluviolacustrine occurrences. Such a pattern would be unexpected for a dedicated aquatic subaqueous predator but is entirely consistent with a shoreline generalist feeder. The phylogenetic distribution of relative traits among spinosaurids shows an increasing trend towards aquaphily.
The new reconstruction is justifiable and reasonable, and the hydrodynamic properties derived from it appear to be sound. The resultant interpretation is much more consistent with the shoreline feeder model than the aquatic subaqueous hunter one.
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Reviewer #3 (Public Review):
The authors attempted to, and succeeded at, testing the recent hypothesis that the large theropod dinosaurs Spinosaurus was a fast and capable swimmer and diver.
The strengths of the paper are the extent of the analyses which address and test numerous aspects of the 'aquatic hypothesis' with some depth, though in places this needs a better explanation of the details of the process and organisation. The results do support the conclusions, though these should be more specific and clear and relate to the recent literature on Spinosaurus habits to say what is, and is not, possible/plausible based on their analyses.
Overall this is likely to be a major step forwards in resolving the biology of this animal (and kin) and add considerable depth to the discussion by adding new data and results.
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