There is a world somewhere between reality and fiction. Although ignored by many, it is very real and so are those living in it. This forum is about the natural world. Here, wild animals will be heard and respected. The forum offers a glimpse into an unknown world as well as a room with a view on the present and the future. Anyone able to speak on behalf of those living in the emerald forest and the deep blue sea is invited to join.
Recent discovery of the tall-spined tail bones of the neotypic skeleton reinvigorated the interpretation of S. aegyptiacus as the first fully aquatic dinosaur (Ibrahim et al., 2020b), here dubbed the ‘aquatic hypothesis,’ which makes three basic propositions. Unlike any other theropod, according to the hypothesis, S. aegyptiacus:
reverted to a quadrupedal stance on land, as shown by a trunk-positioned center of mass (Ibrahim et al., 2014; Ibrahim et al., 2020b), ostensibly knuckle-walking with long-fingered, long-clawed forelimbs;
functioned in water as a capable, diving pursuit predator using an expanded tail as a ‘novel propulsor organ’ (Ibrahim et al., 2020b) or as a ‘subaqueous forager’ (Fabbri et al., 2022); and
fossils would be found exclusively in coastal or deep-water marine habitats, like all large-bodied secondarily aquatic vertebrates, and would not be expected to be found in freshwater inland environments.
We test these three central propositions.
Critique of the aquatic hypothesis thus far has focused on an alternative functional explanation for the high-spined tail as a display structure and largely qualitative functional interpretations of its skeletal anatomy (Hone and Holtz, 2021). Biomechanical evaluation of the aquatic functionality of S. aegyptiacus remains rudimentary. The propulsive capacity of the tail in water was judged to be better than terrestrial counterparts by oscillating miniature plastic tail cutouts in water (Ibrahim et al., 2020b), a limited approximation of the biomechanical properties of an anguilliform tail (Lighthill, 1969; van Rees et al., 2013; Gutarra and Rahman, 2022) that failed to take account of the bizarre anterior half of the animal. The center of body mass, a critical functional parameter, has been estimated for S. aegyptiacus three times, each estimate pointing to a different location ranging from the middle of the trunk (Ibrahim et al., 2014; Ibrahim et al., 2020b) to a position over the hind limbs (Henderson, 2018). Quantitative comparisons have not been made regarding the size or surface area of the limbs, hind feet, and tail of S. aegyptiacus to counterparts in extant primary or secondary swimmers.
Thus, adequate evaluation of the aquatic hypothesis requires more realistic biomechanical tests, quantitative body, axial and limb comparisons between S. aegyptiacus and extant primary and secondary swimmers, and a survey of bone structure beyond the femur and shaft of a dorsal rib. Such tests and comparisons require an accurate 3D digital flesh model of S. aegyptiacus, which, in turn, requires an accurate skeletal model. Hence, we began this study by assembling a complete set of CT scans of the fossil bones for S. aegyptiacus and its African forerunner, Suchomimus tenerensis (Sereno et al., 1998).
Quote:Thirteen principal conclusions can be drawn from this study, all of which may be tested:
Adult S. aegyptiacus had a body length of under 14 m with the axial column in neutral pose.
The reduced hind limb long bones in neotypic skeleton of S. aegyptiacus are infilled likely as an adaptation to weight support on land rather than functioning as ballast to increase density in water.
The segment-crossing caudal neural spines in S. aegyptiacus suggest that its tail functioned more as a pliant billboard than flexible fluke.
S. aegyptiacus, like S. tenerensis and other spinosaurids, was bipedal on land with its CM positioned over its hind feet. The long-clawed forelimbs of S. aegyptiacus were not used in weight support on land.
S. aegyptiacus could wade into shallow water for feeding with flotation occurring at water depth greater than ~2.6 m.
An adult flesh model of S. aegyptiacus has a body mass of ~7400 kg and average density of ~830 kg/m3, which is considerably less than the density of saltwater (1026 kg/m3).
S. aegyptiacus was incapable of diving, given its buoyancy and incompressible trunk. Full submergence would require 15–25 times the maximum force output of its tail, depending on estimated lung volume.
S. aegyptiacus was unstable in deeper water with little ability to right itself, swim, or maneuver underwater. Maximum power from its tail, assuming it could undulate as in Alligator, is less than 700 N, which would generate a top speed of ~1 m/s, an order of magnitude slower than extant large-bodied pursuit predators.
All extant and extinct large-bodied (>2 m long) secondarily aquatic vertebrates are strictly marine, whereas fossils pertaining to Spinosaurus have been found in inland basins distant from a marine coast.
Transition to a semiaquatic lifestyle, as occurred in the evolution of spinosaurid theropods, can occur at any body size. Transition to an aquatic lifestyle among tetrapods, in contrast, has only occurred at relatively small body size (<3 m) with subsequent radiation once in the marine realm into larger body sizes.
S. aegyptiacus is interpreted as a semiaquatic shoreline ambush predator more closely tied to waterways than baryonychine spinosaurids.
Spinosaurids flourished over a relatively brief Cretaceous interval (~35 My) in circum-Tethyan habitats with minimal impact on aquatic habitats globally.
Two phases are apparent in evolution of aquatic adaptations among spinosaurids, the second distinguishing spinosaurines as the most semiaquatic of non-avian dinosaurs.
