Spinosaurus aegyptiacus

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Subaqueous foraging among carnivorous dinosaurs

Secondary aquatic adaptations evolved independently more than 30 times from terrestrial vertebrate ancestors. For decades, non-avian dinosaurs were believed to be an exception to this pattern. Only a few species have been hypothesized to be partly or predominantly aquatic. However, these hypotheses remain controversial, largely owing to the difficulty of identifying unambiguous anatomical adaptations for aquatic habits in extinct animals. Here we demonstrate that the relationship between bone density and aquatic ecologies across extant amniotes provides a reliable inference of aquatic habits in extinct species. We use this approach to evaluate the distribution of aquatic adaptations among non-avian dinosaurs. We find strong support for aquatic habits in spinosaurids, associated with a marked increase in bone density, which precedes the evolution of more conspicuous anatomical modifications, a pattern also observed in other aquatic reptiles and mammals14,15,16. Spinosaurids are revealed to be aquatic specialists with surprising ecological disparity, including subaqueous foraging behaviour in Spinosaurus and Baryonyx, and non-diving habits in Suchomimus. Adaptation to aquatic environments appeared in spinosaurids during the Early Cretaceous, following their divergence from other tetanuran theropods during the Early Jurassic.

www.nature.com/articles/s41586-022-04528-0

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Spinosaurus is not an aquatic dinosaur

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.

elifesciences.org/articles/80092

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So I just wanted to highlight something.



Spinosaurus' estimated bite force was found to be comparable to that of Allosaurus in the anterior region of the jaw, although this was extrapolated by scaling up the skull-width ratio of Suchomimus. (Sakamoto, 2022).

Why is this notable? Because Allosaurus actually had thick, tyrannosaurid-like pachydont dentition at the very front of its jaw. And these teeth were built to and used to bite bone (Lei et al., 2023).

Another aspect that requires attention is the robustness of the crowns of these large bodied theropods. The mesial and lateral dentitions of Marshosaurus are particularly labiolingually compressed (CBR < 0.6), which contrasts with the thicker mesial crowns of Ceratosaurus (CBR∼0.6–0.85), Torvosaurus (CBR∼0.65), and Allosaurus (CBR∼0.8–1.16). The mesial dentition of these three last theropods, therefore, appear to be less liable to break when contacting bone than those of Marshosaurus. The lateral crowns of Ceratosaurus are, however, strongly laterally compressed (CBR∼0.3–0.4) whereas those of Torvosaurus (CBR∼0.48) and Allosaurus (CBR∼0.65; Hendrickx et al., 2020) are thicker (Bakker & Bir, 2004; Hendrickx & Mateus, 2014; Data S4). The mesial and lateral teeth of Allosaurus are, in fact, particularly thick, to a point that this theropod is considered by Hendrickx et al. (2019) and Hendrickx et al. (2020) to have a pachydont dentition similar to that of derived tyrannosaurids. The latter are well-known to have incrassate and robust teeth adapted to bone-biting involving high mechanical stresses (e.g., Holtz, 2003; Snively, Henderson & Phillips, 2006; Reichel, 2010; Hendrickx et al., 2019). Both mesial and lateral dentitions of allosaurids would, therefore, appear to be better able to withstand tooth-to-bone contact than any other carnivorous theropods from the Morrison Formation, and the deepest and numerous tooth marks seen on the largest sauropod bones such as those seen in AMNH FARB 366 (a eusauropod ilium) were probably made by Allosaurus and/or Saurophaganax. Particularly long traces (>5 cm) may also be the result of an allosaurid feeding style which, according to Snively et al. (2013), employed their powerful neck muscles to rapidly move the head downward.

A survey of spalled surfaces and the degree of crown wear in the four large-bodied theropods from the Morrison Formation, reveals that all of the large theropod taxa from the Morrison Formation probably engaged in some tooth-to-bone contact during feeding. Tooth wear nevertheless indicates that the tip of the snout with the mesial dentition in Ceratosaurus, Marshosaurus and Torvosaurus were more often in contact with bones, whereas both the mesial and mesio-lateral/transitional dentitions of Allosaurus engaged in tooth-to-bone contact.

Crown apices of fully erupted premaxillary and mesial maxillary and dentary teeth are indeed usually worn out (i.e., the enamel has fully worn away to expose the underlying dentine) in Allosaurus fragilis and A. jimmadseni. The apices of distal maxillary and dentary teeth can also be slightly worn, but are more often intact in this genus, with the denticles often crossing the tip of the crown.

Maybe another recent paper was onto something proposing that Spinosaurus may have crushed turtle shells or juvenile dinosaur bones. Or that this fast yet absolutely (but not relatively) powerful bite could instantly kill small prey or seriously injure larger prey (Yun, 2023).

[Supercommunist]

There are photos of false gharials and fresh water crocodiles killing turtles:





https://www.reddit.com/r/natureismetal/comments/oiqnb3/an_australian_freshwater_crocodile_eating_the/

Spinosaurus may have needed to hunt heavily armored fish which would have pressured it into developing stronger jaws. Alligator gar, for instance are reputed to have extremely tough scales. There used to be a video of a man striking a gar corpse with an axe and he didn't make any marks on its body.

[Supercommunist]

Size-heterodonty was positively allometric, and spinosaurids overlapped with generalist/macro-generalist crocodylians of similar sizes. Spinosaurid crown shape morphologies overlapped with certain slender-longirostrine crocodylians, yet lacked molariform distal crowns typical of most crocodylians. Spinosaurid rostra and mandibles were relatively deep with undulating margins correlating with local tooth sizes, which may indicate a developmental constraint. Spinosaurines had a particularly long concavity caudal to their rosette of anterior cranial teeth, with a corresponding bulbous rostral dentary. The spinosaurid feeding apparatus was well suited for quickly striking and creating deep punctures, but not cutting flesh or durophagy. The jaws interlocked to secure prey and move it deeper into the mouth. The baryonychines probably did little oral processing, yet spinosaurines could have processed relatively large vertebrates. Overall, there is no indication that spinosaurids were restricted to fish or small aquatic prey.

anatomypubs.onlinelibrary.wiley.com/doi/10.1002/ar.25563