This paper seems to have missed the transitional niche of shallow water, somewhere between dry land and deep water. Even Wikipedia, reports, “It (= Spinosaurus] lived in tidal flats and mangrove forests.”
Sereno et al. report,
“That model shows that on land S[pinosaurus]. aegyptiacus was bipedal and in deep water was an unstable, slow surface swimmer (<1m/s) too buoyant to dive.”
There’s no need to dive. What’s wrong with just shoulder-deep water? Or belly deep water? The Sereno et al. figure 2d (Fig 1 here) shows exactly that. That’s where most humans and other tetrapods spend most of their time when near rivers and beaches.
Too buoyant to dive is: OK! Slow surface swimming is: No problemo!
Sereno et al. report,
“Living reptiles with similar spine-supported sails over trunk and tail in living reptiles are used for display rather than aquatic propulsion, and nearly all extant secondary swimmers have reduced limbs and fleshy tail flukes.”
“Secondary swimmers have reduced limbs.” Wait a minute! This is an argument FOR an aquatic niche, as Sereno et al. acknowledge (Fig 1). Hone and Holtz 2021 made the same set of mistakes.
Sereno et al. make no mention of thermal regulation. We looked at that hypothesis for sail evolution back in 2015 (Fig 2).
Sereno et al. make no mention of the retracted and vestigial external naris
of Spinosaurus (Fig 3). This location and its tiny size are both quite different from most theropods making this yet another overlooked aquatic trait. The narial aperture cannot be closed in most tetrapods, including river and sea turtles. By contrast, hippos, crocs, ‘whales‘ and ‘seals‘ have narial valves to close the naris. Transitional pre-spinosaurs with smaller, more and more retracted nares (Fig 3), likely were not fitted with narial valves based on the evidence of extant birds (which lack narial valves) and the vestige naris of Spinosaurus.
The Sereno et al. taxon list fails to include
Xiongguanlong (Fig 4) and several other spinosaur outgroups, so the Sereno et al. tree topology does not match the large reptile tree (LRT, 2104 taxa) due to taxon exclusion.
While this is not pertinent to the aquatic issue,
it does show a too focused view on the part of Sereno et al., rather than a more appropriate panoramic view, examining all pertinent aspects of this issue. These are clues that Sereno et al. were focused only on what they wanted to document, based on their headline, and their lack of considering the spectrum of niches between dry land and the deepest waters. In summary, it’s okay that spinosaurs got wet. After all, they ate fish. Big, slow fish (Fig. 2).
Sereno et al. report what everyone knows,
“Aquatic vertebrates (e.g., bony fish, sea turtles, whales) live exclusively or primarily in water and exhibit profound cranial, axial or appendicular modifications for life in water, especially at larger body sizes.”
Ironically Spinosaurus exhibits profound cranial, axial and appendicular modifications for life in water compared to spinosaur-related theropods (Fig 3) and theropods in general. Not sure why Sereno et al. don’t view Spinosaurus in this way other than by keeping their blinders on. We’ve seen this over and over and over again in vertebrate paleontology, so it’s a pattern.
What can diving ducks teach us?
According to Ducks.org, “Like other specialized diving birds, diving ducks also have an unusually high tolerance for asphyxia, or lack of air. This “diving reflex” is triggered when water touches special receptors in the birds’ nares (nostrils).”
Duck nares are not tiny nor located back near the eyes. So a diving duck is not a good analog for Spinosaurus.
Jabiru (Fig 5) has tiny retracted nares, like those of Spinosaurus. This wading bird dips its snout in the water to probe for prey and continue breathing with nostrils held high above the surface. Based on such morphological evidence Spinosaurus had a similar lifestyle: hovering its head over belly-deep waters, dipping its snout in from time to time to probe for prey.
A co-author on Sereno et al. 2022
is Don Henderson, curator of dinosaurs at the Royal Tyrrell Museum who earlier (Henderson 2018) challenged the buoyancy, balance and stability of Spinosaurus. Sereno et al. echo many of Henderson’s hypotheses.
Henderson D 2018. A buoyancy, balance and stability challenge to the hypothesis of a semi-aquatic Spinosaurus Stromer, 1915 (Dinosauria: Theropoda). PeerJ 6:e5409; DOI 10.7717/peerj.5409
Hone DWE and Holtz TR Jr 2021. Evaluating the ecology of Spinosaurus: Shoreline generalist or aquatic pursuit specialist. Palaeontologica Electronica 24(1):a03 Online Here.
Sereno PC et al. (8-coauthors) 2022. Spinosaurus is not an aquatic dinosaur. bioRxiv preprint doi: https://doi.org/10.1101/2022.05.25.493395; this version posted May 26, 2022.