Summary for those in a hurry:
A unique morphology + a unique niche + a unique prey assemblage = a unique hunting technique.
From the Hone and Holtz 2021 abstract:
“We conclude that …the pursuit predation hypothesis for Spinosaurus as a “highly specialized aquatic predator” is not supported. In contrast, a ‘wading’ model for an animal that predominantly fished from shorelines or within shallow waters is not contradicted by any line of evidence and is well supported. Spinosaurus almost certainly fed primarily from the water and may have swum, but there is no evidence that it was a specialised aquatic pursuit predator.”
Hone and Holtz pay little attention to the fact that Spinosaurus was the only large theropod that had such short hind limbs and had a dorsal fin much deeper than its ribcage. The authors cherry-picked less obvious traits to support their hypothesis, giving only passing notice to what makes Spinosaurus unique.
From the Hone and Holtz introduction:
“In short, these animals [spinosaurs] acted like large herons or storks, taking fish and other aquatic prey from the edges of water or in shallow water, but also foraging for terrestrial prey and scavenging on occasion.”
In paleontology, if Spinosaurus is going to be compared to large herons or storks, it should look overall like a giant heron or stork. It does not.
Hone and Holtz keep hammering away at a single point:
“The hind limbs of Spinosaurus do potentially provide evidence for aquatic locomotion and even striking at prey underwater, but specifically not in the sense of pursuit predation.”
“Surface swimming is considerably less efficient than submerged swimming and incurs considerable extra wave drag for animals moving at, or just below, the surface.”
The problem is, Hone and Holtz want Spinosaurus to be built for speed, like a sailfish, if they are to grant it a submerged aquatic existence. Unfortunately, the authors are caught in a logic rut based on some sort of straw man. They end up cherry-picking traits less important traits while trying to weave their story away from the larger, unique traits.
Spinosaurus was not built for speed.
It didn’t need to be built for speed. Look at the prey taxa available (Fig. 1). Lungfish, giant bichirs and coelacanths are big, fat and slow-moving fish. Sawfish are lethargic bottom-dwellers. Drag is not a factor when moving slowly, like Spinosaurus.
As the only aquatic dinosaur,
Spinosaurus may have developed a sail to help regulate body temperature while staying submerged (except to lay eggs). It may have never needed to stand bipedally, like its theropod sisters. Hence the small legs and quadrupedal center-of-balance.
The tiny backset naris of Spinosaurus
was on its way to complete closure. That’s not a problem as many extant birds without nares demonstrate. They can all breathe throughout their mouth and throat.
Hone and Holtz summarize their study:
“If swimming to engage prey, based on the drag, performance and body shape it would be limited to lunging attack in shallow waters, not pursuit predation at speed in open water.”
No. Spinosaurus was a slow swimmer, unaffected by drag. It would not be limited to lunging attacks in shallow water, contra Hone and Holtz. Rather, slow, steady underwater predation with its sail exposed to maintain a 99º body temperature in an 80º river is still the best explanation for this unique theropod.
“The information provided through recent discoveries may suggest an increase in aquatic affinities for Spinosaurus, and it may have been able to swim with its tail, and even swim well compared to other theropods, but nothing presented to date contradicts the fundamentals of the ‘wading model’ and does not support active pursuit predation.”
Hone and Holtz failed to consider a semi-active, semi-submerged method of predation. “Nothing presented to date” = failure to consider all options. Spinosaurus is indeed a “highly specialized aquatic predator”, just not a fast one.
Earlier we looked at Spinosaurus in its environment here, its ability to swim deep here and its tiny naris here.
Unfortunately, papers from co-author David Hone are infamous for taxon exclusion, inaccurate observation, and illogical interpretation. Not sure why referees and editors are letting him get away with negating good solid science with bad flimsy science.
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.