Before I get into the depths of this post since I’m sure many people won’t read it all (and it’s good to prep the reader), here’s the TLDR: I am not saying Spinosaurus didn’t or couldn’t swim, or that it could not swim better than most other large theropods, but the evidence presented to date that it was a semi-aquatic animal and a specialist hunter in water (and specifically a pursuit predator) is really not well supported from the currently available evidence. It may even ultimately turn out to be correct, but as things stand, the evidence is weak and there are a lot of gaps and contradictions to this model.
So, there’s the essence of it. The purported arguments that Spinosaurus was some croc-like or even stem-whale like animal spending the vast majority of its time in water, and the way it has been illustrated swimming in deep water, and even diving and pursuing fish does not hold up to scrutiny. Instead, a wading model of a more heron- or stork-like animal that spent a lot of time in and around water, but fundamentally fished while standing rather than swimming, is supported. I really don’t want to go into everything in this post since the paper I have just out on the subject is some 13000 words of text followed by 120 references, so is incredibly long and detailed. The paper is open access so freely accessible and I think it’s fairly easy to read and follow, so a huge post here is rather redundant.
The new paper is an extension of my collaboration with Tom Holtz on spinosaur biology and both follows up and expands on our ideas from our 2017 paper, and tackles a bunch of interrelated hypotheses about the biology of Spinosaurus. In essence, it’s supposed to be unique among theropods (even compared to other spinosaurs) in being highly aquatic and a pursuit-predator of fish in water, powered by a deep tail. This stands in contrast to our own favoured concept (which is not original, but really an extension / modification of the basic ideas put forwards by others) with them standing in water to take primarily piscivorous prey. There is more depth and nuance of course but that’s the basic split, actively swimming after things, or wading and grabbing. There’s no real argument that spinosaurs were generally tied in some way to water and aquatic prey, but how much and how they hunted is the core issue here.
For the paper, we went through the whole functional anatomy of Spinosaurus and the claims and arguments for both models. We collected some original data to analyse but also looked at some possible analogues and a bunch of literature and existing data that is out there too. What follows are a bunch of extremely simplified and reduced points and as such they might not be entirely clear to everyone but the central idea should be clear enough.
A principal components analysis of the shape of the skull show that Spinosaurus is like other spinosaurs (which are not supposed to be as specialised) and also closer to other theropods than crocodiles or other semi-aquatic and aquatic reptiles.
Similarly, the nares and orbits are not dorsally positioned and are not like semi-aquatic taxa.
The posteriorly retracted nares would allow the jaw tips to be in water while foraging as they are in modern storks and herons.
The supposed sensory system of the jaw is similar to that seen in other terrestrial theropods.
The form of the teeth are similar to those of large aquatic and semi-aquatic reptiles, but specifically they conform to a ‘generalist’ feeder type.
Functionally the skull would work very similarly to that of Baryonyx even though they are supposed to be hunting in different ways.
Enamel isotopes data shows that some individuals spent considerable times in terrestrail environments and some individuals ate a lot of terrestrial dinosaur prey which doesn’t fit with an animal that is an aquatic specialist to pursue fish.
The neck is specialised for a downwards action which would fit with an animal standing in water striking down better than an animal already swimming in water which could strike in any direction.
The neck is relatively long and with bracing cervical ribs. Such support would not be needed in water, but is useful if standing and striking.
The possibility of webbing on the toes could help this animal swim but would be equally useful for walking around in mud and other soft substrates.
The pedal unguals are rather flat, but other large theropods have similarly flattened unguals. Only very few modern birds have these are not usually good swimmers and also include a number of waders.
The large neural spines in the tail don’t match those of other swimming animals and do match those of species which use the tail for display.
The efficiency calculations presented show that Spinosaurus was a much less efficient swimmer than crocodiles, but these themselves are not fast and efficient swimmers and are not pursuit predators.
The tail has limited muscle attachments compared to tail-driven swimmers suggesting it was not a fast or powerful swimmer.
The dorsal neural spines would have created massive surface drag meaning that they could only reach efficient swimming if they were submerged in very deep water.
Despite the evidence of high bone density, they still have lots of pneumaticity which would make them unstable in water and make diving a strenuous exercise.
The legs (while apparently reduced) and arms are not anything like as reduced as seen in fast swimming animals.
As I say, there’s even more than these points covered in the paper and there’s additional details and nuances that I’m not going to cover here (and there’s some further ideas we left out of the paper because we didn’t have room). Still, as you can hopefully see there are some major issues with the advocated hypothesis that Spinosaurus was extremely adept in water. As noted at the start, this does remain a possibility, but the arguments put forwards to date are weak and even contradicted by other data. So the idea of a highly-aquatic and pursuit-predator Spinosaurus should be shelved for now, while the wading model is currently well supported by various lines of evidence.
Water was obviously important to the spinosaurs and Spinosaurus does show some traits suggesting greater affinities with water and perhaps reliance on swimming than other members of the group. But fundamentally it’s very similar to Baryonyx and other spinosaurs in lots of important functional ways suggesting they were basically doing the same thing the same way. Spinosaurus shows only the slightest adaptations towards aquatic ecology compared to lots of other semi-aquatic and aquatic animals and doesn’t have lots of ones that have appeared repeatedly in numerous lineages. The analyses to date of its swimming ability have been primitive but a big animal with a sail would have ton of drag and it appears to be a low muscled and inefficient swimmer – that’s not an animal that is going to be actively chasing fish.
We all know there is more material of this enigmatic dinosaur being uncovered and new work is also coming on various other spinosaurs which will help clarify things further. I think it’s reasonable to say this is by far the most in depth assessment of the ecology and behaviour of these animals to date and helps redress the balance of some hypotheses that have been advocated with very little support and gives a much firmer foundation for working out which spinosaurs may have been doing what and how. These are fascinating animals who undoubtedly had unusual ecologies but we can test ideas about their biology and this is, I hope, a major step forwards in that.
Hone, D.W.E., & Holtz, T.R. 2021. Evaluating the ecology of Spinosaurus: shoreline generalist or aquatic pursuit specialist? Palaeontologica Electronica.
The paper is fully open access to anyone can access the link above. I’d like to thank Tom for all his work on this, the four referees and editors who put a lot of effort into reviewing such a big paper and all the various people who contributed little bits of data, papers and images (especially GetAwayTrike on Twitter for the skeleton and Bob Nicholls for the awesome artwork) and those who acted as sounding boards for various discussions.
A quick update, here’s the podcast special episode I did on this paper.
Dave Hone's Archosaur Musings 
Nice work! You certainly seem to cover a lot of different lines of evidence.
But where is Bob’s artwork? I don’t see it in this post.
I can see it, so not sure why its not displaying.
OK, using my 1337 web-dev skills, i have determined that the problem is one of my ad-blockers thinks https://pbs.twimg.com is a tracking beacon, and is blocking the request for that reason. So the problem is at my end. (Though you can be safe by having the blog itself host all the images.)
Ok thanks. My image allowance is full these days and with the way I do stuff on multiple sites it seemed a waste to upload one thing to 5 different servers all the time hence linking like this.
Wow, I didn’t even know there was such a thing as “image allowance”!
An excellent article with several good points. I also believe it was a wading predator like Suchomimus. However, there are some animals that are aquatic and that have roughly similar tail shape: https://upload.wikimedia.org/wikipedia/commons/thumb/e/e0/Kammmolchmaennchen.jpg/580px-Kammmolchmaennchen.jpg
Go and read the paper, we even mention this genus! But the short version is that tail shape is under sexual selection, female newts don’t have it and don’t need it to swim.
What if a similar dimorphism was present in spinosaurs?
Also interesting is the drag the huge back sail would have made under the water. While observing some fishes maneuvering and swimming under water they have the large dorsal fins folded when swimming in straight line and open them only in sudden turns. It seems that so does the sailfish as well: https://www.youtube.com/watch?v=pRyFGSTaQ_Y
In reply to Berislav below: yes, we absolutely wonder about similar dimorphism in spinosaurs, including Spinosaurus. Our sample size for spinosaurs, however, is pathetically small at present.
The idea that the biggest theropod was swiftly chasing down darting fish seemed fishy to me. Paleontologists seem to have trouble communicating to the public how unsure they are. The difference between being 99% sure of something and 65% sure of something is huge. For instance there’s a large chance for error when working with a small crumbling bone versus a perfectly preserved skeleton. The public ends up thinking it was a ‘mistake’ instead of a close call where one bet is slightly more parsimonious then the other. It is and isn’t the media’s fault, they want to report yes or no type stuff, not a researcher thinks an animal might have been a decent swimmer versus a poor swimmer. Americans don’t like soccer because it ends in ties..
I think a lot of people will be disappointed by your post. The idea that the biggest and therefore greatest theropod of all time wasn’t swiftly and gracefully chasing down and tearing apart it’s prey. Instead it was either stumbling around in slippery muck snapping at fish like a common bear. And/or standing around boringly for hours like a boring bird until it can seize and slurp down an unwary muddy mudfish or mudpuppy. You’ve broken the hearts of children everywhere.
That wasn’t the intent of course but is a possible outcome. Hopefully people will look at the data and arguments for what they are rather than what they’d like. And we’re prepared to be very wrong, but it needs to be shown.
So I guess the older Spinosaurus toys aren’t so inaccurate after all. Btw regarding enamel, if some Spinosaurus individuals ate a lot of baby Stomatosuchus, would the biochemical signature be readily distinguishable from that of dinosaurs? The Baharije and coeval environments had the big Carcharodontosaurus, so how might resources have been partitioned if Spinosaurus hunted a lot of terrestrial dinosaurs? Maybe some Spinosaurus individuals did a lot of scavenging, for example when Paralititan suffered high mortality at times (in cases just before spinosaurs succumbed too).
A excellent paper, and it’s great to see some of the things we chatted about at Tetzoocon a few years ago in a full paper, the idea of Spinosaurus as a grizzly bear, or as I like to think of it a mega Heron is a fantastic concept, plus it makes sense for it to be able to feed on multiple sources of prey. Congratulations to you both on a fantastic piece of work!
It’s about time we had a paper on Spinosaurus that follows the data, rather than just what’s cool. You get the idea that Ibrahim et al. just *want* Spinosaurus to be aquatic, and thus are quick to report on anything that superficially corroborates their position, without doing enough rigorous analysis.
However, I think there’s still something weird going on with Spinosaurus’ morphology, especially those short legs. If if was specialized for wading, it would seem more likely to have long legs, like wading birds. (They might not be as long and spindly as a heron, but at least the length of other similarly-sized theropods.) I think some degree of aquaticity (if that’s the right noun form) is still definitely in the range of possibility.
Incidentally, female crested newts do still have a pretty substantial tail fin (https://www.researchgate.net/figure/Sexual-dimorphism-in-the-great-crested-newt-T-cristatus-During-the-breeding-season_fig2_313356227); it’s simply less ornate.
“However, I think there’s still something weird going on with Spinosaurus’ morphology, especially those short legs. If if was specialized for wading, it would seem more likely to have long legs, like wading birds.”
This is addressed directly in the paper. Basically, Hone ‘n’ Holtz posit that Spinosaurus was so big, in absolute terms, that its proportionately short legs wouldn’t have mattered; it would still have been much taller than any wading bird.
Yes, that’s true. I wasn’t implying that short legs prove it wasn’t a wader (and I think it’s quite probable it was); however, they also don’t seem particularly conducive to wading. I think the short legs were most likely an adaptation to something else altogether – probably not merely size, since other similarly sized animals had proportionally larger legs. In fact, the paper does not say that the leg proportions were a result of body size, but that the foot structure was a result of large size combined with small legs. All the arguments I’ve seen merely account for how Spinosaurus could still have been a wader despite limb proportions, not why the limbs would be short to begin with. Now, I’m not necessarily saying that short legs=swimming (and certainly not short legs=aquatic pursuit predator); I’m merely saying we don’t yet know for sure why they are how they are. Perhaps new materials can shed light on this question.
Here’s a naive question. Is it possible that the relatively short hindlimbs of Spinosaurus were an adaptation to bring the jaws (and forelimbs?) closer to the surface of the water, when standing in the water column?
Or to look at it another way, the advantage of long hindlimbs in a fully terrestrial theropod is to increase stride length (such as for cursoriality) or improve the elevation of the head (such as to scan the terrain for prey, or larger predators). Neither would seem to apply to Spinosaurus.
By the way, I really enjoyed this paper by you and Tom. Very concise and comprehensive.
In reply to TimW: That may be a possibility; however, if so, it’s difficult to account for why other spinosaurs (and other waders in general) don’t display the same adaptation, since they would also need to have their jaws and forelimbs near the water. Instead, these waders had long necks and arms so they could reach down without doing a faceplant. As far as we know, Spinosaurus’ neck and arms had similar proportions to other spinosaurs, so there’s not much reason to think it would need an additional adaptation to help it reach the water. Possibly what you’re describing was sort of a “side benefit” resulting from an adaptation with a different primary cause, but I’m not sure it goes all the way to accounting for its ultimate origin.
Also, if Hone & Holtz are correct, and Spinosaurus was also sometimes foraging on land, it probably *did* need to be able to search for prey over relatively large areas. Of course, there’s always going to be some sort of tradeoff involved no matter how an animal adapts, so probably Spinosaurus was gaining an advantage for fishing while becoming less adept at terrestrial foraging. I definitely think more rigorous study should be done on the hindlimbs to clear up these questions.
Why did Spinosaurus evolve its big sail? It may have been for display but I suspect it also compensated for the lowness of S. aegypticus when out of water and quadrupedal. Despite its relatively short legs, Spinosaurus could’ve been a wader since buoyancy would’ve made it easier to maintain a bipedal stance in water. On land, however, S. aegypticus may have been down on all fours hence lower and more likely to seem vulnerable–which could be very dangerous in the same environment as Carcharodontosaurus. The sail may have evolved to compensate for the lowness of a quadrupedal stance i.e. by making Spinosaurus appear taller, it was more likely to deter attack.
My take on the heron thing is that a two legged wading animal weighing over 6000 kg isn’t going to benefit from having long legs, unless it’s going to spend a lot of time on it’s behind. I’d expect it to move towards using all fours which is what was happening. An extra meter isn’t going to disguise all that animal, I think it’s bulk might even attract fish to it’s shade. This is only in regards to hunting strategy. It’s still going to spend most of it’s time in water, swimming from hunting spot to hunting spot and staying cool. It’s only in feeding that it’s an ambush predator. I’d expect crocs, birds, pterosaurs, plesiosaurs, anything that gets close enough is going to get snapped up. It would be exciting if it could suck fish and pigeons up, catfish style. I don’t think so without gills but it could expel the water through it’s nose if it had huge nostrils..
The comparison with herons makes me wonder whether the dorsal sail of Spinosaurus was ever used for purposes similar to the canopy feeding techniques seen in herons. I imagine a Spinosaur’s neck would have needed to be quite flexible laterally to make that work, since the shade cast by the dorsal sail would mostly be to one side of the dinosaur rather than directly ahead of it.
Posit a wet weather pond 100 x 30 x 2 meters – A frolicking adult spinosaur would have every gilled fish in it knocked out cold in minutes. Further, if the water was warm, and warmer than the surrounding air, the sail would be good for dumping heat…
Does not negate any possible social functions of course, but no reason a single trait can’t convey advantage on multiple fronts.
The open water swimming/underwater pursuit never made sense to me considering what I have read about the aquatic environments Spinosaurus was purported to have occupied. I have read that they may have occupied mangroves which at least in contemporary mangroves measure around 8-10 feet in depth? For an animal the size of Spinosaurus it isn’t exactly an Olympic swimming pool. Not to mention when the tide goes out they would be left standing. The same goes for rivers and deltas, they aren’t always massively deep, so outright swimming doesn’t seem 100% necessary. I am still partial to the hippopotamus “punting” as an analogue for Spinosaurus moving in the water.