Depictions of Spinosaurus

New ideas (or at least new papers reviving ideas) on the appearance, behaviour and ecology of dinosaurs are often accompanied by a wealth of new palaeoart as people get invigorated and inspired by potential ideas and want to create them. The flipside of this is that there is a tendency for the previous ideas to be immediately written off as being wrong or out of date. Given the huge volume of images of Spinosaurus that have been produced in the last few years off the back of the swimming hypothesis, I think this is therefore probably worth addressing.

Spinosaurus by Bob Nicholls of Palaeocreations

As we say clearly in the paper (and I said in an earlier post), there’s no reason at all to think Spinosaurus did not swim (pretty much all tetrapods can) and it was hanging around, and in, water really quite a lot. I don’t think it was swimming that much though, and diving would have been strenuous and difficult, but probably not impossible. Even animals we don’t think of as swimmers can at least get in and around water – I once saw a large grey heron in Japan dive into the water from a high river bank, get completely submerged and come up a few seconds later. That doesn’t mean that’s normal for that individual let alone the species, but depicting a heron underwater grabbing a fish would not be wrong.

In short, the vast majority of sketches, drawings, paintings and even videos of Spinosaurus underwater are not now suddenly wrong, inaccurate, or dated. Still, if you think that the wading model is something worth illustrating or if you have seen other ideas and hypotheses in the paper and want to use that on the next round of Spinosaurus (or indeed other spinosaur) illustrations, here’s a few key points and details and suggestions. As above, none of these are inherently correct at this point, but these all come up and would be relevant if you were trying to do an animal according to our ideas in the paper.

We suggest that the tail ‘fin’ may have been a display structure (in addition to the dorsal sail and the head crest) and as such could have been brightly coloured or patterned with strong contrasting colours. We also noted that crocodylians do a head-and-tail-up display posture in water which would be a potential option for a spinosaur.

Based on work done on Irritator, the head may have been typically held at 45 degrees (though there’s lots of variation seen in these value as per the excellent recent bovid study on head posture). This could well be close to a natural posture when foraging with the snout underwater (and we reference this specifically in one of our figures).

This is one that seems to get missed. We don’t have a single good full Spinosaurus skull but we do have a good idea of the nostril position and it’s a long way from the top of the snout. There is a question about how far forwards it is, but it’s very much not near the front or top of the snout. If foraging with the snout underwater, the nostril is usually going to be kept clear of the water.

And on that note, we propose that the snout would likely be below the surface normally, either when waiting for fish to come past that can be attacked, or possibly even shovelling around the bottom. (Come to think of it, we don’t really explicitly state this in quite these words, but we do talk about stork and heron-like behaviours, and show a stork foraging stirring up mud and reference searching for benthic things, so I think we cover both of these possibilities in what we have written).

If the animal is foraging while standing in water would not be too deep so that it wouldn’t float. This would probably be anything up to the belly (or of course much less).

We did also suggest that it may have made crocodile like thrusts into water to grab things, using the legs and tail and probably generating a lot of splash.

It doesn’t have to be in or even near water. There’s good evidence that some were spending considerable periods on land as ‘normal’ theropods.

A lot of things were probably on the menu apart from fish. We know from isotopic data that some individuals were eating herbivorous dinosaurs and the like with little or no aquatic prey in the diet and the tooth and skull structure point to Spinosaurus at least having the capacity to eat things like turtles and crustaceans, while we know other spinosaurs took dinosaurs and pterosaurs, and it probably scavenged too.

In short, there’s lots of opportunities to expand the range of things Spinosaurus is shown doing (OK, so those opportunities were already there, but here’s some specifics with reasons) and hopefully this will help give some hints and tips. I’ve already seen lots of new artwork popping up, in addition of course to the wonderful rendition that Bob Nicholls did for us (shown at the top) that has already been plastered round the web. Coupled with the (revised? alternate?) skeletals that are out there by Scott Hartman and Get Away Trike (which we used in our paper), this should be a productive time for drawing this and other spinosaurs outside of them being deep underwater. Pencils and tablets at the ready!

Testable hypotheses for Spinosaurus

I tried to emphasise in the last post that there is still more to come here. I’m sure other people are writing more about Spinosaurus and its behaviour, ecology and functional biology right now and there will inevitably be further discoveries and data coming out in future. While I am confident in the wading / heron-like idea, further work could easily provide major modifications to that or overturn it. Such is the way of science: things are not settled. Below, I thought I would put together a few ideas of hypotheses or aspects of the model that could be tested or assessed further to show how much more there is to come and move away from the ‘this is settled’ idea that I’m sure at least a few people will have articulated. Some of these are much easier than others to tackle (measuring toes is way easier than working out drag) but all are, I think, something we as a scientific community can tackle in the coming years based on the techniques we have and the available fossils.

What does naris and orbit position look like in an even wider range of reptiles and in birds?

What is the overall density and distribution of mass? In particular, how buoyant would it be?

What was the exact arrangement of the neck muscles?

How flexible is the neck?

How stiff is the dorsal series?

What is the exact arrangement of the dorsal sail?

How much drag would the sail / legs produce when swimming?

How much wave drag would there be at different depths?

How spread were the toes? Is this more than other theropods?

What swimming form would it use? (Whole body or just the tail)?

Would leg thrusts help in propulsion or add more drag than thrust?

How flexible is the tail?

How strong are the caudal neural spines?

How much muscle did it have in the tail and where?

Would increased flexibility help it provide propulsion?

Would a leg and tail driven thrust in water be effective, even for a single thrust?

How efficient would it be walking?

What would the efficiency calculations for the tail look like with a more accurate model with variable flexibility and different degrees of submergence?

Are there more general common features of various aquatic and semi-aquatic reptile lineages and can we quantify things like leg reduction, tail musculature, drag reduction etc. to look at this as more of a continuum than a binary state?

Is ungual curvature driven in part by size / evolutionary relationships / habitats?

What are the isotopic signatures like for teeth that are in situ in jaws?

How many different habitats and environments did Spinosaurus occupy? And what were these like?

I think that all shows that we can push this forwards considerably. The new paper covers all kinds of different bits of anatomy, ecology, mechanics and possible behaviours and some considerations of the environments too, but this is still all just starting points. This of course is all based on existing fossils and any future discoveries (an arm would be nice, or a complete skull for that matter) is only going to provide most data (or fuel to the fire, take your pick). Still, this hopefully provides a few ideas for people to be getting on with.

If you want even more discussion on Spinosaurus (and why wouldn’t you) then the first episode of the new series of my Terrible Lizards podcast is now up and it’s a whole hour of wading hell-herons (copyright Andrea Cau).

The evidence for Spinosaurus being a specialist aquatic predator and good swimmer is weak

Spinosaurus shown as a wading hunter. Artwork by and copyright too Bob Nicholls, used with permission.

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.