Sereno et al. argue Spinosaurus wasn't aquatic

[Sim]

Considering only male basilisks have the sails on the tail, that sail isn't used when swimming, it's just there on the male basilisk.  The paper doesn't ignore newts, it says:

The hypertrophied neural spines of the tail of S. aegyptiacus, in contrast, are ambiguous as an "aquatic adaptation," because expanded tails can function both as aquatic propulsors and terrestrial display structures. For the expanded tail to be an "aquatic adaptation," its morphological construction and biomechanical function demonstrate its primary utility and capability in water, as shown in extant tail-powered primarily or secondarily aquatic vertebrates (e.g., newts, crocodylians, beavers, otters; (Fish et al., 2021). The same must be shown or inferred to be the case in extinct secondarily aquatic vertebrates (Gutarra and Rahman, 2022). We have not found such substantiating evidence in tail form and inferred function in S. aegyptiacus or other spinosaurids for the heightened tail to be substantiated as an aquatic adaptation (see below).

As for the eel comment, eels aren't secondary swimmers.  The paper argues that secondary swimmers don't have bone supported tail paddles.

[stargatedalek]

Basilisks use their tail fins when swimming. It just isn't an adaptation for swimming.

And they may not have ignored newts, but you are definitely ignoring what I said about newts.

Also ignoring fish and newts which propel themselves just fine with thin surfaces for their tails (though newt tails are something of an in-between, being thin but also pliable).

[Newt]

A recent (paywalled) paper suggests basilisks may use their sails to thermoregulate, as long suggested for various prehistoric sail-backed beasts. This is almost certainly not the main reason basilisks have sails - the fact that the sails are sexually dimorphic is pretty clear evidence that they are primarily display structures. Nor does it explain the evolutionary origin of the sails - basilisks are part of the Corytophanidae, or casque-headed iguana family, and their cousins Corytophanes and Laemanctus are highly cryptic animals whose non-sexually-dimorphic crests are pretty clearly silhouette-disruptors whose main function is to make the lizards look less lizard-shaped to potential predators.

All of which is to say, animals will use the tools at their disposal to do whatever they can manage with them, even if it is not the tool's primary function. Spino's tail being a display banner and being a scull are not mutually exclusive.

[Sim]

No, I'm not ignoring what you said about newts, or fish.  The paper isn't arguing thinness is a factor in tail fins as far as I'm aware.

[Sim]

Something I'd like to add about basilisk sails - I know the tail sail is only present in the males of one species and I know the crests/sails of basilisks are sexually dimorphic in more than one species, but I don't know if the tail sail is only present in males of all the species.  That having been said, the paper argues that the tail sail of basilisks isn't used while swimming.

[Newt]

I took these pics at the Zoo today, of a green basilisk pestering a northern caiman lizard - a much more aquatic animal. 

The basilisk tail sail is mainly on the proximal part of the tail, which is not where you want all that surface area for a proper scull - you want the tall part placed distally (if your tail is fairly rigid) or you want the whole tail to be essentially the same height (if your tail is more sinuous). The distal part of the basilisk tail is fairly thin and round - a generic arboreal iguanian tail. The caiman lizard's tail is not only relatively tall and laterally compressed towards the distal end, but also massively muscled. The row of enlarged scales along the dorsal surface also helps push against the water. That's what the tail of an animal that uses its tail for swimming looks like!

@Sim - my understanding is that no female basilisk has a tail crest, though I wouldn't be surprised if an occasional female had a suggestion of it in the same way that the occasional lioness has a bit of a mane.

In terms of what this implies for Spinosaurus - I'm not sure. Spino's tail has the profile of a swimmer's, but to my eye it seems too delicate to propel such a massive animal. A crocodilian in that size range would have far more robust bones and tremendous processes for the muscles to work against. Of course, Spino may have just been a feeble swimmer. If it was terrestrial, it couldn't have been too athletic on land either. Evolution does throw up these things from time to time. How is an astrapothere supposed to work, or a desmostylian? 

[stargatedalek]

It's also worth noting that interpretations of Spinosaurus as an aquatic animal already note it as a relatively feeble swimmer, adapted for floating near the surface. Something which doesn't conflict at all with the observation it would not have been a great swimmer.

[GojiraGuy1954]

It's also worth noting that interpretations of Spinosaurus as an aquatic animal already note it as a relatively feeble swimmer, adapted for floating near the surface. Something which doesn't conflict at all with the observation it would not have been a great swimmer.

It would have had to be at least some wasy competent in the water considering there are spinosaurus fossils from Brazil as well as Africa

[stargatedalek]

And a floating animal can be perfectly competent in the water without being a strong swimmer.

[andrewsaurus rex]

I agree that it's odd that with a huge sail on its back, whatever its shape, that Spinosaurus would also evolve a flattened tail for display too.  Perhaps the tail shape was simply a hangover from the elongation of the back spines, or originally its ancestor had longish spines along  the whole body and tail and the ones over the back elongated further as it evolved, producing an animal with a sail and somewhat elongated tail spines.

However, more likely in my view, is that the tail was used in 2 ways.  For corralling fish and for paddling along the surface.  This is how I see Spinosaurus living.  It spends its time floating along the surface of the water, paddling along with is tail and limbs.  When it finds a school of fish near the surface it corrals them with its long, tall tail bringing them within reach of its mouth.  The sail, other than for display, could be used to help keep the animal warm as it spends perhaps many hours a day in the water....floating along the surface the sail would be fully exposed to the sun and able to soak up much warmth to offset the chilling effect of the water.

[Sim]

I agree that it's odd that with a huge sail on its back, whatever its shape, that Spinosaurus would also evolve a flattened tail for display too.

Yet basilisk lizards do just that.  As for the tail being flattened, flesh and bone is a significant investment to produce so a display structure would be more likely to be flattened.

However, more likely in my view, is that the tail was used in 2 ways.  For corralling fish and for paddling along the surface.

The tail being extended in height anteriorly suggests its evolution wasn't driven by swimming.  Same for the tail sail being supported by bone, as the paper points out secondary swimmer paddles aren't supported by bone.  The tail of Spinosaurus wasn't efficient for swimming either, the new paper says the maximum power from Spinosaurus's tail would make it move at a speed of around 1 m/s.  Quite slow.

This is how I see Spinosaurus living.  It spends its time floating along the surface of the water, paddling along with is tail and limbs.

Adding to what I mentioned previously about the slow speed of tail propulsion in Spinosaurus, the hindlimbs were also inefficient for swimming according to the new paper:

Thus, the crocodylian foot paddle becomes even less effective as a propulsor with increasing body size. Nonetheless, a crocodylian of spinosaur size would have a foot paddle area an order of magnitude greater than is possible in S. aegyptiacus (Figure 6E). Even a fully webbed hind foot in S. aegyptiacus (Figure 6A) is far too small to have functioned as a significant aquatic propulsor or for stabilizing control.

The sail, other than for display, could be used to help keep the animal warm as it spends perhaps many hours a day in the water....floating along the surface the sail would be fully exposed to the sun and able to soak up much warmth to offset the chilling effect of the water.

I see what you're thinking of here, but based on Mark Witton's work on how large animals take very low temperatures to feel cold, I doubt the enormous Spinosaurus would want to be warmed up in the environment it lived in.

[andrewsaurus rex]

well, with the sail on its back, thermoregulation is going to happen whether it needs it or not.  With the sun on it, the body is going to warm up and with breeze on it the body will cool.  Makes me wonder if the sail caused body temperature problems for Spino.  It may have needed to be dormant and in the shade on sunny days for fear of overheating or alternatively to stay in the water to keep cool.  So perhaps, if the sail evolved just for display, that same sail  forced Spino into an aquatic lifestyle to prevent overheating.  A lifestyle it may have not been ideally evolved for but nevertheless could get by with.

[stargatedalek]

Spinosaurus didn't paddle like a crocodile, it was built completely differently and it's feet were webbed for crying out loud. That comparison is completely idiotic and any results coming from a comparison so baseless are not going to show us anything of merit regarding the animals ecology.

[GojiraGuy1954]

https://www.biorxiv.org/content/10.1101/2022.05.05.490811v1

Response paper dropped

[Sim]

Spinosaurus didn't paddle like a crocodile, it was built completely differently and it's feet were webbed for crying out loud. That comparison is completely idiotic and any results coming from a comparison so baseless are not going to show us anything of merit regarding the animals ecology.

The feet of Spinosaurus are not known to be webbed.  It's just thought they may be because its toe claws had flat bottoms and that this foot morphology suggests Spinosaurus evolved to walk on unstable substrate.  You also underestimate people thinking Spinosaurus paddled like a crocodile, I think it's a more widely held opinion than you seem to think.  And on that note, how do you think Spinosaurus swam?

https://www.biorxiv.org/content/10.1101/2022.05.05.490811v1

Response paper dropped

No, that paper is a response to another paper that focused on bone density.  So the paper you linked to focuses on bone density as well and doesn't adress the paper of this thread's topic.

[Bowhead Whale]

What is said about the claws of the feet being relatively flat for earth-dwelling carnivores? Isn't that characteristics better for swimming than for walking on the ground?

[Sim]

I guess the toe claws having a flat underside would be better for swimming, but all I've seen is they've been understood to indicate that Spinosaurus's feet evolved for walking on unstable substrate.

[Bowhead Whale]

I guess the toe claws having a flat underside would be better for swimming, but all I've seen is they've been understood to indicate that Spinosaurus's feet evolved for walking on unstable substrate.

The only unstable substrate I know of is desert sand... how can this animal be half-aquatic-like as well as desert-dwelling-like? It does not make much sense in my mind...

[bmathison1972]

I guess the toe claws having a flat underside would be better for swimming, but all I've seen is they've been understood to indicate that Spinosaurus's feet evolved for walking on unstable substrate.

The only unstable substrate I know of is desert sand... how can this animal be half-aquatic-like as well as desert-dwelling-like? It does not make much sense in my mind...

beach sand and mud are also potentially unstable...

[bmathison1972]

Hippos are bad swimmers. They're even negatively buoyant. You push a hippo off a boat into the ocean, that hippo is going to drown. 100% guaranteed.

And yet they spend the majority of their lives in water. Go figure.

this is a great statement; if hippos were known only from fossils, you can believe they would have been depicted as wholly terrestrial animals