If you try phylogenetic bracketing, the closest living relatives of T. rex can swim quite well, at least the aquatic ones. Some of them use their arms (wings), and some use their legs. When swimming on the surface they generally keep their heads well above the water and over their backs a bit. And the second closest relatives are all aquatic, but all quadrupedal, so not so good a guide.
It seems very likely that spinosaurs could swim. Would that be good enough?
Of course fish can’t swim. Could you swim without any arms? Even if they have a powerful hind end + tail their heads would sink. Just consider some simple biomechanics.
Fish do indeed have arms, or at least the homologous structures (pectoral fins). They rely on these to provide lift at the front end of the body. Unlike T rex they don’t have huge heads, and their bodies are streamlined for movement in water, which is not true of terrestrial tetrapods like T rex.
Fully aquatic tetrapods, like whales and seals, do indeed swim with their tail/hind legs. They are also highly streamlined and they also basically lack a neck, and still retain powerful forelimbs for braking, steering, and adding lift if necessary. Streamlining is extremely important in the dense medium of water, and this issue gets increasingly important with large size, as per T rex. A tadpole can get away with a lot just because it’s small.
The tail of T rex does indeed balance the weight of the head ---- in fact there is some reasonable speculation that the arms are so small because the head is so huge. But that balancing is done by cantilevering the body over the legs in air. In water, the head would sink, as shown in the reconstruction of a swimming T rex. Which isn’t to say it mightn’t be able to paddle for some short periods, but it would have a hard time getting its head above water, if it could manage it at all.
The boy swimming without arms is doing so on his back. This would not be an option for a quadruped like T rex as it would be unable to straighten the legs at the hip to align with the body and they would stick out of the water. Not a viable option for swimming.
All terrestrial tetrapods that are reasonably adept at swimming use their forelimbs to generate lift at the front end so that the head doesn’t sink (and also to generate thrust for propulsion). For example, most likely Spinosaurus.
(Phylogenetic bracketing won’t help here, especially as swimming is done in very different ways in birds and crocodiles [and also in different ways in those few birds that do swim], but functional morphology will. Depending on how one drew the cladogram, phylogenetic bracketing might inform us that humans knuckle-walked, but functional morphology would show that we did not.)
Having tiny arms would be a huge impediment to T rex attempting to swim. It is not an animal that is “designed” to swim, and it is highly unlikely that it was able to to any great extent. SGI footage is not evidence. I stand by my original “intuition”. It’s not “I just sorta feel”, it’s based in knowledge of vertebrate functional anatomy.
Some fish have huge heads.
Some fish aren’t streamlined.
Some terrestrial tetrapods are streamlined and have small heads.
Is that enough counterexamples?
It doesn’t have to be as good at it as a fish, or to have specifically adapted to swimming, to be able to actually do it and still do it well.
Recall that you began by saying that “Of course T rex couldn’t swim.” I just don’t think you’ve established that, and it seems like you’ve walked the claim back now to it not being able to do it to “any great extent”. I don’t think, say, Buffalo can swim to “any great extent”, but they can swim nevertheless and do it well enough to cross a small river. Nobody is saying T-rex should be thought a semi-aquatic animal or anything like that.
“He who allows the alien to live shares its crime of existence.” -Inquisitor Apollyon, Warhammer 40K
The goblin shark is up there with the blobfish and the giant centipede as an absolute abomination and it’s existence is a crime. Someone turn on the gellar fields!
Not so. Birds swimming on the surface usually make no use at all of their forelimbs, with rare exceptions like steamer ducks. Of course their heads are lighter and their necks longer than in tyrannosaurs. But the question to ask is whether the body is balanced over the legs when the head is raised, such that it could stick out of the water. And just how flexible is a tyrannosaur neck? There are a lot of vertebrae, at least.
What do you mean “those few birds that do swim”? True, it’s a minority, but there are three fairly large orders and a few smaller ones. And all of these, except penguins, swim on the surface in the same way; even the wing-propelled divers paddle with their feet and keep their wings folded when on the surface.
A fair criticism.
To All: Short of Biomechanical analysis, I suggest it is more relevant to consider animals that cannot swim, rather than all those oddities which can.
Yup, Some small long-bodied vertebrates (“small” being important here) practice what is called “anguilliform locomotion”. They send a wave of muscular contractions down the entire body. Not quite sure how this helps with an argument about the use of arms/forelimbs in a regular type of terrestrial tetrapod like T. rex, but good try!
Last I noted, buffalo have quite substantial forelimbs. When somebody asks if an animal can swim, they usually mean “can it least swim like a dog?”, not “can it somehow survive if plonked in water?”. But perhaps I misunderstood the question.
My point was about the use of the phylogenetic bracket to establish whether or not dinosaurs could swim. Swimming has clearly evolved convergently in different groups of birds, so it’s not an inherent feature of the archosaur clade, inherited from crocodiles. The compact body shape with basically fused spine/pelvis (yes, a shortcut here) and no tail enable surface swimming as seen in ducks. I doubt that T rex could to this.
Yes, some fish have relatively large heads for their bodies. With the exception of the hammer-head shark (where that head shape provides some front end lift) they are not large animals, and none is nowhere as large as T. rex. Absolute size matters when thinking about biomechanical issues. Also, no fish has a head separated from the body by a neck. This is important when thinking about streamlining in water, which is why cetaceans, true seals and sireneans have basically short necks with fused cervical vertebrae.
The fish you show here that are “not streamlined” are all small fish, and they don’t do much active swimming. Fish, as you may recall, don’t need to worry about keeping their head above the water, unlike a regular terrestrial tetrapod trying to swim. In the latter case, arms are very important (especially as terrestrial tetrapods are, of course, not streamlined).
Many small terrestrial tetrapods (or, semi-aquatic ones as you’ve shown here) are indeed streamlined and with small heads. And your point is?
So, are these good “Gotch’a” examples. I don’t think so.
Ah. Does this help us to understand how T rex could swim with tiny arms, or is the point some sort of “gotcha”?
I’m no expert on teleosts, but pretty good on tetrapods.
I know @Pandora personally and she is a dear friend. She is also one of the authors of the superb textbook Vertebrate Life, which I find to be a really useful survey of the variety and lives of vertebrates of every sort. I also know her reputation – I’ve discussed her work with other paleontologists who regard her as having great insights, specifically in relation to animal locomotion (and yes, I’ve had those discussions when she was out of the room as well as when she was in it, and as well as when she was thousands of miles away). I’ve seen her give talks on the subject to colleagues. And, my goodness – bring up tetrapod taxa and ask her how they move, and you will get back a lot of information.
Now, I am no paleontologist, and perhaps no good judge of these things. But I do think that there are a few odd things going on here.
One is that there is a difference between asking whether T. rex would drown if dunked, or would somehow get by, and asking whether T. rex was reasonably competent at swimming. Those are rather different questions.
There are limits to analogies, and limits to generalizations. Any brief answer to a complex question is incomplete. So when a discussion like this emerges, as this one has, piecemeal, sometimes things get askew.
Now, @Pandora recognizes, I am sure, as well as anyone else does that there may be times when she is wrong. But I don’t think that the sorts of partial analogies on offer here that address only some particular aspect of the problem show that. If the task at hand is evaluating the body size, shape, limb characteristics and orientations and musculature of a creature and understanding how it gets around, she is someone to whose insights I would give great weight.
I see that the answer that I replied to was added to.
No fish have a “neck”. The area between the head and the pectoral fins is filled with gills, and so is immobile.
Elasmosaurs do indeed have long necks. They are not fish. Also, they have substantial pectoral fins (= arms), Who knows how plesiosaurs lived? Who knows what the relevance is to the question about T. rex?
Yes, some small fish that do little in the way of swimming are not streamlined. Remind me of how this relates to the issue of whether T. rex could swim.
Lungfish also have gills. There are not obligate airbreathers (even if some species need access to air occasionally).
Diving tetrapods, like marine iguanas, can certainly hold their breath while underwater (as of course can whales, etc.) However, they do need air after a limited period of time. T. rex was probably endothermic, unlike an iguana, so the need for oxygen would have been more important. No clue what the point is about hippos.
All terrestrial tetrapods (except snakes and some limbless lizards) have forelimbs. Forelimbs = “arms” in this context. Otters are semi-aquatic, BTW. Yes, they are streamlined. Others are not (such as what I think is a yapok in the photo).
Again, how does all of this help us understand how T. rex could have managed to swim with such tiny “arms” (AKA forelimbs). Not at all. Just a “gotcha”.
I didn’t expect a kind of creationist-speak inquisition (i.e., when the identity of the poster is more important than what they have to say). I guess I’ll know in future when posting on this site. If that should occur again.
Swimming is also a learned or instinctive behavior. It is entirely possible the T-Rex could swim if they went through their Red Cross badges, but they were landlubbers, so they all sunk in deep water. Far more people drown with limbs than swim without.
We have to work on our welcoming committee. I have noticed that the number of active contributors has narrowed as of late. As a non-scientist, I have learned a great deal here from some contentious threads where experts differ. You are obviously well founded in your specialty, and I very much hope to see more of your posts in the future.
Lucky you! 
Very true, and a lot of modern animals that get by well enough would have trouble in deep open water.
The researchers found that “giraffes would be very poor swimmers, and that it might be assumed that they would avoid this activity if at all possible. Looked at another way, we conclude that giraffes can swim, but not at all well .”
TL;DR: Don’t take your pet giraffe into deep water.
Ah.
@pandora, I apologise - I seem to have misjudged you. I shall retract the area between my head and where my pectoral fins would be if I had any.
P.S. It is a yapok
The phylogenetic bracketing was a joke. But I’d say if you want to know how a theropod would swim, you need to look at a theropod. Look at a biped with a horizontal backbone and weight balanced over the rear legs. When swimming, those theropods we know of sit horizontally on the water, neck vertical, and paddle with their rear (only) legs. This is quite unlike how mammals swim. Now agreed, extant theropods are quite different from tyrannosaurids in many ways, but they’re still the best model we have. If a tyrannosaur could swim, which I doubt we can know, it would likely swim like that, and the front legs would not be especially relevant.
