Spinosaurus would beat Dreadnoughtus by a factor of 1.6:1 in terms of total body weight. Note: In the above image, “total” refers to the combined body weight of the individual animals. This means that the two are roughly equally massive. The difference is mostly due to the difference in length between the species, not their body weights. I’ve already talked about how that is true in terms of living animals, but the same thing applies to the size of fossilized animals. That is, we can compute the length and weight of the body of the various animals listed here, as well as how their body sizes compare with the length and weight of a dinosaur of their total body size. Here are the sizes of all animals (except for the dinosaur): The most massive animal on the list is a dinosaur from the Cretaceous Period called Eryops, from the island of Madagascar. It’s body was at least 30m long and weighed over 5,000kg, based on estimates of the length and weight of its hip bone. This is roughly the size of a Tyrannosaurus. Let’s consider two hypothetical dinos, one from the age of dinosaurs and one from the Triassic Period. If both are approximately the same length, the one from the Triassic Period would be about 3.5 times smaller in body weight, as its weight would be about 2/3 that of the dinosaur from the age of dinosaurs. (This is an example of relative body sizes, and not absolute ones.) The most massive dinosaurs from the Triassic would be Diplodocus. This dinosaur was approximately 55m long, and based on estimates of its body size, would have had a body weight of at least 50,000kg. This is about the size of a Dreadnoughtus. Dinosaur body sizes are similar to those of theropods from the age of dinosaurs. This is one of my personal peeves. People say dinosaurs were “giants”. Gigantic? Yes, in terms of length. But body size estimates for dinosaurs are always calculated by comparing how their body parts relate to the rest of their body. Consider the following image: The red bar is the length of the femur (thigh bone). As you can see, it’s almost two-thirds the length of the entire femur, as that’s the red bar that is considered the length. The blue bar is the length of the tibia (shin bone). This is the bone just above the thigh bone, at about the same length. As you can see, it’s less than a quarter of the length of the femur. The yellow bar is the length of the humerus (upper arm bone). This is another bone that’s less than a quarter of the length of the femur. This is how they’re calculated. Not by comparing the length of the entire femur to the rest of the body, but how big each part of the body is. What they did when they’re saying “Dinosaur was X metres long” is they’re saying that the diameter of the femur is X times the diameter of the femur, the diameter of the tibia is X times the diameter of the tibia, the diameter of the humerus is X times the diameter of the humerus, etc. This isn’t a measure of the volume of the body; it’s a measure of the diameter of the body. The same goes for theropods. When they say a dinosaur is “100 metres long”, they’re saying that the diameter of the shinbone is 100 times the diameter of the shinbone, the diameter of the humerus is 100 times the diameter of the humerus, etc. This isn’t a measure of the volume of the body; it’s a measure of the diameter of the body. I wish I’d made this post when I wrote the one about bipedalism, but I didn’t, so there you go. :| Note: The red bar is the estimated length of the entire femur. You’d have to divide the yellow bar by four to get the real femur length, but this is easier than estimating that. I also wish I’d made this post when I wrote the one about bipedalism, but I didn’t, so there you go. :| (Note: I’m not talking about humans here. I’m talking about how body sizes are calculated)
Let’s go back to the comparison. Tyrannosaurus and Eryops were both about the same length. As I said above, if Tyrannosaurus was around 30m long, it would have weighed around 5,000kg. That’s a mass of about 10,000kg. Eryops, on the other hand, weighed over 5,000kg. It would’ve weighed around 15,000kg, a mass of 30,000kg.

According to this skeletal by David Peters (Photo by Joe Foy) It also had at least seven strong limbs. The front and back legs were probably similar in size, each with three large toes. The inner two toes are not so clear. It was probably able to walk on its back legs, like a lizard. A skull for a young Tyrannosaurus, about 70 to 80 centimetres in length, was recently discovered in the Canadian province of Alberta. This skeleton included a partial braincase. This braincase had many tiny bone spikes sticking out from the skull, as if the dinosaur had hair. It had a large jaw, with some 70 teeth, with sharp pointed tips. These teeth could easily cut its meaty food, such as thick plant stems. In the end though, it was still weak compared to Eryops, and that is why I think Spinosaurus would beat Dreadnoughtus in a fight.
Thanks for reading!!!