Spinosaurus thermoregulation

Spinosaurus has been recently revised from a long-legged terrestrial big brother to Baryonyx, to a short-legged aquatic giant that probably found it difficult to walk bipedally (Ibrahim et al. 2014; Fig. 1). As the only quadrupedal theropod, Spinosaurus needs to be considered in terms of its environment.

Figure 1. Aquatic Spinosaurus to scale with contemporary Early Cretaceous giant fish.

Figure 1. Aquatic Spinosaurus to scale with contemporary Early Cretaceous giant fish. Click to enlarge. Spinosaurus may have been so large because its prey was so large. As the only aquatic dinosaur, Spinosaurus may have developed a sail to help regulate body temperature while staying submerged except to lay eggs. It may have never needed to stand bipedally, like its theropod sisters.

As the only aquatic dinosaur (until Hesperornis, ducks and penguins came along), Spinosaurus was unlike its closest sisters in several regards. It was larger. It had shorter hind limbs. And it had that famous sail back. If we put Spinosaurus into it proper environment, shallow waters, then the reason for the sail, the great size and the short hind limbs becomes readily apparent.

Sail for thermoregulation
Most dinosaurs did not live in water. Those that do (like aquatic birds) are covered with insulating feathers that keep them warm. Spinosaurus likely did not have feathers, or enough feathers to keep it warm, but it did have that sail. Exposed above the surface to the warmer air, the sail could have helped Spinosaurus maintain a higher body temperature in cooler waters. Overheating was unlikely surrounded by water. Other theropods with longer dorsal spines, like Acrocanthosaurus, show no aquatic adaptations.

Short legs for walking underwater
The hind limbs on Spinosaurus are so short relative to the body that it is difficult to see how it could have walked bipedally like other theropod dinosaurs. Those heavily clawed arms appear to be ill-suited to support the great weight of its forequarters. In an aquatic environment, however, that great weight essentially disappears. Spinosaurus could have walked along the muddy/sandy bottom. It is not known if the hind feet were webbed, but they look like they were best articulated when they were spread (Fig. 2).

Figure 2. The foot of Spinosaurus with PILs and possible webbing. The joints of the foot on the right appear to be better aligned.

Figure 2. The foot of Spinosaurus in ventral view with PILs and possible webbing. The joints of the foot on the right appear to be better aligned.That’s the vestige of digit 5 below metatarsal 4.

Spinosaurus likely preferred water of a certain depth. Deep enough to cover everything but the sail (floating enough to keep weight off its feet), yet just deep enough to touch the bottom with its clawed feet. After all, Spinosaurus did not have flippers or fins. That’s not to say it didn’t swim in deeper waters, or visit shallower waters. After all, it had to lay eggs on land, but it is likely to have been awkward when not supported by water.

Great size
At the same time and in the same waters as Spinosaurus several different types of giant fish co-existed. Many, no doubt, were on Spinosaurus’ menu. Younger spinosaurs would have eaten younger, smaller fish. The snout of Spinosaurus has many small pits. These are thought to have housed pressure sensors to detect prey in murky waters, as in living crocs.

Spinosaurus has been well studied
and there is little else I can add to the data and hypotheses available online here, here and here. The Spinosaurus in Jurassic Park 3 represents the old long-legged, terrestrial version, so best to forget images of Spino attacking T-rex on land. There is great artwork of the new Spinosaurus here, here, here and here.

And I just ran across this beauty.

References
Ibrahim N, Sereno PC, Dal Sasso C, Maganuco S, Fabbri M, Martill DM, Zouhri S, Myhrvold N, Iurino DA 2014. Semiaquatic adaptations in a giant predatory dinosaur. Science. doi:10.1126/science.1258750.

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5 thoughts on “Spinosaurus thermoregulation

  1. ?Plagiarism?? There’s been a lot of crankiness about the spinosaurus in the past year, and so far no one is blaming you!

    • Sorry David — this was a snarky reference to Matt Wedel’s original post in your links — where he complained that Bonadonna must have cribbed from Engh’s earlier painting [and called it plagiarism] because both were fish-eye views with 2 spinosaurs, one turtle, one pterosaur. And this followed what seemed to be hasty and over-the-top criticism last fall on several blogs panning just about everything Ibrahim’s group was doing with spinosaurus….

      • Spinosaurus gave us all a paradigm shift. As they say, there’s no better hypothesis killer than a cold hard fact (in this case short hind limbs).

  2. This is an excellent article. I’m always interested in dinosaur biology (or indeed the biology of any prehistoric animal) and its relation to the environment that the creature lived in. A very pleasant read, well analyzed and well written. Keep up the good work.

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