There is a tendency among paleo-artists to adorn the bodies of theropod dinosaurs with osteoderms. I include myself in this, since I too have been guilty of this practice. As far as I am aware, only two theropod dinosaurs have been found with osteoderms – Ceratosaurus and Carnotaurus. Both of these dinosaurs were, cladistically-speaking, primitive, and were probably closer both in appearance and genetics (and almost assuredly intelligence) to early primitive archosaurs than to later theropod groups.
The word osteoderm literally means “skin bone”. These are small pieces of bone which are embedded in the skin, and sometimes protrude out of it so they look like bony bumps on the dinosaur’s body. The most prevalent example of dinosaurs possessing osteoderms is a group called the thyreophorans, which means “shield-bearers”, a group that includes stegosaurs and ankylosaurs. But other dinosaurs have them to. At least one titanosaurid sauropod, Saltasaurus, has been found with osteoderms, and it is believed that possibly all titanosaurs had osteoderms. As mentioned earlier, only two theropods have been found thus far with osteoderms – Ceratosaurus (western USA and possibly Africa, Late Jurassic) and Carnotaurus (Argentina, Late Cretaceous). Ceratosaurus was found with a single row of small osteoderms running down the middle of its back (NOT multiple parallel rows, as is often shown in some works of paleo-art), extending from the back of the skull and running all the way down to the tip of the tail. Carnotaurus was found with excellent skin impressions on portions of the body, and these showed that the body was covered in non-overlapping reptilian scales, not feathers. The scales themselves were irregular in pattern and arrangement, with some being larger and more pronounced than others. Also, on the back were arranged several parallel rows of osteoderms, spaced at regular intervals. The osteoderms became larger the closer they were to the middle of the body (medially).
As far as I am aware, Ceratosaurus and Carnotaurus are the only two theropod dinosaurs that have been found with osteoderms. Both of these animals belong to the same group of theropod dinosaurs, Ceratosauria. Specifically, Ceratosaurus is a ceratosaurid and Carnotaurus is an abelisaurid – a slightly more advanced line. It may be possible that all theropods within Ceratosauria were adorned with osteoderms. However, some paleo-artists have a tendency of taking a feature found in one or a few specific animals and ascribing this feature to the entire sub-group of dinosaurs. For example, a few paleontologists and paleo-artists believe that many and perhaps all sauropods had a row of keratinous spines running down the neck, back, and tail just because ONE specimen of Diplodocus was found with them. While this proves that this particular species and possibly the genus had this feature, it does not mean that all diplodocid sauropods had these keratinous spines, and it certainly doesn’t prove that all sauropods in general had this feature. The same goes for theropods. Many paleo-artists place osteoderms on their meat-eating dinosaur’s bodies simply because osteoderms have been found on these two carnivores, and they decided to put them on virtually every theropod that they drew or sculpted.
Extrapolation is no sin. There’s nothing wrong about making an observation about something and suggesting that something else which was similar may have had identical properties. Writers and researchers do it all the time. However, I should warn people out there that there are varying degrees of extrapolation. It’s one thing to make an observation based upon the fossils of these two dinosaurs, which, as I stated before, came from the same theropod sub-division, and assume or hypothesize that other species within this particular group may have had this feature as well. It is quite another thing to take that feature and apply it to every theropod genus, from Compsognathus to Tyrannosaurus.
I may sound like I’m being self-righteous and pontificating, but I too am guilty of making wild extrapolations and assumptions when it comes to my prehistoric illustrations. A few years ago, I did an anatomical study of Tyrannosaurus rex in a running pose, and I had it with osteoderms, for no other reason other than so many other paleo-artists had pictured T. rex with osteoderms in the past. I followed the crowd, and illustrated my T. rex accordingly. However, later on when I read about tyrannosaur skin impressions, I learned that these large tyrannosaurs had small pebbly skin with no osteoderms. I revised my drawing.
Another example of where I might have made a mistake is in my drawing “Giganotosaurus head study”, which was showcased in an earlier post on my blog. You can see it here: https://dinosaursandbarbarians.wordpress.com/2013/07/01/giganotosaurus-head-study/. The reason why I had put those rows of bony bumps on its neck and a few on its jaw was because at the time I thought that Giganotosaurus was an abelisaurid, which is a sub-division of Ceratosauria. I later learned that it wasn’t, but I didn’t want to change it – I think it looks nice as it is. However, I’ll be sure to learn all of the information that I can about a certain subject in the future before I draw it.
What I’m trying to do here is caution paleo-artists and aspiring paleo-artists about the dangers of making wild assumptions and extrapolations. Do your homework, do your research, and illustrate your creations as best as current science allows, and don’t do anything that you aren’t able to explain or back up with argument.
Keep your pencils sharp.