Dinosaurs and Barbarians

Home » Paleontology » The Presence and Usage of Osteoderms in Dinosaur Paleo-art

The Presence and Usage of Osteoderms in Dinosaur Paleo-art

Categories

Archives

Many times, paleo-artists take a feature that was found in a few species and ascribe it to entire groups. One of these trends is to portray osteoderms on the bodies of dinosaurs in their artwork. 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 that they look like bony bumps on the dinosaur’s body. Evidence suggests that they were often covered with a keratinous scute. The most prevalent example of dinosaurs possessing osteoderms is a group called the thyreophorans, meaning “shield-bearers”, which includes the stegosaurs and ankylosaurs, but other dinosaurs have them too. At least one titanosaurid sauropod, Saltasaurus, has been found with osteoderms, and it is believed that possibly all titanosaurs had osteoderms as well.

One contentious issue regarding osteoderms is their presence in theropods, or rather, in artwork depicting theropods. There is a tendency among paleo-artists to adorn the bodies of theropod dinosaurs with rows of small osteoderms along their neck, back, and tail, and I too have been guilty of this practice. However, as far as I am aware, only two theropod dinosaurs have been found with osteoderms: Ceratosaurus (western USA and possibly Africa, Late Jurassic) and Carnotaurus (Argentina, Late Cretaceous). 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.

In terms of appearance, 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).

Ceratosaurus. © Jason R. Abdale. April 23, 2012.

 

Carnotaurus skeleton, Museu de Ciências Naturais da PUC Minas. Photograph by Roberto Murta (November 19, 2009). Public domain image. https://commons.wikimedia.org/wiki/File:Dinossauromcnpucminas.jpg.

 

In terms of cladistics, Ceratosaurus and Carnotaurus belong to the same group of theropod dinosaurs, Ceratosauria. Specifically, Ceratosaurus is a ceratosaurid and Carnotaurus is an abelisaurid, which is a slightly more advanced line. It may be possible that all theropods within Ceratosauria were adorned with osteoderms, but we cannot be 100% certain of this. However, as I said earlier, 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 in association with 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 Eoraptor to Velociraptor.

But what about “scutes” on dinosaurs? The term “scute” has two definitions: either it is the scale-like covering over an osteoderm, or it’s simply an unusually large thick scale. Preserved skin impressions from multiple species have shown that some dinosaurs had rows of large, thick, texturally-pronounced scutes arranged on their bodies, but these scutes did not have a bony core – therefore they can’t be classified as “osteoderms”. Examples of animals that have this feature are the stegosaur Hesperosaurus and the ceratopsian Chasmosaurus.

Chasmosaurus. © Jason R. Abdale. March 31, 2016.

 

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. Consequently, I revised my drawing, which you can see here.

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. 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 the picture – 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 back up with researched facts and/or persuasive arguments.

Keep your pencils sharp.


4 Comments

  1. Good post! Then, of course, there is the hypothesis that states dino scales were former feathers and only on the big taxa. Chickens and other birds are naked beneath the feathers. I wonder if basal dinos were scaleless, developing protofeathers, feathers and scales in that order and not everywhere.

    • jrabdale says:

      Thanks for your kind words. Since dinosaurs fall into Archosauria, it’s pretty much taken for granted that even the earliest ones had scales. Both bird feathers and reptilian scales are made of keratin. Also remember that birds DO have scales, just not all over the body – they have scales on the parts that aren’t covered by feathers (the lower leg and the feet), which infers that feathers were originally scales.

      Now, pelycosaurs on the other hand, that’s a different story. Despite their lizard-like appearance, I’ve heard that those giant neckless sail-backed proto-mammals didn’t have scales, but were instead covered in a thick leathery hide. I’m not sure if this claim is true or not.

      If you’d like to read my post “Neckless Wonders”, where I hypothesize about pelycosaur feeding strategies, click here (I actually give a shout-out to you in the post):
      https://dinosaursandbarbarians.wordpress.com/2014/03/02/neckless-wonders-feeding-techniques-of-dimetrodon-and-other-early-terrestrial-tetrapods/

  2. Alejandro Herraiz Gassan says:

    Did carcarodontosaurids and other similar theropods have small bumps on the top of their head or is it another trend to depict them like that?

    • jrabdale says:

      Thank you for your question. Carcharodontosaurus and Giganotosaurus belong to the group Allosauroidea. There is currently no evidence that any member of this group possesed osteoderms on the body. However, the structure of their skulls is another story. Both Carcharodontosaurus and Giganotosaurus had rugose nasal and lacrimal bones. Therefore, the small bumps running along the top of the skull or in a pair of parallel ridges which are depicted in paleo-art appear to be accurate.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: