Ceratosaurus is an iconic dinosaur due to numerous physical attributes that distinguish it from other theropod species: the horn on the end of its nose, the massive teeth, the tiny hands with the four fingers, the wide tail, etc. However, the main focus of this article are its osteoderms – the bony bumps that were on its back. What were they, where exactly on the body were they, and what did they look like?
Despite its instantly recognizable profile, Ceratosaurus fossils are surprisingly rare. Only a handful of skeletons have been found, and all of them are incomplete. Of these specimens the things which are especially unlikely to be preserved are its osteoderms. These small bony lumps (there’s really no other way to describe them) occurred in a row running down the middle of its back, and it is one of this animal’s more distinctive features. It is the only theropod species which is known to have possessed body armor. Yet “armor” is hardly the word that I would use to describe this anatomical attribute, as we shall see later.
In order to make an accurate picture of Ceratosaurus, I needed to get as much information as I could about its osteoderms. So far, nobody has done a comprehensive study of Ceratosaurus osteoderm morphology – there’s a paleontology Master’s thesis that’s just begging to be picked up by someone. There wasn’t much information to go on because written descriptions of the osteoderms are rather scant. Only a few mounted specimens of Ceratosaurus include the osteoderms as part of the display, and I’m not aware of any museum having Ceratosaurus osteoderms housed in its collections departments.
Charles W. Gilmore says the following in his description of Ceratosaurus fossils:
“Several dermal ossifications were found with the type specimen of Ceratosaurus nasicornis, and some of these were so retained in the matrix as to indicate their exact position in relation to the internal skeleton of the living animal. Reference is made here to the row of elongate, irregularly shaped, bony ossicles present above the spinous processes of caudals (fig. 1, pl. 22) 4 to 10 inclusive, and above cervicals 4 and 5 (0, pls. 29 and 30). The position of these ossicles would appear to indicate a continuous row of dermal ossifications, extending along the median line of the back from the base of the skull well down on the tail, if not the greater part of its length…The ossifications above the tail are from 25 to 38 mm above the tops of the spinous processes of the vertebrae, evidently indicating the thickness of the skin and muscles between them and the tops of the spines. Those on the neck are much closer to the vertebrae, and in one instance appears to rest on the spine (figs. 1, 2, and 3, pl. 20). That there were other dermal ossifications is shown by the presence of a small skin plate found with the bones of this skeleton. It had been freed from the matrix when it came into my hands, so there is no evidence as to its probable position in the skin. It is a relatively small subquadrangular plate of bone 58 by 70 mm., with a comparatively smooth ventral and a roughened dorsal surface. The under surface is gently concave in the direction of its shortest diameter, with a low longitudinal swelling extending through the middle of its longest diameter. The roughening of the external surface is without definite pattern” (Charles W. Gilmore, Smithsonian Institution-United States National Museum, Bulletin 110 – “Osteology of the Carnivorous Dinosauria of the United States National Museum, with a Special Reference to the genera Antrodemus (Allosaurus) and Ceratosaurus”. Washington: Government Printing Office, 1920. Pages 113-114).
Unable to examine these osteoderms in person, I did the next best thing – I looked at as many pictures of Ceratosaurus osteoderms as I possibly could, and I made the following observations:
- There was only one row of osteoderms running along the middle of its back.
- The osteoderms are all fairly small.
- The osteoderms are irregularly shaped.
- From overhead, the osteoderms appear to be diamond or lozenge-shaped, elongated anteriorally-posteriorally.
- The osteoderms were smooth underneath, but they had a rugose texture on their upper surface.
- Some osteoderms seem to come to a point on their upper surface, while others come to a low ridge, and others don’t have any raised features at all. This might be due to the fossilization process.
The appearance of the osteoderms was somewhat perplexing to me. In numerous examples of paleo-art, these bony knobs were shown as pronounced features, more or less uniform in shape, often being exposed bone or bone covered with a thin scute. However, the physical evidence doesn’t look anything like the commonly-portrayed iconography. If the osteoderms themselves were used for display purposes (as they likely were, since the use of a single row of small pieces of bone as armor would only be minimally protective), then they would have been much larger, much more pronounced, more uniform in appearance, and would have had a more “finished” look to them in order to make them more visually apparent. As they are, these formless bony lumps would have made a poor sight, and they certainly would have been of little use as armor.
The rough texture of the osteoderm’s dorsal surface implies that they had a covering of keratin atop them. Due to the irregular shape of the osteoderms, it is also implied or inferred that the osteoderms themselves were not the visual focus, but rather, what was on top of the osteoderm was. It’s possible that each of these small osteoderms served as the anchor point for a large keratinous scute which extended upwards from the dorsal surface of the osteoderm, possibly for a considerable distance. The image that comes to mind is that of the spines which are seen running along the backs of some lizards like a crest, such as the iguana.
In 1990, a specimen of Diplodocus was discovered with skin impressions, and among these were a series of iguana-like keratinous spines running along the top of the animal’s vertebrae. It’s therefore possible that Ceratosaurus might have had a similar appearance.
Diplodocus carnegeii. © Jason R. Abdale (May 11, 2020)
With all of this being considered, I decided to revise my Ceratosaurus drawing that I had made in April 2012. In the original drawing, the animal has a single row of osteoderms that form a line of low semi-circular bumps, looking very much like crocodilian armored scutes. You can see that drawing below.
Ceratosaurus. © Jason R. Abdale (April 23, 2012).
Now, I changed the animal’s appearance by extending the osteoderms with the addition of a keratinous scute, shaped like the spines of a lizard (although my impression was that they actually looked more like theropod teeth). I also took the time to touch up the drawing’s overall color and smoothness. You can see the updated drawing below.
Ceratosaurus. © Jason R. Abdale (April 4, 2020)
When I decided to alter the shape of the osteoderms with the addition of the erect spines, I noticed two important changes to the animal’s overall appearance. Firstly, it made the animal taller. In real life, the addition of a few inches of height would have made the animal seem bigger and more imposing than it actually was. Secondly, it gave the animal a much more intimidating appearance, like a “razorback” wild boar. This might have been helpful in disputes over carcasses or competition for mates. It is unknown whether both male and female Ceratosaurus possessed this feature because so few fossils have been found that a sexual compare-and-contrast cannot yet be performed. However, it is almost certain that the males were ornamented in this way.
I hope that you found all of this interesting. Keep your pencils sharp.
Categories: Paleontology, Uncategorized
I love the revised look of ceratosaurus! I’m always forgetting that any bony scute was almost definitely covered in a keratinous sheath, and so it would not be too much speculation at all to extend it in our reconstructions. Thank you for the reminder! 🙂
Thank you. I’m glad that I could help.
One thing to note is that the “bumps” on Carnotaurus and the spines on the Howe Quarry diplodocid are not osteoderms, as they do not have bony cores. Instead these structures are scales that have been termed “feature scales”, specifically “midline feature scales” in the case of the diplodocid (https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0031295). Although the osteoderms on Ceratosaurus certainly would have had keratin coverage, they are not directly comparable to Carnotaurus and the diplodocid.
Thank you for your input, and thank you for letting me know about the mistake that I made concerning Carnotaurus. For many years, I had been told that Carnotaurus possessed osteoderms, not dermal scutes. Therefore, I have removed all mention of Carnotaurus possessing osteoderms on this article and on other articles on this website.
As to Diplodocus, I never said that it had osteoderms. In each article where I write about its spines, I have always stated that they were composed exclusively of keratin without the presence of a bony core.
Concerning Ceratosaurus’ osteoderms and their potential for having elongated keratinous scutes attached to their dorsal surface, I stand by my observations and my hypothesis. I was already aware when I wrote this article last year that the dermal features on Ceratosaurus were not 100% comparable to Diplodocus since Diplodocus’ “razorback” of iguana spikes did not have any bony cores or even bony anchor points at their bases. However, that was not the argument that I was making here. I was saying that, based upon the shape and texture of Ceratosaurus’ osteoderms, I believe that they served as the anchor points for raised dermal scutes which may have been visually similar to those iguana-like spikes that were seen on the back of Diplodocus – similar in appearance, but not necessarily similar in physical composition.