Dermal Armor and Spikes

E. rugosidens, specimen AMNH 5665
Photo ©Shriram Rajagopalan from Vancouver, Canada

The armor of nodosaurs is generally similar across taxa with variation changing sizes, shapes, and numbers of plates and spikes depending on not only the genus or species, but also the individual animal. Edmontonia, for the most part as a genus, possesses armor that is constituted of flat, mostly smoothed, dermal plates starting with laid out rounded rectangles in organized rows from the neck into the tail. The skull and head lack dermal plating entirely. Over the pelvic and pectoral girdle the dermal plating is significantly different, making the armor patterns similar over the torso and abdomen and pelvis and tail. The torso and abdomen pattern consists of large oval plates guarding large areas of the rostral back (dorsum) of the animal whereas from the pelvis to the caudal-most plates the shape of the plates is more spherical and the shapes are more populated. This causes the armor to leave smaller gaps, possibly providing greater overall protection from crushing bites and injuries than the more rostral armor. The trade-off is in the sizes of spikes and mobility; not to mention Carpenter's hypotheses of sexual dimorphism and/or age as judged from the sizes of shoulder spikes. Nodosaurs do not have tail clubs like ankylosauridae genera nor do they have vertical spikes or sheets of armor across their pelves like some polacanthinidae genera. Instead, nodosaurs like Edmontonia possess large lateral spikes across their shoulders with smaller spikes trailing down to the pelvis, a trait that has led to many representations of nodosaurs hopping about to thrust their shoulder spikes at attacking predatory dinosaurs. The smaller links of armor around the pelvis would enable such movements as the plates would not take up as much space and could be compressed well as the animal twisted and turned. Additionally, the large plates could be similarly manipulated to manipulate the shoulders, but larger plates need more space between one another to move in a similar fashion, which could account, in part, for the large gaps between plates in the torso area. These large gaps could have also enabled the animal to look upward at a slight degree as the spacing between rows could be compressed as the head and neck pushed the extreme rostral rows of dermal plates back toward the shoulders.

Please remember that these are hypotheses based on looking at fossils, the papers of others, and generally restating shown interpretations of the animal already distributed via film and screen and that we still have many unanswered questions about these very interesting animals. When traits like spikes and armor plating are highlighted everyone automatically (it seems) thinks of two possibilities: defense or mating. The defensive capabilities of nodosaurs like Edmontonia are fairly clear in looking at the skeleton and associate spikes and dermal plates: a large, but squat, animal with hardened scales on its back and large sharp protrusions of bone was probably very good at getting low and defending itself regardless of how it actually managed it. If its shoulder spikes were used as offensive weapons they were probably used mainly to intimidate as they would otherwise need to be picked up, moved with speed, and very accurately aimed. Any movement that elevated and sped up the body of this animal would have left the unprotected underbelly exposed long enough that it could have been tragic. This leaves us with two possibilities, as I see it: Edmontonia was much more turtle-like in its defense of itself or it was a brash and intimidating animal that attempted to scare away predatory dinosaurs rather than actually fight them. Both of these possibilities are intriguing and the behaviors behind both could be fascinating. Please feel free to discuss the likelihood of either or both scenarios. I enjoy these kinds of conversations and thoughts.