What a pain in the…arms! A record-breaking number of injuries in a theropod dinosaur

Ouch!

That word came to mind a lot while reading a new paper published today in PLOS ONE. In the new paper, authors Phil Senter from Fayetteville State University and Sara Juengst from Appalachian State University, both located in North Carolina, describe the many injuries inflicted upon one poor Dilophosaurus wetherilli.

And to be honest, reading their thorough descriptions actually made me cringe. How many injuries can one dinosaur survive and endure? In the case of this Dilophosaurus, many! In fact, the specimen described by Senter and Juengst has a record-breaking number of injuries present on eight different bones in the pectoral girdle and forelimb.

For those of you unfamiliar with Dilophosaurus, I direct you to an earlier post on the PLOS Paleontology Community blog by guest blogger Robert Gay, who discussed this early theropod in greater detail. The specimen examined in this study is the holotype specimen for D. wetherilli. It resides at the University of California Museum of Paleontology in Berkeley, California, but was collected in the Kayenta Formation in Arizona. Senter and Juengst conducted a surface examination of the bones of this specimen, and found a myriad of maladies.

The world of paleopathology is a bit tricky for a few reasons. For example, you have to parse out pathology from taphonomy (i.e., is the abnormality the result of an injury incurred in life or from post-death distortion?). Also, interpretation of the pathologies has to be inferred from comparison to pathological features from extant organisms, and in the case of dinosaurs, researchers such as Senter and Juengst use birds and reptiles as analogues because of their close relationship as well as similarity in symptoms produced by various ailments as compared to mammals. Senter and Juengst note that using mammals or humans as model organisms can be misleading in diagnosing pathological conditions in dinosaurs and should be avoided when possible.

Another problem that often arises in paleopathology is that the pathological features are often not reported by researchers describing specimens. Either they are not recognized for what they are, or are omitted from the description because they are not characteristic traits of the species or deemed unimportant. But, just like tracks or other trace fossils, pathologies are important in helping paleontologists infer life history and behavior of organisms, and in the case of this unlucky Dilophosaurus, just how much of a beating a dinosaur can take—and live to tell about it.

The many injuries of Dilophosaurus. Art by L. Walters. From Senter and Juengst 2016.
The many injuries of Dilophosaurus. Art by L. Walters. From Senter and Juengst (2016).

So, what did this dinosaur tell Senter and Juengst? Quite a bit. I won’t go into every detail here in the blog (that is what the paper is for), but the short story is that this particular Dilophosaurus had several fractures and fibriscesses (similar to abscesses in mammals). Some of these fractures show no indication of infection. Other wounds were not so lucky: a puncture wound on the left ulna in interpreted by the authors as having become infected, due to the abnormal bony growth present. Likewise, the left proximal phalanx has fibriscesses that may represent lysis following infection that began at the puncture wound located there.

The right humerus tells its own story. Along the long axis, the right humerus shows more torsion along the shaft than the left humerus, which would have caused the right forearm and hand to protrude at an unusual angle in life for this dinosaur. Senter and Juengst interpret this malady as osteodysplasia, a common deformity that occurs in birds with nutritional deficiencies. The explain that if a bird, for example, has an injury to one hindlimb, they shift their weight to the other hindlimb to avoid pain, and if they are suffering from a nutritional deficiency, this shift in weight-bearing can cause distortion in the weight bearing bones. While it can never be known for sure, Senter and Juengst feel confident in inferring that this Dilophosaurus specimen may have done something similar by favoring its right forelimb to avoid pain in the left forelimb. And the unusual protrusion of the forearm and hand due to the osteodysplasia in the humerus, the right forearm couldn’t have been tucked into the body and was likely more prone to injuries. Indeed, some deformities in the metacarpal and finger could have been the result of this weird arm posture.

hand
Hand of UCMP 37302 (Dilophosaurus wetherilli in full flexion, showing pathological orientation of the phalanges of finger III. Note that the third finger is abnormally angled in two places: at the metacarpophalangeal joint and at the first interphalangeal joint. Bones with broken outlines are missing from the right hand and are reconstructed according to their shapes in the left hand. Art by L. Walters. From Senter and Juengst (2016).

So, what happened to this poor Dilophosaurus? Well, these injuries didn’t kill him; owing to the visible healing of these injuries, it survived at least several weeks after incurring them. Senter and Juengst suggest that, based on the placement and types of pathological features present, that this dinosaur encountered either several heavy impacts, or one large high-impact encounter. They speculate that this Dilophosaurus could have collided with a tree or rock wall during a fight with another dinosaur to cause many of the fractures. The puncture wounds could have been caused by another dinosaur, either from another predator or a very defensive prey item. But this dilophosaur lived to see another day, and its bones survived to tell a story to paleontologists, a story that is only known because of the field of paleopathology.

Reference: Senter P, Juengst SL (2016) Record- Breaking Pain: The Largest Number and Variety of Forelimb Bone Maladies in a Theropod Dinosaur. PLoS ONE 11(2): e0149140. doi:10.1371/journal. pone.0149140

Featured image CC-BY by Heather Kyoht Luterman. From Milner et al. 2009.

Published by Sarah Z. Gibson

Dr. Sarah Z. Gibson is a paleontologist and science communicator based in Minnesota. Her research focuses on the evolutionary history of ray-finned fishes from the Early Mesozoic. https://orcid.org/0000-0002-6784-3980

4 thoughts on “What a pain in the…arms! A record-breaking number of injuries in a theropod dinosaur

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