User:Abyssal/Theropod behavior

Theropod fractures
Bruce Rothschild and others published a study examining evidence for stress fractures and tendon avulsions in theropod dinosaurs and the implications for their behavior. These pathologies provide evidence for very active predation-based rather than scavenging diets. Stress fractures are caused by repeated trauma rather than singular events like acute fractures. Since stress fractures are due to repeated events they are probably caused by expressions of behavior. Activity-related fractures are also known from ceratopsians. Pedal injuries could be caused by runnning or migration, but manual injuries would most likely be due to resistant prey items. Stress fractures in dinosaur bones can be identified by studying the bones for diaphyseal surface bulges, usually facing anteriorly on the bone. When viewed under x-rays the fractures exhibit areas of reduced x-ray attenuation that appear as a clear zone angled through the diaphyseal bulge. Usually this zone of attenuation is not visible on the surface of the bone. Allosaurus had a significantly greater number of diaphyeal bumps than Albertosaurus, Ornithomimus or Archaeornithomimus. The fractures "were distributed to the proximal phalanges" and occurred across all three major digits in "statistically indistinguishable" numbers. Pathologies of the distal unguals were only noted among dromaeosaurids, where they represented 50% of manual lesions. The authors refrained from performing a statistical analysis of these injuries in non-dromaeosaur theropods because they were so uncommon that such an analysis would be impossible for all intents and purposes.

Avulsion injuries were only noted among Tyrannosaurus and Allosaurus. Scars from these sorts of injuries were limited to the humerus and scapula. A divot on the humerus of Sue the T. rex was one such avulsion. The divot appears to be located at the origin of the deltoid or teres major. The researchers described theropod phalanges as being pathognomonic for stress fractures, meaning they are "characteristic and unequivocal diagnostically." Lesions left by stress fractures can be distinguished from osteomyelitis without difficulty because of a lack of bone destruction. They can be distinguished from benign bone tumors like osteoid osteoma by the lack of a sclerotic perimeter. No disturbance of the internal bony architecture of the sort caused by malignant bone tumors was encountered among the stress fracture candidates. No evidence of metabolic disorders like hyperparathyroidism or hyperthyroidism was found in the specimens. The thin shell caused by a subperiosteal hematoma was not seen and would have been easily identified under x-ray. Since the lower end of the third metatarsal would contacted the ground first while a theropod was running it would have borne the most stress and should be most predisposed to suffer stress factors. The lack of such a bias in the examined fossils indicates an origin for the stress fractures from a source other than running. The authors conclude that these fractures occurred during interaction with prey. They suggest that such injuries could occur as a result of the theropod trying to hold struggling prey with its feet. The localization in theropod scapulae as evidenced by the tendon avulsion in Sue suggests that theropods may have had a musculature more complex and functionally different than those of birds. The authors suggest that future workers compare the anatomy of a Komodo dragon and crocodile with that of Tyrannosaurus.

The following taxa had no evidence of stress fractures or tendon avulsions among the examined specimens: Herrerasaurus, Coelophysis, Dilophosaurus, Carnosaur (indeterminate), Velocisaurus, Mononykus, Megalosaurus, Marshosaurus, Ornitholestes, Compsognathus, Alectrosaurus, Albertosaur, Gorgosaurus, Utahraptor, Deinonychus, Therizinosauridae (indeterminate), Struthiomimus, Ornithomimus, Archaeornithomimus, Dromiceiomimus, Elmisaurus, and Troodon.

Reference

 * Rothschild, B., Tanke, D. H., and Ford, T. L., 2001, Theropod stress fractures and tendon avulsions as a clue to activity: In: Mesozoic Vertebrate Life, edited by Tanke, D. H., and Carpenter, K., Indiana University Press, p. 331-336.