“Warwick's Duck", an Exceptional Specimen of Edmontosaurus (Dinosauria: Hadrosauridae) with Extensive Caudal Neural Spine Pathologies: Evidence of Mating Injuries in Hadrosaurs?
FREEDMAN FOWLER, Elizabeth A; Dickinson State University, Dickinson, ND. FOWLER, Denver W; Badlands Dinosaur Museum, Dickinson, ND.
Pathologic neural spines have been noted in many specimens of hadrosaurid dinosaurs, which are abundant in Late Cretaceous units, offering a large sample for ecological analysis. Here we describe MOR 3003, a specimen of the Maastrichtian hadrosaurid Edmontosaurus (Hell Creek Formation, Montana) which is exceptional in the extent of the injuries to its caudal neural spines. The otherwise fragmentary skeleton includes 18 articulated and 26 disarticulated caudal vertebrae, which together form a near complete proximal caudal series. Of 42 preserved neural spines, only 3 are not pathological; 23 exhibit healed but displaced fractures and fracture calluses; and the remaining 16 preserve exostoses, remodeled surfaces and/or significant lateral warping. In 19 neural spines, the dorsalmost 5-10 cm have been broken and pushed down 1-6 cm to the left lateral side. Some of these neural spines exhibit a crushing fracture in which the dorsal tip is split vertically, sliding downward over both lateral surfaces of the remaining ventral part of the spine (similarly observed in a second new hadrosaurid specimen, BDM 003 from the Campanian Judith River Formation, Montana). Two adjacent neural spines are truncated approximately halfway, with the dorsal ends completely missing (hypothesized to have been resorbed in life), and the broken dorsal surface heavily remodeled. This truncation is comparable to a previously published Edmontosaurus specimen (DMNH 1493) which exhibits three truncated caudal neural spines in a similar serial position.
Pathologies of caudal and dorsal neural spines are commonly observed in large bodied ornithopod dinosaurs, suggesting that their ecology or bauplan may predispose them to this kind of injury. In the case of DMNH 1493, it was suggested that the neural spines might have been bitten off by a tyrannosaur. However, MOR 3003 and BDM 003 show consistent injuries over such a long section of the tail (as many as 23-39 neural spines) that a biting attack seems implausible. A lateral impact is also considered an unlikely cause as there is no damage to centra or chevrons. The pathology is most consistent with a strong impact onto the dorsal margin of the tail, perhaps being caused by a falling object. However, we consider it more plausible that these injuries were caused during mating, with the position of the injuries consistent with a male pressing down on to the female's dorsal and caudal neural spines while mounting. This hypothesis makes the potentially testable prediction that all individuals bearing these pathologies would be female.
Funding kindly provided by the Smithsonian Institution through Jack Horner. Fieldwork permits granted by U.S. Fish and Wildlife.