A NEW EXAMPLE OF TYRANNOSAURUS BITE MARKS FROM THE HELL CREEK FORMATION

During the spring of 2004 a fragmentary hadrosaur rib from the Upper Cretaceous Hell Creek Formation was found in the collections of the University of Wisconsin-Madison Geology Museum. The rib shows evidence of scavenging, including a deep furrow near the head of the rib and a set of mysterious grooves further distal on the shaft. Similar grooves were found on another rib fragment collected from the same locality in southeastern Montana during the summer of 2003 by a University of Wisconsin field crew. The fragments were found exposed sub-aerially, however it is unlikely that pre- or post-depositional processes such as stream transport or wind erosion are responsible for the formation of these grooves due to their depth, placement and uniform distribution and dimensions. The grooves occur in patches along the anterior face of the rib fragments. They are parallel and regular, roughly 0.7 mm wide and rectangular in cross section. In one area the grooves curve concentrically. The width of each continuous patch of grooves provides an estimated minimum tooth height that eliminates known Hell Creek predators other than the genera Nanotyrannus and Tyrannosaurus. The average width of the individual grooves indicates a minimum tooth denticle size larger than that of Nanotyrannus. Alternatively, the size, shape and spacing of lateral tooth denticles in Tyrannosaurus rex closely match those of the grooves. Previously, most feeding traces attributed to T. rex have been characterized by deep punctures left by teeth crushing large bones, but these specimens exhibit a previously unrecognized feeding behavior for T. rex. These specimens also demonstrate that feeding modifications and behavioral clues can be identified on smaller, fragmentary bones that are often times overlooked in the field or in museum collections.