A partial mandible was discovered in a mass of fossilized stomach matrix near an incomplete Tyrannosaurus rex skeleton found in Dawson County, Montana, in 2009. The dissection of the T. rex stomach contents revealed a partial mandible with a secodont, two-cusped last molar tooth in a damaged socket. The bone of the mandible was structurally damaged, but it was only mildly attacked by stomach acids, strongly suggesting the T. rex died shortly after eating the animal. Electron microscope analysis of the tooth enamel remaining on the partly digested specimen indicated it belonged to a small mammal. The stomach of the dinosaur was filled with fossilized mud and sand that protected the mammal’s jaw from further digestion. This also showed clear evidence that the T. rex died by drowning. In addition, the electron microscope studies of the teeth suggest a slower-than-modern-day development of the mammalian dental enamel prisms.