It was a fearsome predator by any definition.
Interestingly however a 68 million years old fossil of one of these monsters was discovered. The fossil had a small sample of flesh on it (presumably from the t-rex). After DNA studies were conducted it seems the closest living relative to the tyrannosaurus rex is the modern day chicken. See more here.
What's more interesting is the bone structure of the animals. The bone structure of the bird is rather unique, putting walking force on the shin instead of thigh, something the t-rex did as well. (