 Although descended from smaller ancestors , tyrannosaurids were almost always the largest predators in their respective ecosystems , putting them at the apex of the food chain . The largest species was Tyrannosaurus rex , one of the largest known land predators , which measured up to 12 @.@ 3 metres ( 40 ft ) in length and up to 10 @.@ 2 tonnes ( 11 @.@ 2 short tons ) in weight . Tyrannosaurids were bipedal carnivores with massive skulls filled with large teeth . Despite their large size , their legs were long and proportioned for fast movement . In contrast , their arms were very small , bearing only two functional digits . 
