On average, researchers estimate that some 20,000 T. rex lived at any one time and that about 127,000 generations of the dinosaurs lived and died. Those averages imply that a total of 2.5 billion T. rex lived in the species’ native North America, possibly as far north as Alaska and as far south as Mexico, over a two- to three-million-year timespan.
In the last 20 years, researchers have discovered an extraordinary amount about T. rex, including how long it lived (roughly 28 years), when it reached sexual maturity (roughly 15.5 years old), and how much it weighed when fully grown (roughly 15,000 pounds, on average).
If all the T. rex that ever lived—the estimated 2.5 billion—yielded just [32 fossils in total], then only about one in 80 million T. rex fossilized after they died. Even if a higher percentage of the animals fossilized and we have yet to find the remains, the sheer tininess of these odds underscores just how rare it is for a carcass to be buried quickly enough and in the right chemical conditions to mineralize and form a fossil. “If T. rex were a thousand times less abundant—if the total wasn’t 2.5 billion but was 2.5 million—we might never have found it,” [paleontologist Charles] Marshall says.