When I was a teenager at the turn of the millennium, right around the time I became smitten with fossils, the Field Museum in Chicago dismantled its Brachiosaurus and installed a Tyrannosaurus rex. In essence, the institution was trading one dinosaur icon for another. Out went the plant-eating colossus, heavier than 10 elephants, its neck arcing gracefully far above the museum's second-floor viewing gallery. In came the biggest, baddest predator of all time: a bus-sized brute with railroad-spike teeth that shattered the bones of its prey.