Add up all the Tyrannosaurus rexes that ever lived on Earth and you’d get something like the human population of India and China combined — some 2.5 billion apex predators with big teeth and bigger appetites.
That’s the best guess that scientists shared in a new study published Thursday, which estimates that a total of 2.5 billion individual T. rexes have walked the Earth in all of its history.
Two-and-a-half billion T. rexes might sound like a lot, but researchers say there were probably only about 20,000 alive at any given time. The hulking apex predators roamed North America for about 1.2 to 3.6 million years, or what scientists estimate was 127,000 generations.
Researchers used the fossil record, theories about the T. rex’s lifecycle and biology rules around food and population density to come up with their estimate, according to their findings published in Science.
It’s the first time that anyone has tried to make such a guess, and the study authors acknowledge that there is a dinosaur-sized margin for error in their numbers. Their model says there might have been anywhere from 140 million to as much as 42 billion T. rexes in total, with 2.5 billion being the midpoint of that range.
“That’s a lot of jaws,” said the study’s lead author, Charles Marshall, director of the University of California Museum of Paleontology. “That’s a lot of teeth. That’s a lot of claws.”
Kristi Curry Rogers, a paleobiologist at Macalester College, said she was stunned by the estimate.
“Probably like a lot of people, I literally did a double-take to make sure that my eyes hadn’t deceived me when I first read that 2.5 billion T. rexes have ever lived,” said Rogers, who was not part of the study.
The estimate gives paleontologists an idea of just how rare it is to find a T. rex fossil. Fossils from 100 individuals have been found to date, and only 32 of those were complete enough to show that they came from adult specimens.
The study authors say humans have found approximately one fossil for every 80 million T. rexes that ever lived. That ratio is thought to be much higher (roughly one per 16,000) in well-known fossil hotspots.
Marshall says that if the total T. rex population had been only 2.5 million, we might never have known they existed.
“You hold a fossil in your hand and you know it’s rare. The question is, how rare?” Marshall said. “To know that, you need to know how many of them existed.”
Thomas Holtz, a vertebrate paleontologist at the University of Maryland in College Park, told Nature that the calculation is an “interesting speculation.”
“We always knew that the chance of any individual becoming a fossil was exceedingly rare,” he said. “But we lacked the calculation to figure out how rare.”
Marshall and his team at UC Berkeley came up with their estimate based on a variety of factors. They assumed that T. rex reached sexual maturity at 14 to 17 years old, and that they lived to a maximum of 28 years old. The researchers also estimated that a T. rex’s energy needs (i.e. appetite) would fall somewhere between that of a Komodo dragon and a lion.
They also incorporated some general rules of biology, which say that bigger species tend to have bigger appetites and fewer individuals in any given space. It’s the same principle at work in the ocean, where a handful of sharks or whales will co-exist alongside vast schools of tiny fish or clouds of plankton.
Based on the study’s numbers, an area the size of Toronto would have been large enough to support approximately six T. rexes. Calgary would have been large enough for seven individuals, while Vancouver would only have enough room for one.
Peter Makovicky, a paleontologist at the University of Minnesota, told the New York Times that it was “exciting” to see such methods applied to a long-dead species such as the T. rex.
“I don’t think many of us who are sort of the, let’s say, skeletally focused paleontologists have thought this would be possible,” said Makovicky, who was not part of the study. “I think it’s really eye-opening in that sense.”
James Farlow, a geology professor at Purdue University, also hailed the estimate as important — for another reason altogether.
“The truth, as I see it, is that this kind of thing is just very cool,” he said.
—With files from The Associated Press