In total, some 2.5 billion Tyrannosaurus rex may have reached maturity and marauded the Cretaceous landscape, according to a new study published today in the journal Science. Though the researchers indicate a range in the plausible number of the apex predators, all signs point to an “absolute abundance” of the animals, as written in the paper’s title.
Study author Marshall Phillips, a paleontologist at the University of California Berkeley, enjoys asking the big questions, like when the first individual of a dinosaur truly emerged from its egg — a literal look at the chicken-or-the-egg conundrum. One such question emerged as he was looking at the T. rex cast outside the museum at UC Berkeley: What were the odds of the fossil’s survival? In order to figure that out, Phillips said he needed to figure out how many of the animals ever lived.
“I was very keen to make sure I came up with an accurate answer, and that meant accurately assessing the precision,” Phillips said in a video call. “The precision is quite fudgy, but it feels solid. I think it’s a good thing inasmuch as it’s good to know how secure your knowledge is.”
In some ways, T. rex was an evolutionary supernova; a final burst of nature’s ingenuity in the twilight of dinosaurs’ tenure on Earth. The T. rex was at the top of the food chain for 2.5 million years. When the dinosaurs died in the catastrophic fallout of an asteroid’s collision with Earth, the T. rex went out on top and has reigned supreme in our collective memory, thanks to its unique morphology and, of course, pop culture relics like Jurassic Park.
Along with other megatheropods, T. rex have been termed a “morphospecies”; as previously reported by Gizmodo, members of their species filled various ecological niches over the course of their development. For that reason, juveniles T. rex, increasingly treated as a wholly separate predator from their adult counterparts, weren’t included in the team’s recent count. Phillips said the model is an attempt to put brackets on the total sum of T. rex to reach adulthood, in a way that could help paleontologists figure their odds for future excavations of tyrannosaurs or other fossilised animals.
The researchers “suggest investigating how rare a ‘rare’ species has to be in order to escape fossilisation,” said Denver Fowler, a paleontologist at the Badlands Dinosaur Museum in North Dakota who is unaffiliated with the recent paper, said in an email. “This is going to be the big splash that the paper is looking at creating — it will be interesting to see how other scientists use the model, and the sorts of hypotheses that they can test with it.”
The team used a mathematical model that drew on known T. rex finds and extant comparable species, like lions and komodo dragons, to approximate the Cretaceous predator’s geographic range, population density, warm-versus-cold-bloodedness, and growth curve, among other useful variables, to ultimately estimate that about 2.5 billion individuals lived to see adulthood, with about about 20,000 T. rex alive at any one time. The 2.5 billion figure is the middle value of a pretty broad curve, with the 2.5% and 97.5% tails sitting at 140 million and 42 billion. They further estimate that one T. rex fossil fossilises for roughly every 80 million individuals, and one is found for every 16,000 that died in a fossil-friendly deposit, like Montana’s Hell Creek formation.
The model could be a useful heuristic tool, though some paleontologists are careful to indicate the limits of such work. “I am ok with using a model to give a ballpark figure and think about what that means ecologically, but again, it’s not data,” Fowler said.
“I think this is a neat study that nicely shows what kinds of information we can glean from the fossil record,” Jordan Mallon, a paleobiologist at the Canadian Museum of Nature who is unaffiliated with the paper, said in an email. “Even 50 years ago, if you had told a paleontologist that one day we would be able to estimate dinosaur population densities from their fossil numbers, you’d be called nuts … It’s great if you’ve got one T. rex skeleton, but we can learn so much more about the ancient ecology of a species if we collect many, many more.”
Mallon added: “As neat as it is to finally be able to estimate the total number of T. rex to ever walk the planet, it’s important to keep in mind the vast uncertainties in the estimates. No doubt, the question will have to be re-visited over the years as we get new information.”
Going by this study, that’d be roughly two T. rex for every Washington, D.C.-sized area (not odds I’d love as a Cretaceous-era herbivore). More fossils, as always, will sharpen the resolution of life and its abundance in the deep past.