Fossils, comparable to this skeleton of a T. rex on show within the Netherlands, could also be even rarer than scientists realised. Credit: Marten van Dijl/AFP by way of Getty
Ever puzzled how many Tyrannosaurus rex ever roamed the Earth? The answer is 2.5 billion over the 2 million or so years for which the species existed, in accordance to a calculation revealed right this moment in Science1. The determine has allowed researchers to estimate simply how exceedingly uncommon it’s for animals to fossilize.
Palaeontologists led by Charles Marshall on the University of California, Berkeley, used a way employed by ecologists finding out modern creatures to estimate the inhabitants density of T. rex throughout the late Cretaceous interval.
“You hold a fossil in your hand and you know it’s rare. The question is, how rare?” says Marshall. “To know that, you need to know how many of them existed.”
To do this, he and his co-authors turned to a way used to estimate the inhabitants density of residing animals from their physique mass and the geographic ranges that they occupy. Damuth’s Law stipulates that the common inhabitants density of a species decreases in a predictable manner as physique mass will increase; for instance, there are fewer elephants than mice in a given space.
Chances of being fossilized vanishingly small
The group used their estimates of the entire vary of T. rex throughout trendy North America, mixed with their estimates of the dinosaur’s physique mass, to calculate that, at anybody time, round 20,000 T. rex would have been alive on the planet. That interprets to round 3,800 T. rex in an space the scale of California, or simply two T. rex patrolling Washington DC.
Calculating that T. rex survived for about 127,000 generations earlier than changing into extinct, the researchers got here up with a determine of 2.5 billion people over the species’ complete existence. Only 32 grownup T. rex have been found as fossils, so the fossil document accounts for simply 1 in about each 80 million T. rex. This signifies that the possibilities of being fossilized — even for one of the largest-ever carnivores — had been vanishingly small.
These numbers counsel that fossils typically are exceedingly uncommon, and trace that many species that had been a lot much less widespread than T. rex had been in all probability by no means preserved, says Marshall, who provides: “The fossil record is our only direct knowledge of these completely unimaginable past histories of our planet.”
Thomas Holtz, a vertebrate palaeontologist on the University of Maryland in College Park, calls the calculation an “interesting speculation”, including that “we always knew that the chance of any individual becoming a fossil was exceedingly rare, but we lacked the calculation to figure out how rare”.
But he says it could be good “to see someone ground-truth these kinds of estimations against living species to get a better sense of accuracy”. He’d additionally prefer to see comparable research made on extinct species with extra plentiful fossils, comparable to woolly mammoths, Neanderthals and dire wolves, which could permit us to raised perceive historic ecosystems.
