Palaeontology: More than 2.5 BILLION T. Rexes roamed the Earth over their 2.5 million years


Tyrannosaurus rex. Their name means ‘tyrant lizard king’ — and as far as popular culture goes, they certainly ruled the dinosaurs. But just how many were there?

According to experts from California, the answer is a whopping 2.5 billion of the beasts over the 2.5 million years they roamed North America in the Late Cretaceous.

Around 20,000 adult T. rexes were probably alive at any given point during the species’ existence — give or take a factor of ten, the researchers have estimated.

T. rex — along with the rest of the dinosaurs — went extinct in the wake of a devastating asteroid strike on the Earth some 66 million years ago.

Tyrannosaurus rex. Their name means 'tyrant lizard king' — and as far as popular culture goes, they certainly ruled the dinosaurs. But just how many were there? According to experts from California, the answer is a whopping 2.5 billion of the beasts over the 2.5 million years they roamed North America in the Late Cretaceous. Pictured: an artist's impression of T. rexes

Tyrannosaurus rex. Their name means ‘tyrant lizard king’ — and as far as popular culture goes, they certainly ruled the dinosaurs. But just how many were there? According to experts from California, the answer is a whopping 2.5 billion of the beasts over the 2.5 million years they roamed North America in the Late Cretaceous. Pictured: an artist’s impression of T. rexes

WHAT WAS T. REX?

Tyrannosaurs rex was a species of bird-like, meat-eating dinosaur.

It lived between 68–66 million years ago in what is now the western side of North America.

They could reach up to 40 feet (12 metres) long and 12 feet (4 metres) tall.

More than 50 fossilised specimens of T. rex have been collected to date.

The monstrous animal had one of the strongest bites in the animal kingdom.

An artist's impression of T. rex

An artist’s impression of T. rex

‘The project just started off as a lark, in a way,’ paper author and palaeontologist Charles Marshall of the University of California, Berkeley explained.

‘When I hold a fossil in my hand, I can’t help wondering at the improbability that this very beast was alive millions of years ago, and here I am holding part of its skeleton.’

‘The question just kept popping into my head, “Just how improbable is it? Is it one in a thousand, one in a million, one in a billion?” ‘

‘And then I began to realize that maybe we can actually estimate how many were alive, and thus, that I could answer that question.’

Given the incomplete nature of the fossil record, the notion of being able to reliably estimate the population numbers of long-extinct species has long been dismissed as an impossibility — most notably by the US palaeontologist George Gaylord Simpson.

‘As Simpson observed, it is very hard to make quantitative estimates with the fossil record,’ Professor Marshall conceded. 

‘We focused in developing robust constraints on the variables we needed to make our calculations, rather than on focusing on making best estimates.’

The team’s calculation raises questions — such as why, if the species was so numerous, fewer than 100 individual T. rex have been found, many of which are known from single bones only.

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‘There are about 32 relatively well-preserved, post-juvenile T. rexes in public museums today,’ Professor Marshall explained.

‘Of all the post-juvenile adults that ever lived, this means we have about one in 80 million of them.’

‘If we restrict our analysis of the fossil recovery rate to where T. rex fossils are most common, a portion of the famous Hell Creek Formation in Montana,’ he continued. 

‘We estimate we have recovered about one in 16,000 of the T. rexes that lived in that region over that time interval that the rocks were deposited.’

‘We were surprised by this number — this fossil record has a much higher representation of the living than I first guessed.’ 

‘It could be as good as one in a 1,000, if hardly any lived there, or it could be as low as one in a quarter million, given the uncertainties in the estimated population densities of the beast.’

HOW THE TEAM CALCULATED THE TOTAL NUMBER OF T. REXES  

In their study, Professor Marshall and colleagues consulted both the existing scientific literature and the expertise of peers.

They calculated that T. rex likely reached sexual maturity at 15.5 years and it probably lived into its late 20s.

The average body mass of adults of the species was likely 819 stone (5,200 kilograms) — but they could grow to as heavy as 1102 stone (7,000 kilograms). 

These estimates (combined with Damuth’s Law, discussed further below, which links body mass to population density) allowed them to calculated that each generation lasted for around 19 years and that the average population density was about one dinosaur for every 39 square miles (100 sq. km).

After estimating that the geographic range of T. rex was about 0.9 million square miles (2.3 million sq. km) and factoring in their temporal range of around 2.5 million years, the team arrived at a standing population size of 20,000 individual dinosaurs.

Over the total of some 127,000 generations the species would have lived, that equates to some 2.5 billion individuals overall, the researchers concluded.

'It's surprising how much we actually know about these dinosaurs and, from that, how much more we can compute,' said Professor Marshall. 'Our knowledge of T. rex has expanded so greatly in the past few decades thanks to more fossils, more ways of analysing them and better ways of integrating information over the multiple fossils known.' Pictured: a mounted cast of T. rex skeleton on display outside the University of California Museum of Paleontology

‘It’s surprising how much we actually know about these dinosaurs and, from that, how much more we can compute,’ said Professor Marshall. ‘Our knowledge of T. rex has expanded so greatly in the past few decades thanks to more fossils, more ways of analysing them and better ways of integrating information over the multiple fossils known.’ Pictured: a mounted cast of T. rex skeleton on display outside the University of California Museum of Paleontology

The team’s estimate certainty come with large uncertainties. 

For example, while there was most likely around 20,000 adult T. rexes at any given time, the so-called ’95 per cent confidence interval’ — in which there is a 95 per cent chance of finding the real number — ranges from 1,300 to 328,000 individuals.

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Given this, the total number of T. rexes across time could have been anywhere from 140 million to 42 billion, the researchers explained.

The team used so-called ‘Monte Carlo’ computer simulations to determine how the uncertainties in their data led to uncertainties in their final result.

According to Professor Marshall, the greatest uncertainty stemmed from outstanding questions about the exact nature of T. rex’s ecology — including how warm-blooded the creature was.

The calculations relied on data published by ecologist John Damuth of the University of California Santa Barbara which associates body mass to population density in living animals — a relationship dubbed ‘Damuth’s Law’.

While the relationship is strong, Professor Marshall explained, ecological difference can result in large variations in population densities for animals that otherwise have similar physiologies.

For example, hyenas and jaguars are around the same size — but the latter can be found with populations densities  some 50 times greater than the big cats.

‘Our calculations depend on this relationship for living animals between their body mass and their population density, but the uncertainty in the relationship spans about two orders of magnitude,’ Professor Marshall said. 

‘Surprisingly, then, the uncertainty in our estimates is dominated by this ecological variability and not from the uncertainty in the palaeontological data we used.’ 

For their calculations, the team elected to regard T. rex as a predator whose energy requirements lay halfway between those of a lion and a Komodo dragon — which is the largest lizard alive on the Earth.

The team also chose to ignore juvenile T. rexes, which are both underrepresented in the fossil record and — recent research has suggested — may have lived apart from adults, pursued different prey and behaved almost like a different predator species.

The team chose to ignore juvenile T. rexes (one of which is depicted above), which are both underrepresented in the fossil record and — recent research has suggested — may have lived apart from adults, pursued different prey and behaved almost like a different predator species

The team chose to ignore juvenile T. rexes (one of which is depicted above), which are both underrepresented in the fossil record and — recent research has suggested — may have lived apart from adults, pursued different prey and behaved almost like a different predator species

‘In some ways, this has been a paleontological exercise in how much we can know, and how we go about knowing it,’ said Professor Marshall.

‘It’s surprising how much we actually know about these dinosaurs and, from that, how much more we can compute.’

‘Our knowledge of T. rex has expanded so greatly in the past few decades thanks to more fossils, more ways of analysing them and better ways of integrating information over the multiple fossils known.’

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Professor Marshall said that he expects his peers to quibble with many, if not most of the numbers involved in his team’s estimate.

The researchers have made the computer code they used to estimate T. rex numbers available to other researchers — saying that it could lay a foundation for estimating how many species might be missing from our understanding.

‘With these numbers, we can start to estimate how many short-lived, geographically specialized species we might be missing in the fossil record,’ Professor Marshall said.

‘This may be a way of beginning to quantify what we don’t know.’

The full findings of the study were published in the journal Science.

KILLING OFF THE DINOSAURS: HOW A CITY-SIZED ASTEROID WIPED OUT 75 PER CENT OF ALL ANIMAL AND PLANT SPECIES

Around 65 million years ago non-avian dinosaurs were wiped out and more than half the world’s species were obliterated.

This mass extinction paved the way for the rise of mammals and the appearance of humans.

The Chicxulub asteroid is often cited as a potential cause of the Cretaceous-Paleogene extinction event.

The asteroid slammed into a shallow sea in what is now the Gulf of Mexico.

The collision released a huge dust and soot cloud that triggered global climate change, wiping out 75 per cent of all animal and plant species.

Researchers claim that the soot necessary for such a global catastrophe could only have come from a direct impact on rocks in shallow water around Mexico, which are especially rich in hydrocarbons.

Within 10 hours of the impact, a massive tsunami waved ripped through the Gulf coast, experts believe.

Around 65 million years ago non-avian dinosaurs were wiped out and more than half the world's species were obliterated. The Chicxulub asteroid is often cited as a potential cause of the Cretaceous-Paleogene extinction event (stock image)

Around 65 million years ago non-avian dinosaurs were wiped out and more than half the world’s species were obliterated. The Chicxulub asteroid is often cited as a potential cause of the Cretaceous-Paleogene extinction event (stock image)

This caused earthquakes and landslides in areas as far as Argentina.

But while the waves and eruptions were  The creatures living at the time were not just suffering from the waves – the heat was much worse.

While investigating the event researchers found small particles of rock and other debris that was shot into the air when the asteroid crashed.

Called spherules, these small particles covered the planet with a thick layer of soot.

Experts explain that losing the light from the sun caused a complete collapse in the aquatic system.

This is because the phytoplankton base of almost all aquatic food chains would have been eliminated.

It’s believed that the more than 180 million years of evolution that brought the world to the Cretaceous point was destroyed in less than the lifetime of a Tyrannosaurus rex, which is about 20 to 30 years.



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