2.5B T. rex inhabited the planet, researchers say | Hacker News Reader

2.5B T. rex inhabited the planet, researchers say

axios.com

68 pointsjimmy202058 comments

It's not 2.5B T. Rex at the same time.

2.5 billion Tyrannosaurus rex lived and died during the roughly 2.4 million years

I'm in a meeting so I'm not totally paying attenion to this comment as I write it (sorry) but I believe that works out to just a little over 1000 per year. That's not a huge number for an entire species across an entire planet.

Deeper in the article, there's an estimate of a 19 year lifespan.

So ~20k alive at any point in time, assuming a flat distribution during that 2.5B year window.

20k seems unlikely low, even as a median. A population that small would be difficult to sustain for that long through variations in climate, resources, ecosystem, and evolutionary competition.

They need not have been spread across the entire planet, though. 20k in a smaller area might have been fine.

There is a supported theory that at one point there were 10,000 or fewer humans on the planet: https://en.wikipedia.org/wiki/Toba_catastrophe_theory#Geneti...

This claim is not that there was 20K T-rexes at one dire point in history but an average over millions of years.

Humans ended up surviving that disaster, and then went on to populate the whole earth. Also side note, supervolcanoes are scary as hell.

On the contrary, for apex predators, smaller populations are easier to sustain than large populations.

Especially apex predators of that size.

Yeah but the prey was even bigger. Have you seen brontos? Everything was bigger in that era, including plants and trees.

Not sure what your comment has to do with mine. That doesn't mean the total available biomass to predators in aggregate was larger or smaller, just because some big dinosaurs existed.

It doesn't even mean T-rex hunted brontosaurus frequently or at all (given the somewhat different eras), much like the modern lions don't really hunt adult elephants.

T Rex lifespan of what, 10-20 years? That’s a tiny number - about 3 TRexes in the entire country of Belgium

Imagine how much land you need to support one T rex though given how big they were.

Agreed. It's the one thing we know for sure about the ocean and its rather large predators.

sounds a boring meeting.

It's worth pointing out to anyone in the comments before they read the article:

They arrived at the ~20k number living at any one time number first; ~1k per year * 19yr lifespans. They extrapolated it out to that 2.5B based on how long they were around.

Parent reverse engineered it in reverse!

It seems extremely unlikely that there was a flat population for millions of years (considering e.g. the human population has doubled in the last 50 years), and this wouldn't be my starting assumption.

A small, flat population is even more unlikely - any disruption would wipe out the population more quickly than millions of years down the line.

Populations reach equilibrium very quickly on a geologic timescale. Humans had a relatively flat population graph until the past 10kya or so as well (and realistically long after that point as well).

You have to have a flat population over long timescales. If you started with two tyrannosaurs and had a population growth rate of just 0.02% (population doubles every 6600 years), there would be approximately 500 billion tyrannosaurs on the planet at the same time by the point they went extinct.

It's likely the population fluctuated up and down slightly on short timescales, but the average would have been constant. They were spread out over a continent so any disruption would need to be very widespread to take them all out - which eventually of course is exactly what happened.

Humans are unique in that we can develop technology to exceed our carrying capacity.

https://en.wikipedia.org/wiki/Estimates_of_historical_world_...

This graph is smoothed out first by the logarithmic nature, and second by the fact that "estimates for the 1st century suggest uncertainty of the order of 50%" (and that's only 2000 years ago).

The graph does not show us that anybody thinks the human population was flat before a certain point.

When I first saw the headline I imagined flocks for T-Rexes devouring everything. 2.5B for all of history is more reasonable

They were not spread across the entire planet, though, far from it. They lived in what is now Western North America and used to be an island continent called Laramidia[1].

[1] https://en.wikipedia.org/wiki/Laramidia

In that case then "2.4m years" is the unbelievable number.

I'm frankly astonished that the species lasted for two and a half million years all while being highly endangered the whole time.

Like, nevermind a comet, a poorly timed burp from a volcano could have sufficiently reduced diaspora enough to make them disappear.

One of the threads that make me wish we had downvotes here... Very deceiving clickbaity title!

> I'm in a meeting so I'm not totally paying attention to this comment

I'm in a meeting too and your comment made me chuckle. I love the idea of HN-goers going through the motions of professional life, when in reality they just want to talk about dinosaurs and the nuance behind dinosaur research.

You think that is fun, you should get some biotech friends. We actually talk about what it would take to recreate Jurassic park in real life.

Humans are currently up to 105 Billion Total (since 50,000 BC)

https://en.wikipedia.org/wiki/Estimates_of_historical_world_....

So 7.5% of all humans born in 52.000 years are still alive.

Ergo, humans have an average lifespan of 4.000 years.

Or there has been a massive increase in human survival in the last 100 years.

I would even argue it has been so since the industrial revolution which brought along medical advances. So ~300 years

> the researchers were able to approximate that about 20,000 T. rex were alive at any given time

low

Yep the most endangered elephant is the Asian elephant and there are about 50,000 left in the wild right now, in total there are half a million elephants in the world of all species having only up to 20K at any given time for a species that was pretty much on top of the food chain is quite surprisingly low.

Maybe the other dinosaurs were quite dangerous for the king? I mean the ones with spike tails etc.

T. Rex was a predator, however, and an apex predator at that. Elephants are herbivores. That changes the expected numbers quite a bit. Prey populations typically outnumber their predators by two to three orders of magnitude.

Google says there are about 50,000 killer whales right now. They are apex predators and similar in mass. Of course sea creature versus land animal, but also I believe we are at a low point for killer whale population.

I would put dolphins up there too and their numbers are dwindling due to humans directly fishing them for centuries and now destroying their habitats by over fishing.

Two issues:

1. Water covers ~70% of the Earth's surface. Whales migrate of course but can exist from the poles to the equator;

2. Of the 30% land, for pretty much any species, some portion of that is uninhabitable. Dinosaurs being reptiles likely require hot and wet environments. As predators, T. Rex would be limited by where their food could live too.

The dinosaur era was largely one of a super-continent (Pangaea). A good portion of that was likely too cold or too dry to support such a large predator.

Note that Pangaea had already started breaking apart a hundred million years before Tyrannosaurus rex, which lived right at the end of era of non-avian dinosaurs. Indeed T. Rex is much closer to our time than Pangaea was to T. Rex's time!

They lived in what is now Western North America, but was then an island continent called Laramidia [1] so yes, they were geographically quite limited.

[1] https://en.wikipedia.org/wiki/Laramidia

Many dinosaurs were warm-blooded; fossils of dinosaurs have been found in Antarctica (granted, that’s from a time when the global climate was warmer and Antarctica was ice-free, but still...).

The elephant might be a herbivore but it is prey only for humans.

Yes, but the point is we should expect elephant population to be larger than that of an apex predator.

Their method of estimation is not far from guessing and they even say themselves, that accuracy is "low".

Looking at the paper, it looks like the authors didn't correct for the fact that CO2 levels during the late Cretaceous period were around 1000pm, so at the time there would have been significantly more available calories per square mile. If each individual really did require 40 square miles, it's hard to understand how they would successfully reproduce.

40 Square Miles is only 6.3 miles to a side. That's not that far to travel for mating.

There are plenty of animals with less density, that reproduce successfully. T-Rex was probably a solitary animal, and perhaps akin to a modern Tiger in its matting patterns.

The spatial distribution and population density of tigers in mountainous terrain of Bhutan "Based on our top model, the estimated tiger population size (N) was 91 individual tigers with 60 adult females and a density of 0.24 tigers/100 km2 "

Another interesting comparison between T Rex and tigers: the diameter and crushing force of a T Rex's jaws are pretty closely matched by the killing triangle of a tiger's front paws and fangs. Citation: John Vaillant's amazing book "The Tiger: a True Story of Vengeance and Survival". (Highly recommended)

Only 2.5B over 2 and a half million years that seems astonishingly low. I wonder if we have any idea about their reproductive lifecycle and lifespan.

There are about 1 billion cows on the planet today mostly because we breed them and there are nearly half a million elephants living currently while being hunted by pesky humans.

T. rex were apex predators, which tend to have fairly low populations compared to herbivores. They're also megafauna, which tend to have fairly low populations compared to smaller animals. So comparisons with cattle aren't really adequate. While I'm sure the error in the population estimate is greater than the estimated value, it does seems to make sense that their population would be quite small.

On the other hand, T. rex probably had a significantly larger range than we've found evidence of.

Someone already mentioned this but there are far more killer whales right now and dolphins both who are apex predators and we don’t make it easy for them to survive.

There are 25-20,000 lions left after we’ve hunted them to nearly extinction.

That doesn't seem like a crazy-high number. If there used to be 100k tigers at any one time, mostly before the problem of too-damn-many-humans, 20k T-Rexes doesn't seem like a stretch. I wonder how broad their range was.

I'm dubious. What do we know about the actual lifespan of the T. Rex? At what age did they reproduce, and how often? What was the survival rate? Mating practices? Territorial behaviors?

bones can tell a lot...

same way you can tell the age of a tree when it was cut, but just looking a the cut trunk.

The age estimate is one of the more solid variables. The wildly uncertain variable is population density (which is why the error bars in the paper are orders of magnitude).

Also, remember that T-Rex only lived in the western US and it was a large island. 20,000 individuals is not unreasonable for a predator population.

There were other Tyrannosaur species in other places and times.

Wouldnt they have evolved in that amount of time? Wouldnt that be visible in fossil records? What did they evolve into?

My son will be pleased.

Data point: 100B humans have inhabited the planet.

There was a calculation on what if Xkcd that every water droplet at one point in history was inside a dinosaur

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