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.
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!
A small, flat population is even more unlikely - any disruption would wipe out the population more quickly than millions of years down the line.
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.
The graph does not show us that anybody thinks the human population was flat before a certain point.
So ~20k alive at any point in time, assuming a flat distribution during that 2.5B year window.
Humans ended up surviving that disaster, and then went on to populate the whole earth. Also side note, supervolcanoes are scary as hell.
Especially apex predators of that size.
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.
Like, nevermind a comet, a poorly timed burp from a volcano could have sufficiently reduced diaspora enough to make them disappear.
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.