I hear this question quite a lot, but the answer to 'how many dinosaurs were there?' kind of depends on what you mean. Is it how many species were there, how many genera there were how many individuals existed in total, how many existed in a single ecosystem etc.
Well, why not answer all of those questions? Let's take a look at how many dinosaurs existed.
How many species/genera were there?
In terms of non avian dinosaurs, around 900 genera (the dinosaur's first name) have been found with around 1000 species being named. If we want to count all extinct dinosaurs, including the avian ones, we can bump that number up, but let's stick with the non-avian ones for now.
Having a little trouble figuring out what the heck I mean by genera and species? I got you:
How many of each species were there?
Now, this is a tough one. This is an extraordinarily difficult question to answer with living species, let alone extinct ones. A few considerations need to be taken into account when musing how many members of a species there were.
Firstly, at what point in time? Just like today, a species would have hit a peak at some point before declining into extinction. This would have happened with most species of dinosaur long before the end-Cretaceous mass extinction event.
It also depends largely on the species economic niche, or what kind of role they filled in their environment. Generally speaking, species that have smaller bodies can normally maintain higher numbers, since they are using up less resources. It can also depend on whether they are a predator or herbivore. Within a given ecosystem, there must be more prey organisms than predators, or it is simply not sustainable and those greedy predators will eat themselves out of existence. The ratio for an ecosystem will, again, vary depending on the animals in question, but a rough estimate for a terrestrial ecosystem is usually between 1-4 predators for every 100 prey animals.
The short answer? Anywhere between 2 and 10,000,000 at a given moment in time. Told you it wasn't an easy one.
How many could exist in a single ecosystem?
The answer to this one is much the same as the previous question: it depends.
The number of dinosaurs that could exist in a single ecosystem is likely to be somewhat similar to the ecosystems of mammals (that is, without human influence). In any given terrestrial ecosystem, this is again very difficult to figure out, even when it is extant (still living). Still, you can probably expect to find around 20-40 species of mammal in a single biome, so you could probably expect to find similar numbers of dinosaur species in a Mesozoic biome.
How many in total across all of time?
Don't be silly...oh you're serious?...ok...you have been reading this, right?
Well, let's look back to the group that currently dominate the Earth: mammals. It is estimated that there are currently 130 billion mammals on Earth today. Now, that does include humans, which is a species with a biomass that is simply unheard of for a terrestrial faunal species of our body size, so you could probably go ahead and minus off most of the human population for a more natural number, so let's say around 123.5 billion.
We could probably go ahead and add a few again, come to think of it, since those humans wouldn't have reduced the numbers of the others in the first place. Let's round it up to a nice 126 billion.
If dinosaurs were much the same (though there is absolutely nothing to say they definitely were), we would then need to look at the population growth over time, which is impossible. But, if the average dinosaur's lifespan was around 30 years (though the range of that average is huge given how varied each group's potential lifespan was) and the population was steady until another species came along and took over those numbers, we could probably keep that 126 billion as a 30 year average.
Non-avian dinosaurs existed for around 164 million years (though that number is probably a little bigger, since the earliest dinosaur we know doesn't necessarily mean the first), so if we add 126 billion on top of itself every 30 years for 164 million years, here is my estimate of how many non-avian dinosaurs existed across the entire Mesozoic era:
6.888 x 10¹⁵
In human speak, that's 6,888,000,000,000,00 or 688.8 trillion. Silly number, but you asked for it....hang on, why the hell are these things so difficult to find?!
However, as I'm sure you read, that number has come after god knows how many layers of rough estimations, presumptions and flat out 'we don't know's, so who knows how accurate that number is...or how useful come to think of it. But, hey, it's a fun (albeit oversimplified) thought experiment.
Now, that was a lot of maths for one day...I'm off for a lie down.
Until next time!