How many Mesozoic dinosaur species were there?

How many dinosaur species where there?

More precisely, how many dinosaur species from the Mesozoic Era were there? This specifier excludes the Cenozoic forms (which are all birds), as they generally aren’t within the scope of the usual intent of the question.

Of course, the actual number is unknown. But we could make a guess, of the order of magnitude at least.

The number of currently valid Mesozoic dinosaur species known number somewhere between 700-1000. But that’s the amount currently known – due to the incompleteness of the fossil record, especially that from the Mesozoic, the amount that actually lived during their heyday era is clearly higher. But how much higher? There are actual published attempts to answer this, and the most recent one was in 2016 by Jostein Starrfelt and Lee Hsiang Liao. Using a statistic analysis called “True Richness estimated using a Poisson Sampling Model”, TRiPS for short, they estimated that 1543-2468 species lived during the timespan of the Mesozoic.

However, simply by looking at modern dinosaur diversity today, that number appears to be a severe underestimate. As Steve Brusatte puts it:

I would take these numbers with an ocean full of salt”, he said. “There are over 10,000 species of birds – living dinosaurs – around today. So saying there were only a few thousand dinosaur species that lived during 150+ million years of the Mesozoic doesn’t pass the sniff test. That’s not the fault of the authors. They’ve employed advanced statistical methods that take the data as far as it can go. The problem is the data. The fossil record is horrifically biased. Only a tiny fraction of all living things will ever be preserved as fossils. So what we find is a very biased sample of all dinosaurs that ever lived, and no amount of statistical finagling can get around that simple unfortunate truth.

Brusatte hits the nail head on. A direct extrapolation from the very incomplete record is bound to produce problematic results. Starrfelt, the leading author of the paper itself, acknowledges the issues, and from the same article:

Our estimate of total dinosaur richness of approximately 2000 species was done attempting to combine the sampling probabilities from all stages of the Mesozoic and should be interpreted with caution, and my gut feeling is that the total number of dinosaur species for the whole Mesozoic is higher than our total estimate suggests.

A simple check of numbers support Starrfelt’s gut feeling and Brusatte’s heavy scepticism of the estimates in question. Consider this: based on morphological analysis, an estimated ~18,000 species of birds live today, and molecular analysis yields an even higher estimate of ~22,000. 18,000 or more living species of dinosaurs, just in our comparatively tiny slice of time, and after several recent species such as moa, elephant birds, and passenger pigeons have already gone extinct. And many live within currently stressed ecosystems of today, while we are fresh out of a mass extinction at the end of the Pleistocene, and with birds being at the evolutionary sidelines with mammals dominating many of the key niches. And based on the Starrfeld & Liao 2016 estimate, only 1543-2468 – less than 15% of the living bird diversity – Mesozoic dinosaur species in total lived during the ~180 million years from their debut to the KT, all while dominating over terrestrial ecosystems rather than being on evolutionary sidelines, and sustained by myriads of healthy ecosystems that must have existed in that period? It is immediately clear that this number just isn’t a plausible estimate in practice.

A rough approximation of the magnitude of the actual number of Mesozoic dinosaur species that has ever lived can be performed given the following variables:

• The number of species at any given time.
• The average time in which species are replaced.
• The total time covered by the scope of the estimation.

Then it becomes a simple equation of:

Total number of species = species per given time (total time/replacement time)

Say that dinosaurs first arose some 245 million years ago and the KT took place 66 million years ago, we have 179 million years, rounded to 180 million for simplicity. Thus, we already have the “total time” figure.

The number of dinosaur species at any given time during the Mesozoic is a number for which we unfortunately do not have any data on. Let’s assume it was comparable to modern dinosaur diversity, say, a round 20,000 for the sake of calculations.

Then we need the average replacement rate. The estimated average lifetime of an animal species is around 1-10 million years depending on taxonomic group. In particular, species from endothermic groups, such as mammals and birds, have average lifetimes of around ~1-2 million years, and the same likely applied to nonavian dinosaurs as well. For our calculations, we use a round 2,000,000.

Now, bringing in the inputs:

Total number of species = 20,000 (180,000,000 years/2,000,000 years)

We end up with an estimate of 1.8 million total dinosaur species from the Mesozoic. This number is very rough and could be easily off by a good amount, mainly due to the fact that we needed to make a sweeping assumption about the number of species for every given time, which is probably an underestimate as Mesozoic dinosaur niche diversity would have surpassed that of modern forms. Nevertheless, it gives a good idea of the magnitude involved – the probable total number of dinosaur species across the expanse of Mesozoic time likely lies not in the thousands, but in the millions.

We can also use this simple formula to test the numbers estimated by Starrfelt & Liam (2016): based on their estimates, and given the total time and average species lifetime we used here, the average number of dinosaur species that lived at any one time during the Mesozoic would be around 27 or less. Clearly, this is not a realistic number. And it gets even worse if we consider the ideas of Jack Horner and co. who push the idea that we are supposedly overestimating Mesozoic dinosaur diversity – if we assumed that the known species we have are all that ever existed – we would have only 10 or less dinosaur species living at any given time, smeared out across the world! Needless to say, that idea is very unlikely.

To sum it up, the total amount of dinosaur species that existed during the Mesozoic, while it can’t be known with certainty, can be shown by simple mathematics to most likely lie in the order of millions (10⁶). The portion currently known so far can thus be safely expressed as very likely less than 0.1%.

References

• Starrfelt & Liao, 2016, “How many dinosaur species were there?”
• Barrowclough et al., 2016, “How many kinds of birds are there and why does it matter?”