Most Dinosaur Species Are Still Undiscovered

Just about every two weeks, we meet a new dinosaur species. Some come fresh from the desert. Others have been hiding in museum collections for decades, or were misidentified as different species. However they’re found, though, dinosaurs are stomping out onto the public stage at a greater rate than ever before. Just last week, for example, paleontologist Hans-Dieter Sues and colleagues named a new, tiny tyrannosaur that once scampered around prehistoric Uzbekistan.

And if the latest estimate is correct, we’re not even close to hitting Peak Dinosaur yet.

We’ll never know precisely how many non-avian dinosaurs roamed the planet between their origin 235 million years ago and their decimation 66 million years ago. The fossil record is not complete—animals that lived in upland environments scoured by erosion had poor chances of being preserved, for instance—and that’s not accounting for sampling bias dictated by researcher interests and what field specialists can actually remove from the rock.

Even then, most of what we know about dinosaurs comes from their skeletal remains. This lets us tell the difference between Tyrannosaurus and Triceratops, but, like some modern birds and reptiles, some non-avian dinosaur species may have differed only in color, geographic range, or other squishy features that we just don’t have access to. Even if we had the bones of every single dinosaur, we’d still probably underestimate the true number of species.

Still, given these caveats, University of Oslo researchers Jostein Starrfelt and Lee Hsiang Liow have created a new model they call TRiPS to estimate how many dinosaur species were around during the Triassic, Jurassic, and Cretaceous chapters of their history.

Drawing from known dinosaur records in the Paleobiology Database, the researchers extended the known record into estimations of origin and extinction for dinosaur species throughout their history and included a simulation of how likely it’d be for species to enter the fossil record.

In all, Starrfelt and Liow write, the heyday of the dinosaurs saw the comings and goings of about 1,936 different species. About half this count are expected to be theropods—the lineage that includes T. rex and birds—with the rest split between the long-necked sauropodomorphs and ornithischians such as the armored, horned, and duckbilled dinosaurs.

Starrfelt and Liow acknowledge that they’re dealing with estimates and that refinements will likely alter their dinosaur count. But, for a first run, the results came out similar to what’s been proposed before. In 2006 paleontologists Steve Wang and Peter Dodson estimated that around 1,844 genera of dinosaurs lived during the Mesozoic. While the categories are different—a genus can contain multiple species, like Triceratops horridus and Triceratops prorsus—many dinosaurs described so far are what paleontologists call monospecific, or have only one species in a genus. This affects estimates drawn from the known span of dinosaur discoveries, and might be why the species count isn’t even higher.

The last time anyone did a major count, about eight years ago, paleontologists recognized about 648 valid genera and 675 species of Mesozoic dinosaur, including birds. Those numbers have continued to shift. In 2010, eight new dinosaur species were found in Utah alone, and debates over lumping genera or species continue, as tortured Torosaurus knowsas tortured Torosaurus knowsas tortured Torosaurus knows. And if the current estimates of dinosaur diversity are correct, discovery and debate will keep a frantic pace for decades to come. We’ve only just started to find all the dinosaurs, much less understand the lives of these impressive creatures.

Reference:

Starrfelt, J., Liow, L. 2016. How many dinosaur species were there? Fossil bias and true richness estimated using a Poisson sampling model. Philosophical Transactions of the Royal Society B. doi: 10.1098/rstb.2015.0219