On the Number of Dinosaur Species
by Neal L. Larson
Last week I was received an email from the Jurassic Park Institute with a listing of all of the known dinosaur species to date. I was dumbfounded. As I went through this enormous listing of nearly eight-hundred names, I wondered where I had been hiding while so many new dinosaur names had been described. The largest problem with the publishing of new species descriptions, is that there are hundreds of appropriate journals, periodicals and papers throughout the world where they may be described and named. Unless an organization, such as the Jurassic Park Institute, is available to find and record these names in one source, it could take years to realistically search for, and find, all of the current descriptions. Fortunately, there are relatively few dinosaur researchers in the world, and news of their discoveries usually circulates rapidly.
As I was looking at the list, I wondered how many more new species are out there to discover and describe. With nearly eight hundred species known, how many more could there possibly be? Surely with the number of dinosaur hunters and the number of scientists working on them it shouldn't take too long to find and describe all of the species.
Many people often confuse species with genera. This confusion stems perhaps from confusion about the terminology used in naming newly discovered animals, or a lack of adequate education about the process. Paleontologists, who are supposed to be educating the public, often refer to the genera as species, thus adding to the confusion. Species are the last category on the list of animal classification. For instance, rex is a species, from the genus Tyrannosaurus, from the family Tyrannosauridae, and the suborder Theropoda.
Peter Dodson tried to calculate the number of dinosaur genera (genera is the plural of genus) in 1990, and again in 1997. He came up with some interesting statistics. He estimated that a dinosaur genus had an estimated life cycle of about 7.7 million years, or about the same time as a geological stage (i.e., the Maastrichtian Stage of the Late Cretaceous lasted from 71 MYA till 65 MYA or about 6 million years). This suggested that the fauna in every geological stage would most likely be completely different from the stage before it. In 1990, he estimated that there were probably no more than 100 dinosaur genera living at one time. Based on these calculations, he figured that probably no more than 1200 genera ever existed. Considering that there are 27 geological stages from the Late Triassic Carnian, till the Late Cretaceous Maastrichtian, and 160 million years in the process, his estimate could be low.
As we contemplate the number of dinosaur species that could have existed, we have to look not only at the past, but at the present for clues. We have to figure how many species may have existed at any one time on the Earth by using the number and kind of extant species on all continents and islands. We also have to then calculate how long a species would probably last, and how many years that there were dinosaurs on the planet. When we calculate all of these factors together, we should be able to come up with some logical figure for this unknown number.
The last part is easy. We have a record of dinosaurs on this planet from 227 MYA until 65 MYA or about 160 million years give or take some. This takes in dinosaurs from Eoraptor (Carnian Stage of the Late Triassic) to Tyrannosaurus (Maastrichtian Stage of the Late Cretaceous). Most large animal species today last for about one million years, with some subtle changes due to environment and evolution. The number of species that existed at a given time is another problem. However, it may have taken a lot of time for dinosaurs to diversify, perhaps ten million years or so, so I will use 150 million years as a round number for the time of dinosaur diversity.
There exist today about 8,600 species of birds. Most paleontologists are convinced of a close dinosaur/bird relationship. However, most birds are small, and most of the dinosaurs that we have found are large, or at least they were greater than ten pounds in live weight. That doesn't mean that small dinosaurs didn't exist, just that we don’t find the remains of them very often, and when we do it is very minimal. Did 8,600 species of dinosaurs exist at any time during the Mesozoic? Maybe, but from the information in the fossil record, it doesn't appear so. If there were that many species of dinosaurs that existed at any time, and we multiply that number (the known species of birds) times the number of years that dinosaurs lived on the earth (about 150 million) divided by one million (for the approximate time span of a species) there could have been more than 1.2 million species of dinosaurs. But, from our knowledge gained to date from the collecting and the study of dinosaur species, that doesn't seem probable.
Drawing from the acknowledgment of the close bird/dinosaur relationship, I chose to look at only the larger (5 pounds and greater) bird species that exist today to draw some conclusions. In calculating that figure, I came up with 771 species of larger birds. These include the Ratites (6 species), Penguins (18 species), Albatrosses (13 species), Pelicans (60 species), Frigates (39 species), Herons (60 species), Cranes (37 species), Turkeys (2 species), Flamingos and Shoebills (49 species), Ducks (140 species), Loons (19 species), Grouse (18 species), and Birds of Prey (300 species). Assuming that perhaps the same number of dinosaur species could have existed at any moment in geological time, that would translate out to 771 species times 150 or about 115,650 possible species of dinosaurs. We could refine this number by removing the Ducks, Loons, and Grouse and we are left with about 600 species of birds, or about 90,000 probable dinosaurs species that may have existed through time, using the previous analogy.
How about reptiles? There are about 6,250 species of reptiles known to exist on this planet. Again, most of them are quite small, so are they a good number to go by? Probably not. However, there are a number of paleontologists that insist that dinosaurs are descended from reptiles, and that dinosaurs were cold-blooded reptiles. First, to come up with some logical comparisons of dinosaur species numbers using reptiles, we should eliminate turtles, crocodiles, and snakes from our calculations. I also did not include the Geckos (675 species) because they bear no resemblance to dinosaurs. I instead chose the reptiles that look like dinosaurs. These include the Iguanas (600+ living species), the Agamids (300+ species), and the Chameleons (85 species). If you do the same calculations as we did before, then there could then have existed at any given time perhaps as many as 985 to 1000 species of dinosaurs. Take that number by 150 and you come up with about 150,000 species of dinosaurs that could possibly have existed through time.
How about species of larger land mammals that live on the planet. If mammals are supposed to be the animals that replaced the dinosaurs in the relative order of the food chain, than this number might be close to correct. Remember, we are in the midst of an extinction event, and we are losing species every day.
For the record, there are over 4000 known species of mammals. For use of calculating the possible number of mammals that may have taken the place of the dinosaurs, I eliminated all marine mammals, primates, and small mammals like rodents, weasel, mustelids, otters, bats, etc. Instead, I chose to count Kangaroo (57 species), Sloths (5 species), Ant Eaters (4 species), Armadillos (20 species), Dogs (35 species), Bears (7 species), Raccoons and Pandas (18 species), Hyenas (3 species), Cats (35 species), Elephants
(2 species), Horses (8 species), Rhinos (5 species), Tapirs (4 species), Pigs (8 species), Peccaries (3 species), Hippopotamuses (2 species), Camels (4 species), Deer (41 species), Giraffes (2 species), Pronghorn
(1 species) and the Bovids (123 species). That makes 397 species of large land mammals, excluding primates. For simplicity sake, I shall add three for human, gorilla, and orangutan, bringing the number up to 400. Doing the same calculations as before, I suppose that we could assume that there may have been as many as 60,000 species of dinosaurs that could have existed through time.
What then are my conclusions? That the approximately 800 described species of dinosaurs is an absurdly low number. We, as paleontologists, geologists, and scientists have a long way to go to understanding the fascinating dinosaurs, or the Earth for that matter. The great finds and strides in dinosaur discoveries over the last two hundred years are but a small fraction of what existed. I believe that the number that actually existed is somewhere between 45,000 (300 species at a given time) and 90,000 species. Unfortunately, our wonderful planet and all of her mysteries will probably never reveal all of these fascinating secrets to us. It is up to all of us to try to discover as many as we can.
Macmillan Illustrated Animal Encyclopedia, 1984. Edited by Dr. Philip Whitfield. Pub. by Macmillan Publishing Company, New York. 600p.Dodson, P., 1990. “Counting Dinosaurs: How many kinds were there? Proc. National Acad. Sci. 87, pp. 7608 - 7612.Dodson, P., 1997. “Distribution and Diversity” in, “Encyclopedia of Dinosaurs” Currie, P. J., and Padian, K., (eds.), Academic Press, New York. pp. 186 - 188.