What is a Dinosaur, Really?
What do you think of when you hear the word 'dinosaur'? I imagine most of us would think of dusty bones in museum collections or scaly monsters chasing down action movie protagonists. Perhaps some will think of gasoline, of palm trees, and of a past so long ago we can hardly imagine it. But what, really, is a dinosaur?
Out of the eight animals displayed in the gallery above, only one is a dinosaur: the ruby-throated hummingbird Archilochus colubris, as many of you may be aware. But why are birds dinosaurs, while all the other animals in the gallery are not? To learn why, we'll have to do some history.
The order Dinosauria was named in 1842 by palaeontologist Richard Owen to encompass three animals: Megalosaurus, Iguanodon and Hylaeosaurus1. When Dinosauria was coined, the theory of evolution by natural selection had not yet been proposed and so most Western naturalists believed that all species had been individually created by God. They grouped based on physical similarities and not evolutionary relatedness2. The features that Owen chose to define Dinosauria include a whole lot of minutiae regarding the ribs, spine and pelvis, as well as "extremities of large proportional size... [which] more or less resemble those of the heavy pachydermal Mammals, and attest, with the hollow long-bones, the terrestrial habit of the species1."
So 'dinosaurs' were originally a set of reptiles of large size bearing certain skeletal features and terrestrial habits. Thus , , and other such aquatic reptiles are not and were never intended to be considered dinosaurs, despite contrary assertions in popular culture. Indeed, Owen excluded the long-necked Cetiosaurus from his Dinosauria because he thought it was aquatic1, though better fossils eventually revealed sauropods were terrestrial and did have the characteristics of Owen's Dinosauria.
Archaeopteryx lithographica: A proto-bird, a reptile, and a dinosaur.
H. Raab (CC BY-SA 3.0)
The first big revolution in how we understand Dinosauria came in 1859, when Charles Darwin published On the Origin of Species and brought evolution by natural selection into the scientific consciousness. Shortly after the book was published, the proto-bird Archaeopteryx lithographica was dug out of the limestone quarries of Germany and dinosaurs were pulled into the evolution debate. Thomas Huxeley, a biologist and one of Darwin's allies, noticed that Archaeopteryx was similar to the dinosaur Compsognathus longipes from the same fossil deposits. In fact, they were nearly identical save for the feathers preserved with Archaeopteryx's skeleton. From this similarity, Huxley concluded that Archaeopteryx and Compsognathus must have been closely related, and that birds had in fact evolved from members of Owen's Dinosauria3.
At the same time, a flood of fossil discoveries from North America, including such household names as Brontosaurus, Stegosaurus, Allosaurus, Brachiosaurus and Diplodocus filled out our picture of dinosaur diversity. So dinosaurs were no longer just a bin of giant terrestrial reptiles; they were now a special evolutionary lineage of vigorous, warm-blooded animals3 who encompassed the forerunners of birds. With Dinosauria defined as a lineage, things like and crocodiles which weren't closely related to Megalosaurus and such would have been excluded from Dinosauria regardless of superficial similarities.
But these ideas were not long for this world. In 1926, palaeontologist Gerhard Heilmann wrote The Origin of Birds, in which he argued that birds could not be dinosaurs and that their similarities were the result of 4. It was impossible for dinosaurs to be bird ancestors, Heilmann said, as dinosaurs lacked collarbones and birds did not. It would be exceptionally unlikely for dinosaurs to re-evolve collarbones after they had lost them.
In the following years, interest in dinosaurs was minimal. A consensus emerged that the various groups of dinosaur were actually not related to each other and independently descended from an asssortment of primitive 'thecodont' reptiles who also independently gave rise to crocodiles and birds5. This rendered Dinosauria the way Huxley saw it scientifically meaningless, as its constituents were no longer thought to be the descendants of a singe ancestor. Under this paradigm, which lasted until the 1970s6, there were no animals that could be truthfully called 'dinosaurs'.
A swamp-dwelling brontosaur: the quintessential slow, sluggish 1900s ‘dinosaur’.
Charles Robert Knight (Public Domain)
With 'dinosaur' rendered a colloquialism with no scientific use, it started to drift from its original meaning. Scientists in the 1930s thought of dinosaurs as very stereotypical reptiles, slow and sluggish and associated with the tropics5. The giant sauropods were placed in rivers to support their weight7, while the bipedal and were put in awkward kangaroo-poses8 to let them to stand upright with a sluggish reptilian metabolism. The association of sauropods with water brought in the marine reptiles, while the 'giant, scaly and sluggish' idea brought in , crocodiles and Dimetrodon, which were thought of the same way.
Of course, the palaeontologists of the 1900s were quite wrong about dinosaurs and the scientific community would eventually get around to correcting their errors. In 1969, palaeontologist John Ostrom discovered Deinonychus antirrhopus, a small dromaeosaur ('raptor') with huge claws, long legs and a stiff tail. Deinonychus had extremely birdlike anatomy—even more birdlike than Compsognathus. This was clearly not a sluggish, tail-dragging reptile9, but it was indisputably related to theropods like T. rex. Palaeontologists had to reconsider everything.
John Ostrom, his student Robert Bakker and other palaeontologists reexamined old museum vaults and dug new fossils out of the ground and found that much of what we thought we knew about dinosaurs was wrong. For one, they did indeed have collarbones—the old fossils just didn't preserve them well and no one thought to look after Heilmann's book10. For two, biomechanical studies revealed it was impossible for sauropods to dive11 and in fact they would suffocate if they tried to use their long necks as snorkels12. For three, the old kangaroo-postures would have broken dinosaur tails13, as long tendons helped hold them straight off the ground. For four, many dinosaurs showed growth rates and activity levels that could only be explained by a vigorous, warm-blooded metabolism8. And for five, new fossils revealed that all dinosaurs really were related to each other after all.
Mounted skeleton of Deinonychus antirrhopus. This would have been an exceptionally bird-like animal in life, yet no one would dare say it is not a dinosaur.
Jonathan Chen (CC BY-SA 4.0)
And this is when Huxley's Dinosauria was revived and finally given a definition, which stands to this day. In this system, Dinosauria is the group of animals that contains the last common ancestor of Megalosaurus, Iguanodon and Cetiosaurus, its descendants, their descendants, and all of their descendants forevermore14. And herein lies the key to why Dimetrodon, crocodiles and company are not dinosaurs. They did not descend from the last common ancestor of Megalosaurus, Iguanodon and Cetiosaurus, having spun off on their own evolutionary journeys millions of years before that ancestor first appeared. Crocodiles and pterosaurs are indeed close cousins of the dinosaurs15, but mosasaurs are true lizards16, plesiosaurs might be relatives of turtles17, ichthyosaurs are their own thing18 and Dimetrodon is actually a mammal ancestor19 all the way on our side of the vertebrate family tree. But the descendants of the last common ancestor of Megalosaurus, Iguanodon and Cetiosaurus include one more group not traditionally thought of as dinosaurs: the 10,000+ species of modern birds.
In the 1990s, a flood of feathered fossils poured out of China and made it utterly incontrovertible: birds are the descendants of theropod dinosaurs similar and related to Deinonychus20. And since birds are descended from dinosaurs, they must therefore be dinosaurs as well.
Some people might object to to the inclusion of birds in Dinosauria on the grounds that they so different from their ancestors that they must be something new and separate. And to those people I ask, what differences? Scales versus feathers? The famous Velociraptor had full-fledged wings21, yet it is absurd to disqualify it from the ranks of Dinosauria. Large size versus small size? Compsognathus wasn't large at all, yet no one says it's not a dinosaur. Teeth versus beaks? Oviraptor had a toothless beak, while many Mesozoic birds still had teeth. Long tails versus short tails? Caudipteryx had a short, feather-fanned tail and yet it's a flightless Oviraptor-relative. Prehistoric age versus modern age? Modern-looking birds like Asteriornis were around at the same time as T. rex22. We know now that the animals we know as dinosaurs occupied a spectrum of forms from completely un-bird to completely bird. So what use is really there for a group that contains Diplodocus, Stegosaurus and Velociraptor but excludes Archaeopteryx and chickens?
So, what are dinosaurs? The way I would put it, dinosaurs are a single lineage of animal which ruled the lands of Earth for more than half the time land animals were even a thing. In their tenure they conquered every environment from rainforests to deserts to polar ice caps and produced some of the biggest titans, most fearsome hunters and most graceful fliers known to man. If Dimetrodon fits the historical picture of a dinosaur—scaly, sluggish and sprawling—more than any actual dinosaur, that just means that our vision of dinosaurs must evolve into the future, into forms foreign yet just as majestic—just like dinosaurs themselves.
References
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3 Huxley, T. H. (1868). On the Animals which are most nearly intermediate between Birds and Reptiles. The Annals and Magazine of Natural History, 4(2), 66-75.
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17 Lee, M. S. Y. (2013). Turtle origins: insights from phylogenetic retrofitting and molecular scaffolds. Journal of Evolutionary Biology, 26(12), 2729–2738. https://doi.org/10.1111/jeb.12268
18 🔒Motani, R., Minoura, N., & Ando, T. (1998). Ichthyosaurian relationships illuminated by new primitive skeletons from Japan. Nature, 393, 255–257. https://doi.org/10.1038/30473
19 Angielczyk, K. D. (2009). Dimetrodon is not a dinosaur: Using tree thinking to understand the ancient relatives of mammals and their evolution. Evolution: Education and Outreach, 2, 257–271. https://doi.org/10.1007/s12052-009-0117-4
20 🔒Ostrom, J. H. (1973). The origin of birds. Nature, 246, 136. https://doi.org/10.1038/242136a0
21 Turner, A. H., Makovicky, P. J., & Norell, M. A. (2007). Feather quill knobs in the dinosaur Velociraptor. Science, 317(5845), 1721. https://doi.org/10.1126/science.1145076
22 🔒Field, D. J., Benito, J., Chen, A., Jagt, J. W. M., & Ksepka, D. T. (2020). Late Cretaceous neornithine from Europe illuminates the origins of crown birds. Nature, 579, 397–401. https://doi.org/10.1038/s41586-020-2096-0
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