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, so groups of species were drawn 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 the original concept of dinosaurs included all reptiles of large size bearing skeletal features and terrestrial habits. The latter condition means that mosasaurs, plesiosaurs, mosasaurs and other such aquatic reptiles are not and never were considered dinosaurs, despite their association in popular culture. Indeed, Owen excluded the long-necked sauropod Cetiosaurus from his Dinosauria because he (erroneously) thought it was aquatic1, though sauropods were eventually found to be terrestrial when better fossils were discovered later on and were henceforth included in 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. No longer were dinosaurs just a bin of giant terrestrial reptiles; they were now an exclusive evolutionary lineage descending from a single common ancestor, including the forerunners of today's birds. Scientists like Huxley thought they were vigorous, warm-blooded animals3 who ruled the ancient world with their awesome size and might.
But these ideas were not long for this world. In 1926, artist 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 convergent evolution4. 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 crocodilians and birds5. This essentially rendered Dinosauria the way Huxley saw it scientifically meaningless, as its constituents were no longer thought to be the descendants of a singe ancestor. Until the mid-70s some scientists said believed that there was no such thing as a 'dinosaur'6.
A swamp-dwelling brontosaur: the quintessential slow, sluggish 1900s ‘dinosaur’.
Charles Robert Knight (Public Domain)
With 'dinosaur' rendered a colloquialism and not a scientifically meaningful term, 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 meat-eating theropods and plant-eating ornithischians 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 things like pterosaurs, 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 carnivorous dinosaurs 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, new trackways showed that sauropods were terrestrial13 and in fact would suffocate if they tried to use their long necks as snorkels14. For three, the old kangaroo-postures would have broken dinosaur tails15, 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 metabolism16. And for five, new fossils revealed that all dinosaurs really were related to each other after all. Huxley's Dinosauria was revived and officially defined: the ancestor of Megalosaurus, Iguanodon and Cetiosaurus*, and all of its descendants17.
By the 1990s, a flood of feathered fossils poured out of China and made it utterly incontrovertible: birds are the descendants of dinosaurs18. A lot of people accept that birds are dinosaurs today, but others might say that while birds are descended from dinosaurs, they don't have the features we associate with dinosaurs and so they shouldn't be considered 'true' dinosaurs. And to those I ask, what features? Scales? Many meat-eating dinosaurs certainly possessed feathers and even full-fledged wings19, and there's even some evidence that the ancestor of all dinosaurus had them as well20. Large size? Compsognathus wasn't large at all, yet no one says it's not a dinosaur. Teeth? Many dinosaurs had toothless beaks, while uncontroversial birds like Ichthyornis still had teeth. A long tail? Caudipteryx had a short tail and yet it's further from birds than Velociraptor is21. Prehistoric age? Fully modern birds like Asteriornis22 were around at the same time as T. rex. What use is there really in a group that contains Diplodocus, Stegosaurus and Deinonychus but not Archaeopteryx and chickens?
So, what are dinosaurs? Scientifically, 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
1 📰Owen, R. (1842). Report on British fossil reptiles. Part II. In Report of the Eleventh Meeting of the British Association for the Advancement of Science; Held at Plymouth in July 1841 (pp. 60–204). John Murray.
2 📖Secord, J. A. (2000). Victorian Sensation: The Extraordinary Publication, Reception, and Secret Authorship of Vestiges of the Natural History of Creation. University of Chicago Press.
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.
4 📖Heilmann, G. (1926). The Origin of Birds. H. F. & G. Witherby.
5 Bakker, R. T., & Galton, P. M. (1974). Dinosaur monophyly and a new class of vertebrates. Nature, 248, 168-172. https://doi.org/10.1038/248168a0
6 🔒Thulborn, R. A. (1975). Dinosaur polyphyly and the classification of archosaurs and birds. Australian Journal of Zoology, 23(2), 249-270. https://doi.org/10.1071/ZO9750249
7 📖Romer, A. S. (1966). Vertebrate Paleontology (3rd ed.). University of Chicago Press.
8 📖Desmond, A. J. (1976). The Hot-Blooded Dinosaurs: A Revolution in Palaeontology. Dial Press.
9 Ostrom, J. H. (1969). Osteology of Deinonychus antirrhopus, an unusual theropod from the lower Cretaceous of Montana. Peabody Museum of Natural History Bulletin, 30, 1-165.
10 Camp, C. L. (1936). A new type of small theropod dinosaur from the Navajo Sandstone of Arizona. Bulletin of the University of California Department of Geological Sciences, 24, 39-65.
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