Dr. Tom Holtz is a dinosaur expert, specializing in the phylogeny and functional morphology of theropod dinosaurs, especially Tyrannosauridae. His current research focuses on the differences between mammal and dinosaur ecosystem structures due to their different styles of reproduction and parental care and on describing new dinosaur species from Maryland and Argentina.
Q: Happy Thanksgiving, Dr. Holtz! When did the idea of an evolutionary relationship between birds and dinosaurs begin?
A: “The hypothesis of the dinosaurian origin of birds goes back to the late 19th Century, as paleontologists such as Thomas Huxley and O.C. Marsh observed specific similarities in the skeletons (especially in the hips and hindlimbs) of birds and dinosaurs. This hypothesis was mainstream paleontology for a while, but it was deflected due to some misinterpretations by early 20th-century researchers.
The idea was revived during the early 1970s by John Ostrom and colleagues, largely due to the discovery of small-bodied “raptor” dinosaurs such as Deinonychus and Velociraptor, with their numerous detailed anatomical similarities throughout their skeletons with early birds such as Archaeopteryx.
In the 1990s, the first fuzzy and feathered dinosaur specimens were discovered, and now we have a rather complete series of transitions between small-bodied fuzzy carnivorous dinosaurs to fully feathered forms to early flying birds.”
Q: Have recently discovered fossils supported this view?
A: “In fact, it is important to point out that no fossils (recently found or otherwise) contradict this view!
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We do not have true feathers present in any animals other than theropod dinosaurs (including birds).
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We do not have furculae (wishbones) in any animals other than theropod dinosaurs (including birds): in fact, we have wishbones in giant predators like Tyrannosaurus and Allosaurus.
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We do not have the half-moon-shaped wrist bone (semilunate carpal block) in any animals other than theropod dinosaurs (including birds).
Anatomical feature by anatomical feature, we find that birds are deeply nested in the theropod dinosaur family tree.”
Q: At one time, birds were classified in the Class Aves and dinosaurs in the Class Dinosauria. Do we still use this classification?
A: “The main change is the abandonment of the use of ranks like ‘Class’ and ‘Order’ in biological classification. These were metaphysical concepts from pre-evolutionary modes of thought. How do we tell if a group is a ‘Class’ rather than an ‘Order?’ It was entirely arbitrary!
Modern biological classification puts evolution front and center. All living things are part of an ever-branching family tree of life, where a lineage of organisms can branch into smaller subdivisions and so on. All groups are part of larger groups, but they don’t stop being part of that larger limb.
So, Aves is a clade (branch) within the larger clade Dinosauria, just as Chiroptera (bats) are part of Mammalia. In other words, birds are a kind of dinosaur, not a separate group. Birds are not descended from dinosaurs; they ARE dinosaurs, just as much as Tyrannosaurus, Triceratops, and Stegosaurus are.”
Q: We’ve known that birds have existed since the Jurassic with the discovery of Archaeopteryx in the Solnhofen Limestone. Is Archeopteryx still considered the first bird?
A: “‘Bird’ as a vernacular word is somewhat imprecise. At what point we consider a particular type of dinosaur as being a ‘bird’ is fairly arbitrary. Is it the first feathered ones? The first flying ones? (And at what point is aerial locomotion truly ‘flying’ rather than semi-powered gliding?) The first toothless flying ones? Each of these would point to a more or less inclusive group, and a different first member.
Later Late Jurassic Archaeopteryx is one of the oldest flying members of the bird lineage, and most studies put it as more closely related to (sharing a more recent common ancestor with) modern birds than to Deinonychus and Velociraptor. But early Late Jurassic Anchiornis was fully feathered and is also closer to modern birds than to Deinonychus, although probably not as good a flier as Archaeopteryx. Was Anchiornis the first bird? Or do we reserve that English word for powered fliers?”
Q: Birds seem like fairly fragile creatures, yet some must have survived the K/Pg extinction event. What type(s) of bird survived and why?
A: “Many types of birds, including the enantiornithines (the most common birds of the Cretaceous), perished in the great extinction. Only neornithines (the modern bird group, which are distinguished by many traits, such as an entirely toothless beak in which the upper jaw is composed almost solely of the bone that makes up just the front tip of other dinosaurs) survived.
There are various speculations as to why Neornithes survived and other bird groups didn’t:
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Many Cretaceous neornithines seem to be ecologically tied to the near-shore environment, feeding on shoreline invertebrates, small fish, and so forth. Sediment-dwelling invertebrates were much less hard hit by the extinction than were swimming sea life, so perhaps this meant the neornithine food supply was less affected.
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Others suggest that the toothless beaks of mainland neornithines allowed them to feed on worms, seeds, and other food stored in the soil in ways their toothed kin (which required more “fresh” food like small vertebrates or fruit) could not.
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Or maybe enantiornithines were more tree-based nesters, and the neornithines were ground-nesters: With the global forest collapse seen as a consequence of the asteroid impact, it might be that the neornithine’s “nurseries” were less affected.
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Or it might be a combination of these, or sheer luck.
Compared to their non-bird kin among the dinosaurs, though, birds of all groups were smaller and thus needed less food to survive the hard times of thermal pulses and impact winter than their larger cousins did.”
Q: Are fossils of feathered theropods on display anywhere for the public to view?
A: “Yes. The best displays are in China, where the majority of these fossils are found. But many museums now have displays of either actual feathered theropods or casts of the same. For instance, the Smithsonian National Museum of Natural History has casts of early fuzzy and feathered non-bird dinosaurs as well as early birds in their Deep Time Hall.”
Q: So, gazing at our Thanksgiving turkey, smothered in gravy and garnished with cranberry sauce … are we eating a dinosaur?
The bottom line: “Absolutely! To paleontologists, a dinosaur is any descendant of the most recent common ancestor of Iguanodon, Megalosaurus, and Diplodocus. As every species of modern and extinct bird is one of those descendants, they are all dinosaurs! The Thanksgiving turkey, the Christmas goose, and all the other birds of the world are indeed dinosaurs.”
Bon Appetit! 😋🦃
