Every night, before he goes to bed, my wife and I read a book to my son. Last night, he picked out one of the books he got from the library. I don’t recall the title, but it was full of cartoon dinosaurs accompanied by facts about them. A lot of those facts were things I’ve learned in a lifetime of obsession with the “terrible lizards”, and a good number of them are things he’s learned from Dinosaur Train and from other books we’ve read. But that really isn’t the point of the evening ritual. If he learns something, that’s great. For me, though, it’s the opportunity to read to him.
“What’s a theropod?” my son asks.
“It’s a type of dinosaur,” I say, realizing that I’ve forgotten what a theropod is or what the other type of dinosaur is.
“Why is it called a theropod?” he continues.
“I don’t know,” I tell him.
“Can you find out?”
Of course I can.
Let’s start off with the basics. You may have heard that dinosaurs aren’t reptiles. This is not strictly true, as Dinosauria is a clade of the class Reptilia – a class that in modern times includes turtles, crocodilians, snakes, amphisbaenians (aka worm lizards), lizards, and tuatara. Dinosauria contains two Orders, Ornithischia (primarily herbivorous dinosaurs with a hip structure similar to birds) and Saurischia (primarily carnivorous dinosaurs with a hip structure similar to lizards), although it appears that many contemporary paleontologists classify Ornithischia and Saurischia as clades rather than orders.
A clade can be thought of as a branch on the tree of life.
More specifically, a clade consists of a single ancestor species and all the species that evolved from it. In broad strokes, all life is a clade because everything traces back to a single organism.
“Theropod” derives from Greek, and means “wild beast foot”, and was coined in 1881 by O. C. Marsh as a suborder to hold Allosaurus. They are ancestrally bipedal, ancestrally carnivorous dinosaurs – “ancestrally”, because some species of theropods evolved omnivorous or herbivorous diets, or quadrupedal gaits, or both. Depending on the “age” of the species and the specific lineage they may have had scales or feathers or both. The order includes many of the famous, “sexy” dinosaurs, such as the Tyrannosaurus Rex, the “four-winged” Microraptor, and chickens.
Yes, chickens. See, one clade within Theropoda was Averostra (“bird snouts”), and Averostra had a couple of clades survive the Cretaceous–Paleogene extinction event: Ceratosauria and Orionides. Within Orionides was the clade Maniraptoriformes, which was the parent clade for Avialae, which was the parent clade for Aves – modern birds. So, yes, birds are “lizard-hipped” dinosaurs. Go figure.