Dinosaur articles, movies, and museum displays are common enough that you might consider all dino-mysteries solved, but the Smithsonian reports that major ones still remain. Among them:
- Who was first? Nobody knows which dinosaur species came first, partly because fossil records provide fragmentary insights rather than "the entire reel," notes the Smithsonian.
But skeletons in Tanzania and tracks in Poland tell us that dinosaurs may have started about 245 million years ago—with the slim and "dog-size" Nyasasaurus.
- Hot- or cold-blooded? Several experts say dinosaurs were hot-blooded, but now some suggest they were "mesotherms," warming their bodies with their muscle activity and experiencing changing body temperatures.
- Who was biggest? The fossil record isn't clear, but titanic sauropods evolved a few times into huge creatures like the Supersaurus, Diplodocus, and Argentinosaurus, each of which reached about 100 to 110 feet in length.
- How did they mate? Well, they hatched from eggs.
We assume females had a cloaca—an orifice that functions for reproduction, excretion, and urination in today's crocodiles and birds—and males had an "intromittent organ" found in ostriches and ducks.
But we can't know without clear fossils of the organs.
(Here's one theory on the latter.)