Dinosaurs are often mentioned in discussions about evolution, yet many people do not know how dinosaurs evolved.
Some of the most popular dinosaurs were highly-specialized genera that lived about 160 million years ago to 65 million years ago, the latter half of the "Age of Dinosaurs." Indeed, many famous dinosaurs like Tyrannosaurus, Triceratops and Ankylosaurus were relatively large, highly ornamented, or could otherwise be described as strange. They evolved from earlier ancestors that we might say were "plainer" and did not have some of the same special traits. These earlier dinosaurs are often less well-known to the public because they are not as spectacular as their later relatives, and their fossils are also quite rare. Surely there are many that have yet to be discovered.
Paleontologists do not expect to find the direct ancestors of later dinosaurs, it is important to understand. There were definitely more dinosaurs alive in the past than we know of now, or may ever know, and what we might call a direct ancestor today might be shown to be a more distant relative when a later discovery is made. Instead, fossils are compared to others to help figure out what the actual ancestral dinosaurs might have looked like. This is why "transitional form" is an important phrase. Even if we cannot directly identify one kind of dinosaur as ancestral to another, we can at least figure out the change in form that happened in different lineages.
An even better understanding of the evolution of dinosaurs will have to come from further fossil finds. The authors note that certain sites in China, for example, have yielded some of the oldest members of familiar dinosaur lineages like the horned dinosaurs and tyrannosaurs. Even then, though, there are even older sites all around the world that require more study. Who knows what has yet to be found? The broad pattern of dinosaur evolution is coming into view, but there are still many details yet to be filled in.