As I heard one person say, "Every fossil is a transitional fossil."
Some, however, are particularly important, like Darwinopterus.
Transitional Pterosaur: A Missing Link No Longer Missing
It isn't going to help that they named the genus Darwinopterus in honor of the 150th anniversary of the publishing of Origin of Species and the 200th anniversary of Darwin's birth. That will irritate anti-evolutionists worse and make them oppose it more.
It's an awesome find, however, being intermediary between earlier pterosaurs, longer in tail and smaller in size, and later pterosaurs, shorter in tail and larger in size. There's more differences, though, and Darwinopterus modularis falls right in the middle.
Older forms had "small heads, short necks, short wrists, a long tail and a long fifth toe on the foot," he said. In later, more derived types, "the skull, neck and wrist became relatively long, but the tail became short and the fifth toe dwindled to a small nub or was lost altogether."
Lu added: "Darwinopterus captures a moment in that evolution from primitive to advanced forms. But contrary to what we expected, it has the head and neck of an advanced pterodactyloid while the rest of the skeleton is like that of the primitive rhamphorhynchoids (flying reptiles)."
That's from Discovery News, quoting Junchang Lu, lead author of the study.
It's pretty interesting because this is not a gradual shifting, like the picture we get from human evolution. Pterosaurs apparently evolved in rapid jumps, body part by body part. The picture we get is one wholly evolved part, or a few, then the rest wholly evolved. We don't get a medium-length tail, for example.
Of course, keep in mind what this means. This means that the shift from a long tail to a short tail happened rapidly in the sense of hundreds of thousands or even a few million years, and as a result we don't have the intermediary fossils from 160 million years ago.
Relatively few dead animals are preserved, and the longer ago the animal died the less likely we are to find it. Even well-preserved lineages like the whale have produced a few hundred fossils over 52 million years of evolution. A couple of well-preserved individuals from each million years would be thrilling. Instead, we get individuals preserved far more rarely than that, and we get bits and pieces more often: a tooth here, a leg here, etc.
So the find of Darwinopterus modularis, which produced about 20 individuals, is incredible. It is also one more missing link that is no longer missing.