For feathered dinosaurs the late Cretaceous period may have been a very itchy time. Lice—the tiny wingless insects that feed on dead skin, and sometimes blood—were just beginning to dig in about 100 million years ago, and the epoch's small furry mammals, early birds and dino-birds would have provided ample food.

The louse fossil record is relatively sparse. So far, only a 100 million-year-old book louse and a 44-million-year-old bird louse have been uncovered. By pairing these specimens with modern louse genetic sequences, however, a team of researchers led by entomologist Vincent Smith of London's Natural History Museum has reconstructed the little hitchhikers' evolutionary history. According to their analysis, published April 6 in Biology Letters, lice lineages began to split and diversify during the late Cretaceous.

Exactly which animals played host to lice back then is unknown. No one has yet found an exquisitely preserved feathered dinosaur carrying fossilized lice. But the various lice lineages of the period would have been feeding on something, and dinosaurs were almost certainly on the menu. After all, birds are the descendants of small, feathered dinosaurs, and the fact that the 44-million-year-old bird louse closely resembles its modern cousins suggests that lice had become well-adapted to feeding on feathered organisms at a much earlier time. Lice have jumped to new hosts as they co-evolved, but the close resemblance of living and fossil lice is a testament to their evolutionary success.

Lice are not the only successful parasites with a deep fossil record. During the past several years, paleontologists have recognized the telltale traces of multiple parasite species. Although millions and millions of years old, these prehistoric body invaders look very much like their modern counterparts—in fact, their close resemblance to modern species is what has allowed scientists to identify so many of them—making parasites some of the greatest evolutionary champions of all time.

View a slide show of these hard-bitten epochal survivors