Researchers have found the fossils fleas of Jurassic era in China and have found that those were very large as compared to the present day fleas.
The present day fleas are of the size of 1-10 mm but the primitive fleas of Jurassic and Cretaceous era were 8-21 mm long. They have no jumping specializations and were lacking in smooth jaws of modern fleas as they were armored with saw-like projections in siphoning mouthparts.
“The mouthparts are certainly overkill for piercing the hides of early mammals and birds,” says Engel. “It really appears as though they were specialized for working their way into some heavy hides, such as those on dinosaurs.”
George Poinar, a palaeoentomologist at Oregon State University in Corvallis, agrees that without jumping legs, the fleas “would have to crawl or run to find a host”. He adds that “they could have attacked dinosaurs or possibly pterosaurs”.