There is a species of flea evolved to feed on most terrestrial animals; cat fleas feed predominantly on cats, dog fleas feed predominantly on dogs, human fleas feed predominantly on humans and bird fleas feed predominantly on birds.
It seems that in the Jurassic period, even the dinosaurs had dinosaur fleas. And the dinosaur flea was huge compared with its tiny modern descendants. Dinosaur bloodsuckers were 2 cm long - eight times the size of today's fleas, researchers reported in the journal Nature.
Flea fossils 125 million to 165 million years old found in China are evidence of the oldest fleas. Their very long proboscis, or hypodermic needle-like mouth, had sharp, saw-like, serrated edges for cutting through the tough skin of their dinosaur hosts. However, the ancient dinosaur fleas had one big difference from modern ones: They could not jump the way modern fleas do. The leap of modern fleas is prodigious; they jump to height equivalent of the man jumping over a high jump bar 60 metres high, or over the Leaning Tower of Pisa.
Perhaps today's fleas seem less of a problem if one thinks of what a dinosaur flea could do to you if they were still around. But they went the way of the dinosaurs.