These were insects much larger than modern fleas, and from the size of their proboscis, we can tell they would have been mean. ~ American zoologist George Poinar, Jr.
Dinosaurs had fleas. Big ones: 10 times the size of the modern flea.
It really appears as though they were specialized for working their way into some heavy hides, such as those on dinosaurs. ~ American palaeoentomologist Michael Engel
In subsequent evolution, fleas downsized. This required acquiring the strong spring-legged jump that lets modern fleas leap 100 times their height.
Without the incredible jump, dinosaur-era fleas had to scurry and hop on their prey. Alternately, ambush-style, they may have dropped from vegetation.
The flea also adapted to suck blood without causing so much pain, thereby allowing getting a full meal without unduly agitating its host. This involved going from a jaw with saw-like projections to smooth mouth parts.