Giant Jurassic fleas measuring more than two centimetres (about an inch) may have fed on feathered dinosaurs, say scientists.
Fossils of several of the blood-sucking insects were unearthed at two sites in China.
The largest females were 20.6 millimetres (0.81in) long, while males grew to 14.7 millimetres (0.58in).
Besides being much larger than modern fleas, they lacked their characteristic jumping hind legs.
One group of fleas from Ningcheng County, Inner Mongolia, dated back to the Middle Jurassic period 165 million years ago.
The other, from Liaoning Province, was 40 million years younger from the Lower Cretaceous period.
The most striking feature of the fleas was their blood-sucking "siphonate" mouthparts which were both unusually long and sturdy.
This would have been used to pierce the hides of their hosts, said an international team of scientists reporting the finds in the journal Nature.
Why the fleas were so large, and armed with such powerful weaponry, is a puzzle, because only small shrew-like mammals existed during the Jurassic and early Cretaceous periods.
Modern fleas feed exclusively on animals with fur and feathers.
One possibility is that the insects fed on feathered dinosaurs, a number of which have been discovered in China.
The scientists, led by Dr Andre Nel, from the Museum of Natural History in Paris, France, wrote: "The early mammals were small animals, making the large size of these Mesozoic species and the robustness of their mouthparts seem mismatched.
"It is... possible that the hosts of these early fleas were among the feathered dinosaurs of the period that became well known from the same deposits."
As placental mammals evolved and diversified, so did the fleas that fed on them, "eventually giving rise to the plague-carrying species that have so markedly altered the course of human history", the researchers added.