Dry traces, of course. Reduce your expectations. This is big anyway.
Although scientists have found fossils of suspected blood-sucking insects, the creatures’ feeding habits have mostly been inferred from their anatomy or the presence of blood-borne parasites in their guts. But Greenwalt’s fossilized mosquito contains molecules that provide strong evidence of blood-feeding among ancient insects back to 46 million years ago. …
It’s hard to see what else the mosquito could have fed on (blood or sap), given its anatomy, but more such finds may enable some insight into the physiology of the mosquito’s extinct victims. You can only get so far with anatomy (fossils).