The result, recently discovered in some oil shale from northwestern Montana, is the first fossil of a mosquito found still engorged with ancient blood.

The discovery was announced in this week’s issue of the Proceedings of the National Academy of Sciences.

According to the lead researcher of the study, Smithsonian Institution paleontologist Dale Greenwalt, this is only the fifth instance of blood-eating, or hematophagy, by any insect to be revealed in the fossil record, and it’s the first in a mosquito with traces of blood that his team calls “incontrovertible.”