BackgroundFleas, the most notorious insect ectoparasites of human, dogs, cats, birds, etc., have recently been traced to its basal and primitive ancestors during the Middle Jurassic. Compared with extant fleas, these large basal fleas have many different features. Although several fossil species with transitional morphologies filled the evolutionary blank, the early evolution of these ectoparasites is still poorly known.ResultsHere we report a new flea with transitional characters, Pseudopulex tanlan sp. nov., assigned to Pseudopulicidae, from the Lower Cretaceous Yixian Formation of Liaoning Province, China. Different from the previously described pseudopulicids, P. tanlan has relatively smaller body size but lacking any ctenidia on the tibiae or body, while the male with comparatively smaller and shorter genitalia. On the other hand, P. tanlan has some characters similar to the transitional fleas of saurophthirids, such as, a small head, short compacted antennae, small pygidium and many stiff setae covering the body.ConclusionsEven though other possibilities can not be ruled out, the female specimen with extremely distended abdomen suggests that it might have consumed its last meal before its demise. Compared with other reported female flea fossils, we calculate and estimate that P. tanlan sp. nov. might have consumed 0.02 milliliter (ml) of blood, which is about 15 times of the intake volume by extant fleas. These new findings further support that fleas had evolved a broad diversity by the Early Cretaceous.