Pseudopulex
Pseudopulex is a genus of extinct primitive fleas that lived between the Middle Jurassic and Early Cretaceous periods in what is now modern-day China. The Latin root for the name Pseudopulex roughly translates to "false fleas". Taxonomy 

There are currently four identified species in this genus that have been discovered through fossil remains: Pseudopulex jurassicus, Pseudopulex magnus, Pseudopulex wangi, and Pseudopulex tanlan. The majority of current research focuses on P. jurassicus and P. magnus. Fossils of P. jurassicus date back to the mid-Mesozoic of China, more specifically the middle Jurassic-aged Jiulongshan Formation, dating back to around 165 million years ago, while P. magnus were discovered from the Early Cretaceous Yixian Formation, around 125 million years ago. Potential hosts for P. jurassicus include Pedopenna daohugouensis and/or Epidexipteryx hui, while P. magnus may have parasitised Sinosauropteryx prima or Microraptor gui. These organisms have similar features to other studied ectoparasites, and they share traits with present-day fleas, most notably in their wingless bodies and stylets that are very long and sharp, enabling them to pierce through thick skin and hide. They also exhibit traits (for example their thinner and more elongated clawed appendages and extended mouthparts) that can distinguish them from similar parasites, such as lice. Although they are similar to modern crown fleas, they exhibit traits that are different from modern fleas, which may be an indication of this genus possibly having an early evolution that resulted in a dead-end lineage. P. tanlan is considered to be a transitional organism between P. jurassicus or P. magnus and existing fleas as it has a smaller body plan, more compact antennae, and other features associated with extant fleas. A difference from extant fleas, such as crown fleas, is the lack of enlarged hind legs, prohibiting them from jumping or running, unlike most extant fleas. Presently, there has been a total of six other flea-like fossils that have been discovered along with this genus. Description 
Pseudopulex species have flattened bodies, similar to common ectoparasites like ticks or bedbugs. However, even between the species of Pseudopulex, there are distinct differences in morphology. P. jurassicus was slightly longer and contained a shorter stylet, while P. magnus was thicker and had very long mouthparts. P. tanlan was found to be smaller than other Pseudopulex species at about 10 mm long, with a relatively small head and thoracic cavity. The body of P. tanlan also exhibits very short and stiff setae. Compared to P. magnus and P. jurrasicus, P. tanlan has relatively small male genitalia and short tibias in females. P. jurassicus was much larger, with a body length of 17 mm and a mouthpart length of 3.4 mm, which was twice the size of its head. This species had very small eyes, antennae, and a short torso covered in long thin bristles. Their legs were also quite long and each ended in a pair of long claws. P. magnus was 22.8 mm long with 5.2 mm long mouthparts. Their heads were relatively small, and their bodies were compressed and stout compared to P. jurassicus. This species also exhibited antennae, dense setae, and claws on the ends of their legs, but had a distinctively large abdomen. P. wangi females were about 14.8mm long and had small heads, relatively short mouthparts, and short antennae. Males, however, were much smaller and had longer bodies with larger genitalia, indicating P. wangi was more sexually dimorphic than other species in its genus. Paleobiology  
While these parasites have some similar characteristics to modern fleas, they show major differences in body morphology and size, as well as in hosts. They possessed serrated stylets, which were likely used for feeding on blood through thick layers of skin. Moreover, these organisms are estimated to have been about fifty times larger than the dog flea. Pseudopulex may have later adapted to parasitizing birds or mammals, altering their morphology to more closely resemble modern-day fleas. References 

Fossil taxa described in 2012
Enigmatic insect taxa
Middle Jurassic insects
Late Jurassic insects
Cretaceous insects
Parasitic insects
Parasites of reptiles
Insects described in 2012
Prehistoric insects of Asia
Prehistoric insect genera
Fleas