GraduateUniversityofChineseAcademyofSciences,Beijing100049, ChinaSummaryParasite-host associations among insects and mammals orbirds are well attended by neontological studies [1]. AnEocene bird louse compression fossil [2, 3] and severalfleaspecimensfromEoceneandOligoceneambers[4–8],re-ported to date, are exceptionally similar to living louse andflea taxa. But the origin, morphology, and early evolutionof parasites and their associations with hosts are poorlyknown[9,10]duetosparserecordsofputativeectoparasiteswith uncertain classification in the Mesozoic, most lackingmouthpart information and other critical details of the headmorphology [11–15]. Here we present two primitive flea-likespecies assigned to the Pseudopulicidae Gao, Shih et Renfamilia nova (fam. nov.), Pseudopulex jurassicus Gao, Shihet Ren genus novum et species nova (gen. et sp. nov) fromthe Middle Jurassic [16] and P. magnus Gao, Shih et Rensp.nov.fromtheEarlyCretaceousinChina[17].Theyexhibitmany features of ectoparasitic insects. Large body size andlong serrated stylets for piercing tough and thick skin orhides of hosts suggest that these primitive ectoparasitesmight have lived on and sucked the blood of relatively largehosts, such as contemporaneous feathered dinosaurs and/or pterosaurs or medium-sized mammals (found in the EarlyCretaceous, but not the Middle Jurassic).ResultsHerein, we report from the mid-Mesozoic of China two well-preserved fossil ectoparasitic insects, exhibiting manyfeatures of ectoparasitic insects, e.g., wingless body coveredwith stiff, posteriorly directed setae and bristles, short andbeaded antennae, reduced eyes, absent ocelli, robust mouth-parts with serrated stylets (laciniae), scythe-like pretarsalclaws, and ctenidia on the tibiae. These specimens sharewith extant fleas, especially with the most basal flea lineageTungidae [18], some striking similarities, but given a lack ofsome diagnostic features of Siphonaptera, their systematicpositions are deemed as Order incertae sedis.TaxonomyOrder Incertae sedis.Family Pseudopulicidae Gao, Shih et Ren fam. nov.Diagnosis. Robust and elongate piercing-sucking mouth-parts with serrated stylets; antennae with more than15 segments, flagellomeres compact; thorax relatively small,apterous; legs elongate with numerous stout setae, distinctctenidia on the tibiae, and pretarsus with strong scythe-shaped claws; body covered with stiff, posteriorly directedsetae and bristles.Remarks. Different from extant fleas in the absence of pro-notal and genal ctenidia on body (except for Tungidae), lackof the uniquely modified jumping hind legs, distinct ctenidiaon the tibiae, more developed eyes, detailed structure ofmouthparts with serrated stylets, antennae with more than15 segments, not laterally compressed abdomen, and largerbody size. Although Tarwinia [11] possesses many similardiagnostic characters, it is tentatively excluded from Pseudo-pulicidae fam. nov. due to lacking mouthpart information.Pseudopulex Gao, Shih et Ren, gen. nov.Type species. Pseudopulex jurassicus Gao, Shih et Ren sp.nov.Etymology. The generic name is a combination of prefixpseudo-, indicating ‘‘with visual similarity to,’’ and Latin wordof Pulex, meaning ‘‘flea.’’ Gender masculine.Species included. The type species and Pseudopulexmagnus Gao, Shih et Ren sp. nov.Diagnosis. Same as familial diagnosis.Pseudopulex jurassicus Gao, Shih et Ren, sp. nov., seeFigures 1A–1J and Figures S1A–S1C available onlineHolotype. CNU-NN2010001, a well-preserved almost com-plete body in ventral view (Figures 1A and 1B; Figures S1Aand S1B). Deposited in the Key Lab of Insect Evolution andEnvironmental Changes, Capital Normal University, Beijing,China.Locality and Age. Jiulongshan Formation, the Late MiddleJurassic (Bathonian-Callovian boundary, 165 million yearsago [Mya]) [16]; Daohugou Village, Ningcheng County, InnerMongolia, China.Etymology. The specific epithet jurassicus derives fromLatin ‘‘jurassicus,’’ referring to the age of this fossil.Diagnosis. Different from P. magnus by the number ofantennal segments, presence of tibiae ctenidia, and maxillarylaciniae. Serrated teeth of P. jurassicus cover the entiresurface of stylet, whereas those of P. magnus cover only theedges.Description. Body length 17.00 mm excluding antennae.Head small and trapezoid shape. Eyes reduced and ocelliabsent. Antennal flagellum 16 segments visible, short andstout, slightly compressed, connected to both sides of headin clear depressions (Figure 1G). Mouthparts specialized,long and strong (at least 3.44 mm), much longer than twicethe length of head, labrum unclear, maxillary palps threesegments visible as preserved, labial palps four segmentspreserved, but the exact number unknown, maxillary laciniaeforming some deeply notched serrations, extending frombase to tip (Figures 1A–1F). Thorax small, about one-fifth ofthe abdomen in length. Abdomen not laterally compressed,with eight segments based on eight spiracles (segmentationis not clear), covered with dense and long bristles directingposteriorly, possessing a pygidium with sensilia on the ninthtergite (Figure S1C). Legs unusually long, coxae slightly