Current Biology 23, 1261–1266, July 8, 2013 ª2013 Elsevier Ltd All rights reserved http://dx.doi.org/10.1016/j.cub.2013.05.040 Report New Transitional Fleas from China Highlighting Diversity of Early Cretaceous Ectoparasitic Insects Taiping Gao,1 Chungkun Shih,1 Alexandr P. Rasnitsyn,2,3 Xing Xu,4 Shuo Wang,4,5 and Dong Ren1,* 1College of Life Sciences, Capital Normal University, Beijing 100048, China 2Paleontological Institute, Russian Academy of Sciences, Moscow 117997, Russia 3Department of Palaeontology, Natural History Museum, London SW7 5BD, UK 4Key Laboratory of Evolutionary Systematics of Vertebrates, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing 100044, China 5Graduate University of Chinese Academy of Sciences, Beijing 100049, China Summary Fleas are a group of highly specialized blood-feeding ectoparasites whose early evolutionary history is poorly known [1, 2]. Although several recent discoveries have shed new light on the origin of the group [3, 4], a considerable gap exists between stem fleas and crown fleas. Here we report a new transitional flea, Saurophthirus exquisitus sp. nov., assigned to a new family Saurophthiridae fam. nov., from the Lower Cretaceous Yixian Formation of northeastern China. Saurophthirids are more similar to crown fleas than other stem fleas in having a relatively small body size, relatively short and slender piercing-sucking stylet mouthparts, comparably short and compact antennae, rows of short and stiff bristles on the thorax, and highly elongated legs. The new finding greatly improves our understanding of the morphological transition to the highly specialized body plan of extant fleas. However, saurophthirids also display several features unknown in other fleas, and some of these features are suggestive of a possible ectoparasitic relationship to contemporaneous pterosaurs, though other possibilities exist. The new fossils, in conjunction with previous discoveries, highlight a broad diversity of ectoparasitic insects in the mid-Mesozoic. Results Fleas are a group of highly specialized blood-feeding ectoparasitic insects characterized by small size, laterally compressed body, highly modified mouthparts specializing in blood feeding, and long and robust legs specialized in jumping. The early evolutionary history of fleas is poorly known due to a sparse fossil record. Six definite flea fossils have been described from Eocene and Miocene ambers: Palaeopsylla klebsiana Dampf, 1911; Palaeopsylla dissimilis Peus, 1968; Peusianapsylla baltica Beaucournu et Wunderlich, 2001; and Peusianapsylla groehni Beaucournu, 2003 from Baltic amber [5–9]; and Pulex larimerius Lewis et Grimaldi, 1997 and Eospilopsyllus kobberti Beaucournu et Perrichot, 2012 from the Dominican Republic [10, 11], all of which are very similar to extant fleas. *Correspondence: rendong@mail.cnu.edu.cn Riek in 1970 [12] reported Tarwinia australis from the Early Cretaceous of Koonwarra, Victoria, Australia and considered it to be a flea-related insect, but the lack of preserved mouthparts, its antennal flagellum with 15 segments, and its long, slender legs with moderate-sized hind coxae have fueled debate regarding its classification as a flea [12, 13]. In 1976, Ponomarenko described Saurophthirus longipes as a preflea and presented its long legs, haired body, and other important ectoparasitic characters [14]. Recently, two flea-like fossils, Pseudopulex jurassicus and P. magnus assigned to Pseudopulicidae, were reported from the Middle Jurassic and Early Cretaceous deposits of northeastern China; they had very long and strong blood-sucking mouthparts with robust serrated laciniae [3, 15]. Huang et al. reported three taxa, Pseudopulex wangi, Hadropsylla sinica, and Tyrannopsylla beipiaoensis, from the same stratum of China and classified them to Pseudopulicidae in Siphonaptera [4, 16]. These flealike fossils display some features seen in crown fleas but are still considerably different from extant fleas in many salient features, including their large body size, markedly projecting male genitalic structures, ctenidia on the legs, etc., which hinders our understanding of the evolution of the highly specialized body plan of extant fleas. Herein we describe three fossils from the Late Cretaceous Yixian Formation of Liaoning Province, China, assigned to Saurophthirus (Insecta: Siphonaptera) (we place Saurophthirus into Siphonaptera; for details see the Supplemental Results available online), which has been established based on a well-preserved fossil, but one lacking some detailed structures of the mouthparts, from the Early Cretaceous of Baissa in Siberia [14]. The new findings demonstrate that Saurophthirus is more similar to crown fleas than other Mesozoic fleas or flea-like insects and provide significant new information on the transition to crown fleas. Based on well-preserved male and female specimens, we are able to document a striking dimorphic difference of the species. Taxonomy Insecta Linnaeus, 1758. Order Siphonaptera Latreille, 1825. Family Saurophthiridae Gao, Shih, Rasnitsyn, et Ren, fam. nov. Type genus Saurophthirus Ponomarenko, 1976. (For diagnosis and other details, see Supplemental Results.) Saurophthirus exquisitus Gao, Shih, Rasnitsyn, et Ren, sp. nov. (Figures 1 and 3). Diagnosis. Head length and width subequal; eyes small; antenna at least 12-segmented, fusiform in female and subclavate with palmate penultimate segment in male; hind basitarsus longer than the following tarsal segments combined; abdomen distinctly darkened apically. In contrast, S. longipes has female head clearly longer than wide, hind basitarsus much shorter, and abdomen not darkened apically. Type Material. Holotype female, No. CNU-LL2010016P/C; paratype female, No. CNU-LB2010017; paratype male, No. CNU-LB2010018. Locality and Age. The holotype was collected from the Early Cretaceous Yixian Formation, Dawangzhangzi Village, Current Biology Vol 23 No 13 1262 Figure 1. Holotype of Saurophthirus exquisitus sp. nov. Part, No. CNU-LL2010016P, from the Early Cretaceous Yixian Formation of Northeastern China (A) Habitus. (B) Head. (C) Basal part of the mouthparts. (D and E) Terminal of the mouthpart under different direct oblique light. (F) Line drawing of (B). (G) Line drawing of the terminal of the mouthparts, part of (F). Scale bars represent 2 mm in (A); 1 mm in (B) and (F); 0.2 mm in (C)–(E) and (G). Lingyuan City, Liaoning Province, China. The paratypes were collected from the Early Cretaceous Yixian Formation, Huangbanjigou Village, Beipiao City, Liaoning Province, China. Etymology. The specific name is from the Latin ‘‘exquisitus,’’ meaning ‘‘perfect and beautiful.’’ Description. Holotype female (Figures 1, 2, and S1). Body 8.46 mm long excluding antennae, somewhat dorsoventrally compressed (Figures 1A, 2A, and 2B). Head nearly round, 1.35 mm wide and 1.42 mm high. Compound eyes poorly developed, very small, ovoid, located near the antennary foveae (Figure S1A); ocelli absent. Antennae with 12 segments visible; several segments in the middle much thicker (nearly 2-fold) than the basal ones, but shorter. Piercing-sucking beak (1.22 mm) slightly shorter than the head in height (Figures 1B–1G and 2C); labial palps with three segments visible (Figure S1), reaching beak apex, rather loose (forming neither tight stylet sheaths nor syphonate tube); no mandible traceable; maxillary laciniae thin with small notches at the edge; epipharyngeal stylet wide and sword-like (Figures 1C, 1D, 1E, and 1G). Thorax about as wide as head, with morphology unclear; thoracic and one to four abdominal terga with transverse stripes of stiff bristles (Figures 1F and 2C). Legs covered with long and stiff setae (Figures 2D and 2E); all legs longer than the body. Coxae short, nearly round, all in sublateral position; hind coxae slightly longer than the fore and mid ones. Fore femur slightly longer than fore tibia; basitarsus as long as the following three tarsal segments combined; fourth tarsal segment shortest; fifth tarsal segment nearly equal to the second tarsus; fourth tarsal segment of the mid legs very short (just half of the pretarsus). Claws narrow and gently curved, with a small basal lobe (Figure 2E). Abdomen covered with long and soft setae; exact number of segments unknown; cerci small with long tufts of hairs about 1.80 mm long (Figure 2F). Paratype Female. Figures 3A–3D, S2A, and S2B. Body 9.74 mm long, excluding antennae (Figure 3A). Head 1.72 mm wide, 1.70 mm high (Figures 3A, 3B, S2A, and S2B); eyes small; piercing-sucking mouthparts short (1.10 mm long), with palpus at least three segments; many hairs present around the segmented boundary (Figures 3B, S2A, and S2B). Antenna compact, short (1.10 mm long), 16-segmented, fixed in the antennal fovea, with every segment bowl-shaped and width nearly twice the height (Figure 3C). Abdominal segments unclear, covered with soft and long hairs (Figure 3D). Paratype Male. Figures 3E–3K, S2C, and S2D. Body 6.9 mm long, including male genitalia but excluding antennae (Figure 3E). Head with two dark sclerites of obscure homology; eyes not unequivocally visible (Figure 3G); mouthparts about 0.41 mm long, details unclear. Antenna 16-segmented, 1.22 mm long; basal segments thin; apical segments cupshaped and over twice as thick as the basal segments; the top one or two segments palmate (with narrow lobes extending apically; Figure 3F). Thorax much wider than head (Figures 3G and 3J). Pronotum roundly cruciform, nearly equal to head Cretaceous Transitional Fleas 1263 Figure 2. Holotype of Saurophthirus exquisitus sp. nov. Counterpart, No. CNU-LL2010016C, from the Early Cretaceous Yixian Formation of Northeastern China (A and B) Photograph (A) and line drawing (B) of habitus. (C) Mouthparts. (D and E) Claws of the left hind leg (D) and right middle leg (E). (F) Terminal of the abdomen showing the cerci. Scale bars represent 2 mm in (A) and (B); 0.4 mm in (C)–(F). See also Figure S1. in width and height, covered by rows of short and thick bristles (Figures 3G and 3J); transverse stripes of such bristles occur through the fourth abdominal tergum (Figures 3G, S2C, and S2D). Abdomen about as wide as thorax, with eight segments external; the tergal plates not reaching lateral on many abdominal segments, and at least some of these segments possessing distinct boundaries; the fourth segment widest, about 1.90 mm; the ninth segment internalized and of sophisticate form, possibly bearing internalized cerci. Male genitalia (Figure 3K) long, extending far beyond the body, with elongate gonocoxa and gonostylus; gonostylus and external part of gonocoxa (internal gonocoxa indiscernible) of subequal length, each much longer than any abdominal segment; visible (external) part of penis wide, lanceolate, extending beyond apex of gonostylus (Figures 3K and S2D). Legs covered by numerous hard setae (Figures 3H and 3I); all legs longer than body. Coxae small, nearly round, just slightly swollen, hind pair approaching midline. Claws long, sharp, narrow, and gently curved (Figures 3H and 3I). Fore femur slightly longer than fore tibia, the latter slightly longer than fore basitarsus; basitarsus nearly equal to the following three tarsal segments combined; fore pretarsus slightly longer than mid and hind pretarsus. The new species displays a pronounced sexual dimorphism. The female body is long and narrow, with antenna compact and fusiform, legs particularly long and thin, all coxae sublateral, abdomen weakly sclerotized and supposedly far distensible, with indistinct segmental boundaries, and ending in two hair tufts issued by short cerci. In contrast, the male body is shorter and wider, more sclerotized with abdominal segmentation easily visible, antenna longer and subclavate with the penultimate one or two segments palmate, hind coxae approaching each other, and abdomen with no cerci in apical position. Discussion Ponomarenko reported the first Saurophthirus insect, S. longipes, but did not describe the mouthparts due to a lack of detailed preservation [14]. New morphological data and information from our study clearly indicate that saurophthirids share some putative synapomorphies with extant fleas that are indicative of a hematophagous ectoparasitic style. These features include a wingless and small thorax, compacted and short antenna, reduced eyes, haired body, etc. Particularly important is the supposedly unique flea synapomorphy of having piercing-sucking mouthparts formed by an epipharyngeal stylet and a pair of maxillary stylets while the mandibles are reduced. These features are also seen in other flea-like insects or stem fleas. However, compared with Pseudopulex, Saurophthirus is more similar to crown fleas in having a relatively short and slender piercing-sucking stylet mouthpart and much-swollen hind coxae. Several transitional features are also noteworthy. First, saurophthirids have medium body size (ranging from 6.9 mm for male and from 8.5 to 12.0 mm for females), between that of large Pseudopulex (ranging from 8.0 mm for male and from 14.8 to 22.8 mm for females) and small modern fleas (less than 5 mm). Second, extant fleas possess rows of posteriorly directed ctenidia on the body; saurophthirids lack ctenidia on the tibiae or body, Current Biology Vol 23 No 13 1264 Figure 3. Paratype of Saurophthirus exquisitus sp. nov. from the Early Cretaceous Yixian Formation of Northeastern China (A–D) Female, No. CNU-LB2010017. (E–K) Male, No. CNU-LB2010018. (A) Habitus of female. (B) Mouthparts under alcohol. (C) Left eye and antenna under alcohol. (D) Terminal of the abdomen under alcohol. (E) Habitus of male. (F) Right antenna. (G) Head and pronotum. (H and I) Claws on the right legs. (J) Thorax. (K) Terminal of the abdomen, showing the male genitalia. Scale bars represent 2 mm in (A) and (E); 0.2 mm in all other panels. See also Figure S2. but they have rows of short, stiff bristles on the body (Figures 1F, 2B, 2C, 3B, 3G, 3J, S2C, and S2D), which probably represent the early evolutionary stage of body ctenidia in crown fleas. These bristles cover the thorax and the abdominal segments 1–4 (Figures 1F, 2C, 3G, S2C, and S2D), and the remaining segments are covered with long and soft hairs, a distribution pattern similar to ctenidia in crown fleas. Finally, from the description of Huang et al., Pseudopulex wangi and Tyrannopsylla beipiaoensis have large and wide male genitalia of different structure, not at all invaginated, with gonocoxa short and thick, gonostylus much narrower and attached at an angle, and penis short [4, 16]. This is also a plesiomorphic Cretaceous Transitional Fleas 1265 Figure 4. Phylogeny of Saurophthirus and Related Clades Clade A is supported by a long proboscis with siphonate or stylet mouthparts. Clade B (flea-like insects) is supported by a wingless and small thorax, especially reduced eyes, compact antennae, posteriorly directed setae, and long legs with claws. Clade C (Aneuretopsychidae) is supported by mouthparts forming an annulated siphonate organ, with labial palps lost or nearly lost. Clade E (Pseudopulicidae) is supported by tibial ctenidia, laciniae with very sharp serrated teeth, and totally external male genitalia. Clade D is supported by the absence of ctenidia on the tibia, relatively short piercing-sucking mouthparts with notched laciniae, highly elongated legs, much-swollen hind coxae, and partially internal male genitalia. Clade F (modern fleas) is supported by saltatorial hind legs, ctenidia on the body, laterally flattened body, and completely hidden male genitalia. Clade G (Saurophthiridae) is supported by extremely long legs and body covered by bristles with regional distribution. Representative species: circle, Pseudopulex jurassicus; diamond, Pseudopulex magnus; star, Tarwinia australis; triangle, Saurophthirus exquisitus sp. nov. See also Figure S3. state in contrast to the long, narrow, partially invaginated genitalia with long but thick penis in Saurophthirus, whereas the genitalia are highly reduced in extant fleas. Based on these features, we think the genitalia of Saurophthirus might be in a transition from totally exposed to entirely invisible. These findings greatly improve our understanding of flea evolution (Figure 4; see also Figure S3). Aneuretopsychidae has been suggested as a sister group of Siphonaptera by the presence of a long proboscis with siphonate versus stylet mouthparts with serrate laciniae (Figure 4, clade A) [4], which is totally different from what is known in Diptera [17]. After the evolution of the nonmandibulate piercing-sucking mouthparts, other characteristics of fleas have appeared, including a wingless body, compact antennae, and reduced eyes, which are seen in stem fleas such as Saurophthirus, Pseudopulex, and Tarwinia (Figure 4, clade B). However, the latter two taxa uniquely share the presence of the ctenidia on the tibiae, suggesting that they represent an extinct lineage in early flea evolution (Figure 4, clade E) [13]. Compared with Pseudopulex, Saurophthirus has smaller body size, relatively short piercingsucking mouthparts with notched laciniae, highly elongated legs, much-swollen hind coxae, and partially extended male genitalia; these features indicate that Saurophthirus is more closely related to modern fleas than to Pseudopulex and Tarwinia (Figure 4, clade G). This suggests that Saurophthirus further evolved into the modern fleas during the Cretaceous and Paleogene periods. Following these changes, characteristics of crown fleas such as laterally compressed body and robust legs specialized for jumping have evolved (Figure 4, clade F). Our findings shed new light on the diversity of ectoparasitism and host relation for Early Cretaceous insects. The legs of Saurophthirus are very long and slender; every leg is longer than the body, in contrast to the aforementioned flea-like insects and true fleas. The coxae of the newly described species are swollen and nearly round, and the fore femora are thicker than the mid femora, which in turn are thicker than the hind femora. Pseudopulex insects have very long and strong piercing-sucking mouthparts with two rows of sharp serrated teeth on their laciniae. The length of the mouthparts is at least 3.44 mm for P. jurassicus and 5.15 mm for P. magnus, both occupying over 1/5 of the body length [3], but for P. wangi (holotype, female), the length of the mouthparts is about 2.5 mm, which is about 1/6 of the body length [4, 16]. Such strong structures are considered to have been used for sucking blood from feathered dinosaurs and/or pterosaurs, because these vertebrates had thick and less-sensitive body coverings [18, 19]. S. exquisitus sp. nov. possesses a blood-sucking mouthpart (Figures 1 and 3) that includes a wide epipharyngeal stylet and a pair of relatively reduced laciniae, protected by at least three segmented palpi. The laciniae are not as robust as those of Pseudopulex but are similar to those of extant fleas, lacking the sharp saw-like teeth, except that the edges have some weak notches. The piercing-sucking stylet mouthparts of S. exquisitus sp. nov. are about 1.22 mm long in the holotype and about 1.10 mm long in the paratype, occupying only 1/7 to 1/9 of the whole body length. Compared with Pseudopulex, saurophthirids have much shorter and weaker piercing-sucking mouthparts (Figure 4, clade G). Since saurophthirids have very long and thin legs with sharp claws, they represent adaptation similar to that of various extant parasites dwelling on bat wing membranes [14]. No Mesozoic bats have been discovered thus far, so pterosaurs might be the most possible candidate hosts. It should be noted that all the described Saurophthirus species were collected from the Early Cretaceous. Many pterosaurs began to diversify in the Early Cretaceous, e.g., Haopterus gracilis [20], Feilongus youngi [21], Boreopterus cuiae [22], Gegepterus changi [23], etc., and these pterosaurs had body sizes suitable for such relatively large ectoparasites to live on. However, we are not able to rule out the possibility that Saurophthirus might have preyed on other coexisting vertebrate hosts such as feathered Current Biology Vol 23 No 13 1266 dinosaurs, birds, or mammals. The reported Early Cretaceous ectoparasites now include Saurophthirus, Pseudopulex and its close relatives, Tarwinia, and maybe strashilids and vosilids [24, 25] (but see [26] for a different opinion). These insects display considerable morphological differences, which suggest different ectoparasitic habits and hosts. Together, they highlight the diversity of ectoparasitism in the Early Cretaceous. Experimental Procedures These type fossils are deposited in the Key Laboratory of Insect Evolution and Environmental Changes, Capital Normal University, Beijing, China. The specimens were examined under a Leica MZ16.5 dissecting microscope. Line drawings were prepared with CorelDraw X6 (version 16.0.0.707) and Adobe Photoshop CS 6.0 (version 13.0.1). Supplemental Information Supplemental Information includes Supplemental Results and three figures and can be found with this article online at http://dx.doi.org/10.1016/j.cub. 2013.05.040. Acknowledgments We are grateful to Y.Y. Yao and Y.J. Wang for suggestions and to Q. Yang and C.F. Shi for discussions and specimen preparation. D.R. was supported by the National Basic Research Program of China (973 Program; grant 2012CB821906), the National Natural Science Foundation of China (grants 31230065, 31272352, and 41272006), Project of Great Wall Scholar and KEY project of Beijing Municipal Commission of Education Project (grant KZ201310028033), and China Geological Survey (grant 1212011120115); X.X. was supported by the National Natural Science Foundation of China (grant 41120124002); and A.P.R. was supported by the Russian Academy of Sciences Presidium program ‘‘Biosphere Origin and Evolution of GeoBiological Systems.’’ Received: April 17, 2013 Revised: May 8, 2013 Accepted: May 22, 2013 Published: June 27, 2013 References 1. Rasnitsyn, A.P., and Quicke, D.L.J. (2002). History of Insects (Dordrecht: Kluwer). 2. Grimaldi, D.A., and Engel, M.S. (2005). Evolution of the Insects (New York: Cambridge University Press). 3. Gao, T.P., Shih, C.K., Xu, X., Wang, S., and Ren, D. (2012). 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