Siphonaptera, commonly known as Fleas, are one of the widely known blood-sucking ectoparasitic Insects, comprising more than 2500 described species in 16 extant families. Up to now, the fossil Siphonaptera contain 16 species in five families from Cainozoic amber and Mesozoic compression fossils. All Cainozoic Fleas have been placed in two families, the Ctenophthalmidae and Pulicidae, while all Mesozoic Fleas, grouped in three families, the Pseudopulicidae, Saurophthiridae, and Tarwiniidae, which together form an extinct superfamily, the Saurophthiroidea, comprising nine species within five genera. Saurophthirus longipes, an unusual insect from the Early Cretaceous Zaza Formation of Baissa in Siberia, was described by Arnold Ponomarenko in 1976. He reported that its similarities with Fleas are piercing-sucking proboscis and soft distensible abdomen. A family of Saurophthiridae was established by Ponomarenko ten years later in 1986, including only this species. Another species of this family, Saurophthirus exquisitus, was described in 2013, based on three specimens, from the Early Cretaceous Yixian Formation of Liaoning Province, China, and it was suggested that the Saurophthiridae represent a transitional group from basal to extant Fleas. These fossil Fleas display some features seen in crown Fleas but are still considerably different from extant Fleas in many morphological characters, e.g., the absence of pronotal and genal ctenidia (comblike structures) on the body, the lack of the uniquely modified jumping hind legs, distinct ctenidia on the tibiae, more developed eyes, antennae with more than 15 segments, absence of laterally compressed abdomen, presence of medium body size, swollen hind coxae and partially extended male genitalia. These features indicate that Saurophthirus is more closely related to modern Fleas than to the Cretaceous genera Pseudopulex and Tarwinia.
In a paper published in the journal Acta Palaeontologica Polonica on 21 February 2020, Yanjie Zhang of the College of Life Sciences at Capital Normal University, Chungkun Shih, also of the College of Life Sciences at Capital Normal University, and of the Department of Paleobiology at the Smithsonian National Museum of Natural History, Alexandr Rasnitsyn of the Palaeontological Institute of the Russian Academy of Sciences, and Dong Ren and Taiping Gao, again of the College of Life Sciences at Capital Normal University, describe a new species of Saurophthirus from from the Yixian Formation of Liaoning Province, China, which produces Insects ranging in age from about 128.2 to 121.6 million years, with the majority of the specimens dated to about 125 million years ago (Early Cretaceous).
The new species is named Saurophthirus laevigatus, meaning 'smooth' in Latin. It is described from a single, male, specimen with body almost perfectly fusiform and 9.8 mm long excluding antennae, almost completely-preserved ventral view, slightly dorsoventrally compressed.
Saurophthirid Flea Saurophthirus laevigatus, male (holotype, CNU-SIP-LL2015001) from the Early Cretaceous Yixian Formation of Northeastern China. (A) Part, habitus in general view (A₁), line drawing (A₃), enlargement of antenna (A₂), details of genitalia (A₄), (A₅); 7, 8, 9, the seventh to ninth abdominal segments. (B) Counterpart, general view (B₁), enlargements of claws (B₂), (B₃), arrows. (A₅) photographed under alcohol. Scale bars: (A₁), (B₁) 1 mm; (A₃) 2 mm; (A₂), (A₄), (A₅), (B₂), (B₃) 0.5 mm. Zhang et al. (2020).
Extant Fleas display peculiar morphological characters including relatively small body-size, laterally compressed body, the uniquely modified jumping middle and hind legs. However, the morphological characters of the stem-group Fleas are primitive compared to the crown Fleas. The fossil Fleas known from the Mesozoic possess the following features to support their Flea affinity: the piercing-sucking mouthparts with serrate stylets to penetrate thick and body coverings; relatively large body with long but thin legs and scythe-shaped claws for living on a large surface; body and legs with stiff spines and setae all directed backward imply that they adapted to fix and move on a surface covered with hairs or feathers. Each of the above features separately can be probably found in some other insect groups. However, for insects having all these characters combined, they were most likely adapted to blood sucking of vertebrate host with outgrowths like hairs or feathers. Saurophthirids have been suggested to resemble crown-group Fleas. The new species has the 'transitional' characters of Saurophthiridae and particularly genus Saurophthirus, including the medium body size, short piercing-sucking stylet mouthparts, slender legs (enlarged hind coxae) and the half-retracted male genitalia.
Up to now, three families, four genera with seven fossil species of basal Flea insects have been reported from the Cretaceous of Australia, Russia, and the Northeastern China. In contrast to the species from the Jurassic, the taxa from the Cretaceous have a higher degree of richness, suggesting the Cretaceous is an important stage of evolutionary radiation after the origination of basal Fleas in the Jurassic.
For ectoparasitic insects living in feathers of feathered Dinosaurs, Pterosaurs, or Birds or in hairs of Mammals, their small body size would have provided advantage for concealment in the host and reduced probability for being detected and removed by the host. Furthermore, Zhang et al. consider that the reason for decreasing body size of basal Fleas was to reduce the blood intake and minimize Flea’s demand for food and energy inputs. The blood consumption of the flea is positively correlated with its weight, and the blood consumption of female Fleas are significantly higher than males. The blood consumption of the female Cat Flea, Ctenocephalides felis, per day is equivalent to 15.15 times of their body weight. So, Zhang et al. believe that the Early Cretaceous Fleas, especially the transitional Fleas, became smaller in order to avoid host detection and reduce blood intake, which is the adaptation to ectoparasitic life in the early stage of evolution.
In addition, the evolution of the male genitalia is clearly indicated in fossils. The basal taxa from the Jurassic have entirely exposed genitalia with broad gonostylus articulated at apex to form a wide clasping organ, in contrast to the half-retracted genitalia from the Early Cretaceous. Therefore, Zhang et al. believe that the new species with more retracted genitalia might have provided concealment and protection.
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