The largest fossil of the segmented spider Parvithele muelleri Wunderlich, 2017 (Mesothelae: Parvithelidae) from Cretaceous Burmese amber

A new specimen of the fossil spider Parvithele muelleri Wunderlich, 2017 (Araneae: Mesothelae) is described from the mid-Cretaceous (ca. 99 Ma) Burmese amber of Myanmar. Like the original holotype, the new specimen is a mature male, which clearly preserves a series of tibial macrospines, similar to the condition in the extant genus Liphistius Schiødte, 1849. Given that the single extant mesothele family, Liphistiidae, is predicted to have radiated ca. 39–58 million years ago, the occurrence of somewhat modern-looking male pedipalp spination in older fossil mesotheles is of considerable interest. The new specimen also shows spinnerets in a relatively posterior position, although variability observed in extant species suggests that this trait may be of limited systematic value. Of particular note is the presence of a large and well-developed anal tubercle, a feature seen in other Burmese amber mesotheles, but absent or much reduced in modern taxa. This structure may represent a vestige of the ancestral pygidium and could provide evidence for an intermediate stage following the loss of a segmented pygidium plus a flagellum as seen in the tailed spiders belonging to the chimerarachnid group. The combination of characters in P. muelleri —especially the prominent anal tubercle—supports Wunderlich’s resurrection of the family Parvithelidae from its synonymy with the otherwise Carboniferous family Arthromygalidae.