Eustheniidae

Abstract

3.1.6. Eustheniidae Eustheniidae fossils are known from the Southern Hemisphere by two genera, † Stenoperlidium Tillyard and † Mesontoperla Riek. These placements were supported by similarities in the wing and nymph of † Stenoperlidium to the extant Stenoperla McLachlan (Tillyard 1935), and the presence of many crossveins throughout the wing in † Mesontoperla (Riek 1954). The assignment of both ancient genera (Permian and Triassic, respectively) to an extant family was considered uncertain by the authors, and is still questioned (Aristov et al. 2013; Carpenter 1992; Grimaldi & Engel 2005; Riek 1954; Tillyard 1935). Interestingly, Eustheniidae is one of the few families with an autapomorphy in the wing: the posterior margin of the hind wing is smoothly rounded, without a notch separating the anal fan from the remigium (Zwick 1979, 2000). Unfortunately, the hind wing is not preserved, or this character is not visible, in all fossil representatives of this family including the Northern Hemisphere † Boreoperlidium Sinitshenkova (Aristov et al. 2013; Riek 1954; Sinitshenkova 2018; Tillyard 1935). Extensive crossvenation is plesiomorphic (Fig. 5) to the order (Béthoux et al. 2011) and the character state “CuA reaching wing margin in three branches”, suggested by Aristov et al. (2013), cannot be accepted as synapomorphic because it differs in many extant representatives of the family (Tillyard 1935). Furthermore, all † Boreoperlidium fossils have very few or no crossveins in the (ScP+) RA–RP field (Aristov et al. 2013; Sinitshenkova 2018). The loss of crossveins in this field is an autapomorphy of Arctoperlaria (Cui et al. 2015), making the assignment to Eustheniidae highly unlikely. Instead, a narrow space between the ScP and anterior margin of the wing, and many crossveins in the basal region of the costal field both support an assignment to stem-Systellognatha (Cui et al. 2015). As such, there is no evidence to support the placement of any of these genera in Eustheniidae. Nevertheless, the wings of † Stenoperlidium and † Mesontoperla are similar to those of extant Eustheniidae. The distal half of the wing has extensive crossvenation, including in the RP field. In † Mesontoperla, the poorly preserved CuA appears to join the wing margin distally. These are both potentially homoplasious characters, but nevertheless provide some evidence for the placement of these genera among stem-Antarctoperlaria. The same cannot be said for † Boreoperlidium, in which neither is present. Tillyard (1935) described the nymph of † Stenoperlidium with simple abdominal gills. These gills are not clear in the published photograph, and the validity of this character cannot be assessed. If these gills are indeed present, they are an autapomorphy of Eusthenioidea (Avelino-Capistrano et al. 2018; Zwick 2000).