Mesodiphthera Tillyard, 1919

Lambkin, Kevin J., 2019, Mesodiphthera Tillyard, 1919, from the Late Triassic of Queensland, the oldest cicada (Hemiptera: Cicadomorpha: Cicadoidea: Tettigarctidae), Zootaxa 4567 (2), pp. 358-366 : 359-360

Mesodiphthera Tillyard, 1919

Type species. Mesodiphthera grandis Tillyard, 1919 , by original designation.

Diagnosis. Tegmen elongate (c. 3 times longer than wide), over 20 mm long, apex fairly evenly rounded; base of pre-nodal area, costal space and clavus coriaceous; clavus strongly punctate, base of pre-nodal area and the costal space apparently less so; costal space broad, about two times width of CuA cell; nodal line well beyond ½ tegmen length; basal cell strongly narrowed, closed apically by the short base of M and a very short fusion of M with CuA; primary R and M forks at about same level, midway between apex of basal cell and nodal line; RP angled at nodal line, M1+2 and M3+4 not so; RA1 very close to node; RA2 with 4 or 5 terminal branches; radial and medial crossvein series (ir, r–m1, im and m–cua) closer to nodal line than tegmen apex, apical cells thus quite long; ir upright or slightly inclined; angle of primary M fork acute; im backwardly inclined, between M2 and M3; M3 and M4 both simple; CuA sharply angled at nodal line which runs parallel to it and base of CuA2; CuA1 more or less straight or slightly sinuous, directed towards apical margin, about same length as M4; CuA1 cell quite narrow; apical section of CuA2 sharply angled away from nodal line, about ½ as long as CuA1; the cell bounded by the nodal line, wing margin and apical section of CuA2 long and very narrow; clavus quite large, occupying a significant area of tegmen, extending well beyond ½ tegmen length, claval veins well-developed;

Notes. The diagnosis is based on an analysis of the descriptions and figures of the 25+ fossil genera ascribed to the Tettigarctidae ( Moulds 2018, Jiang et al. 2018, Fu et al. 2018). Their diagnoses are almost entirely based on the structure of the tegmen which displays remarkable diversity in shape, colour and venation. Within this diversity, Mesodiphthera stands apart in the combined presence of three characters: the primary forks of R and M at about the same level, midway between the basal cell and the nodal line; RA2 with four or five terminal branches; and the inter-medial cross-vein backwardly inclined and running between M2 and M3. The primary fork of R is just beyond the basal cell and distinctly proximal to that of M in all genera except the Jurassic Hirtaprosbole Liu, Yao and Ren (Liu et al. 2016 , fig. 1E) and Cretaceous Hpanraais Jiang, Chen, Jarzembowski and Wang (Jiang et al. 2018, fig. 1B) in which the forks are similar to Mesodiphthera , and the Cretaceous Architettix Hamilton and Cretotettigarcta Fu, Cai and Huang , in which the R fork is quite distal to that of M ( Menon et al. 2007, fig. 11.46c, Fu et al. 2018, fig. 4A). RA2 has at most three terminal branches in all other genera except the Jurassic Cicadoprosbole Bekker-Migdisova which has four ( Evans 1956, fig. 20), the Cretaceous Hylaeoneura Lameere and Severin which apparently has five, and the aberrant Cretaceous Elkinda Shcherbakov , in which all post-nodal main veins are extensively polymerized ( Shcherbakov 1988, fig. 2i). The inter-medial cross-vein is upright or forwardly inclined in all genera except Macrotettigarcta Chen and Wang (Chen & Wang 2016, fig. 2 ), Hypanraais ( Jiang et al. 2018, fig. 1B) and Cretotettigarcta ( Fu et al. 2018, fig. 4A). These have the cross-vein backwardly inclined similar to that of Mesodiphthera , but in all three it is between M1+2 and M3, rather than M2 and M3.

Shcherbakov (2009) proposed a subfamily and tribal classification of the Tettigarctidae , based on tegmen shape and venation, comprising two subfamilies, the Cicadoprosbolinae and Tettigarctinae , each with three tribes. With the costal space much wider than the CuA cell, the basal cell strongly narrowed apically, and the post-nodal cross-vein series closer to the nodal line than the apex, Mesodiphthera clearly falls within Shcherbakov’s Cicadoprosbolinae . Since that time, however, the remarkable proliferation of new fossil tettigarctids (14 species in nine genera) has revealed significant additional structural diversity in the tegmen, as well as an array of new combinations of putative diagnostic characters. This has thrown uncertainty onto the higher classification of the family, as noted by several recent authors ( Chen et al. 2014, Chen et al. 2016, Jiang et al. 2018). The tegmen of Mesodiphthera has now further expanded this structural diversity. The apparent trend line in the evolution of the tettigarctid tegmen appears to be the narrowing of the costal space, the basal cell becoming rectangular rather than narrowed apically, the movement of the post-nodal cross-vein series towards the apex, and a substantial reduction in the relative size of the clavus (e.g. compare Mesodiphthera with Tettigarcta (as illustrated by Kaulfuss and Moulds 2015, fig. 2)). Most members of the Cicadoprosbolinae have the plesiomorphic states of these characters and the subfamily is thus more than likely paraphyletic. Clearly, and as alluded to by recent authors, a determination of the generic relationships and any resultant higher classification of the family requires an updated analysis which would obviously benefit from the finding of new revelatory fossil material.