The oldest fossil of the family Issidae (Hemiptera, Fulgoromorpha) from the Paleocene of Menat (France)

The taxa Cubicostissus palaeocaeni gen. et sp. nov. are described from the Paleocene of Menat (Central France) as the oldest representative of the family Issidae for which all currently known fossils are reviewed. With a unique combination of characters, the specimen represents the first fossil described for the tribe Hysteropterini. Its occurrence in the European Paleocene shows that its lineage, and therefore the Issidae, were already present at least some 60 Ma. It allows to discard the hypothesis of an Eocene origin for the family, which is in accordance with a recent molecular calibration of the family, reporting the tribe as old as the Upper Cretaceous.


Introduction
Records of fossil Issidae Spinola, 1839 are rare, making their recognition and knowledge important landmarks in the understanding of the evolution of the family and essential for the calibration of a molecular phylogeny of the family. The oldest issid fossil is reported from Canadian amber 78-79 Ma (Skidmore 1999in McKellar et al. 2008. It was collected in southern Alberta, Canada and K.G.A. Hamilton determined it as a possible issid nymph (C.A.S. # 1121, Canadian amber inclusions in the Canadian National Collection of Insects). However, this identifi cation remains doubtful, needs to be confi rmed and will not be considered in this paper.

R e s e a r c h a r t i c l e
urn:lsid:zoobank.org:pub:28C76456-50AA-4DD7-A143-052433A51DD2 Only eight other issid fossils have been described to date (Bourgoin 2020 Szwedo, 2019. However, none of these four imprints exhibits a typical issid conformation and their place within Issidae is problematic. In the absence of a revision of these fossils, which is out of scope for this paper, we regard them as being in an incertae sedis position within Issidae, although they should be excluded from this family.
We, therefore, report here only the fi fth other fossil issid specimen. It originates from the well-known Paleocene deposit of Menat in France and represents the oldest Issidae fossil taxon known, additionally belonging to the Hysteropterini tribe for which no fossil data was available until now.

Material and methods
The type material (a single forewing imprint of 1 cm, close to another beetle imprint) is deposited in the

Diagnosis
Cubicostissus gen. nov. separates from other Hysteropterini genera by their forewing more than 2.5 as long as wide, wider before mid-length, with anterior margin slightly and regularly convex and posterior margin slightly concave at the end of the clavus. The postcostal cell forms a narrow band, slightly narrower than the radial cell band, and as wide as C1 cell. ScP+RA and RP fork late with ScP+R more than 2.5 times as long as basal cell. MP forks in C3 at the same level of Pcu and A1 fusion and CuA remains single up to the apical level of the clavus, then forks in an open C5. CuP is almost straight, connected by transverse veins to CuA. Pcu and A1 are connected in the last quarter of the clavus.

Etymology
Arbitrary combination referring to the forewing CuA vein (cubito costa) single and the generic name Issus.

Note
Previously placed in Issinae sec. Wang et al. (2016), Hysteropterini were recently regarded deserving a subfamily rank (Zhao et al. 2019

Diagnosis
Described from a single imprint from Menat (Fig. 1). Separated from all Hysteropterini species by its elongated forewing, the narrow postcostal band and the conformation of the venation.

Etymology
The specifi c epithet means 'from the Palaeocaenum (Paleocene)' and refers to the age of the deposit where the fossil was discovered.

Description
Forewing (Figs 1-2) 2.6 times as long as wide, wider before mid-length: 4.04 mm long, 1.63 mm wide before mid-length at MP forking level. Basal cell (bc) short, 0.29 mm long. No hypocostal plate. Anterior margin slightly and regularly convex, maximum convexity at level of MP fork. Posterior margin slightly concave at the end of the clavus. Common stem ScP+R more than twice as long as basal cell (0.67 mm long). Postcostal cell forming a narrow band, slightly narrower than radial cell, itself narrower than basal cell. ScP+R forking late: ScP+R 2.7 times as long as basal cell. Radial cell open, as wide as C1 cell before mid-length of tegmen, with ScP+RA and RP running parallel. Vein MP 1.33 mm long, forking late in cell C3 at same level of fusion of Pcu and A1; MP1+2 in straight line with vein MP; MP3+4 diverging posteriorly. CuA almost straight, single up to apical level of clavus, separated from MP and MP3+4 by C5 cell open. CuA connected with parallel CuP before mid-length of tegmen by four transverse veins. Just before reaching clavus, apically forked with CuA1 starts running parallel to the tegmen margin and CuA2 joining tegmen margin. Pcu and A1 connecting late in the last quarter of a long closed clavus. A1 closely parallel to posterior margin. Traces of brownish coloration and small micro-granulations are visible in cubito-anal and subcostal areas (Fig. 1).

Discussion
The short basal cell and the Y-shaped Pcu-A1 veins place the fossil within the Fulgoromorpha (Shcherbakov 1996). The issidized tegmen (Gnezdilov 2013a) and the tegmen venation are specifi c for genera of Issidae: Hysteropterinae: Hysteropterini, particularly characterized by a CuA single, forking only at the extremity of the clavus, with CuA1 running parallel to the wing margin. Unfortunately, the distal part of the tegmen is not suffi ciently preserved to further refi ne the comparison with other genera of Hysteropterini. However, the combination of the following characters separates the specimen from all other Hysteropterini as a new species in a new genus: the late forking of ScP+R, the late fusion of Pcu and A1, and the particularly elongated tegmen with a narrow postcostal cell.
Although the entomofauna of the Paleocene volcano-sedimentary maar of Menat (Puy-de-Dôme, France) is very diverse and rich in insect fossils (Wedmann et al. 2018), only a few planthoppers have been described in Cixiidae (Szwedo et al. 2006) and in Lophopidae (Stroiński & Szwedo 2012), but no Issidae have yet been documented. Moreover, issid fossils remain rare, as only four of them have been described to date and with an unequivocal placement among the Issidae. The discovery of the oldest fossil of Issidae and the fi rst one of the tribe Hysteropterini is therefore notable. Particularly, it allows dating for both the Issidae and the Hysteropterini from at least the Paleocene, discarding the hypothesis of Issidae as an Eocene lineage (Gnezdilov 2013b).
The age of the Menat beds was previously reported as Thanetian (58.7-55.8 Ma) (Kedves & Russel 1982; PaleoBioDB 2019) based on basalts associated with the Menat maar. Since then, it has been re-evaluated as being Selandian (61-60 Ma) based on magnetostratigraphy (Wappler et al. 2009;Mayr et al. 2019). This age is in agreement with the results of a molecular calibration analysis (Bourgoin et al. 2018) that Charcoal occurs in almost all horizons in the Menat beds, which testifi es to the frequent occurrence of wildfi res during the Paleocene in the vicinity of the Menat paleolake (Wedmann et al. 2018), supporting a subtropical climate with dry and wet seasons. Such a palaeoenvironment is in accordance with current environmental characteristics of Issidae: Hysteropterini being particularly well distributed in circum-Mediterranean dry habitats.  Zhao et al. (2019) and labelled with molecular calibration data from Bourgoin et al. (2018). Red dots refer to the fossils known with their minimal age according to their deposit.