The Middle Triassic palaeontomofauna of Monte San Giorgio with the description of Merithone laetitiae (†Permithonidae) gen. et sp. nov.

The Triassic is considered a crucial period for the establishment of the modern insect fauna and fossil records from this period are fundamental for understanding the real impact that the end Permian Mass Extinction events had on these animals. Here, we review the insect fossils from one of the main deposits of this period in the world, Monte San Giorgio, which is considered one of the nine main insect Fossillagerstätten. In this Lagerstätte, located on the border between Switzerland and Italy, a total of 273 fossil insects have been collected in five localities. The fossils found in Val Mara site D, one of the two richest insect fossils sites of Monte San Giorgio, present peculiar features, such as extraordinary sizes and phosphatisation of internal tissues revealing fine internal details. In contrast, the Val Mara site VM 12 fossil record (248 specimens) is dominated by small to medium size insects, usually almost intact, preserving details such as setae on wings and compound eyes. Besides these exceptional features, these fossil insects are of extreme evolutionary importance, since they represent the first or the last occurrence for their lineage. In this regard, their use to calibrate nodes in a phylogenomic dating analysis led to backdating the origin of many insect lineages, including Diptera and Heteroptera. Up to now, a total of five species from Monte San Giorgio have been formally described, belonging to the orders Archaeognatha (†Monura and Machilidae), Ephemeroptera, Hemiptera (Tingidae) and Coleoptera (Adephaga). A further species, Merithone laetitiae (†Permithonidae) gen. et sp. nov., whose fossil is included among the recent findings in Val Mara site VM 12, is described in the present work.