Pangusyndicus gen. nov.: a new mid-Cretaceous scydmaenine with reduced antennae and prothoracic gland (Coleoptera, Staphylindiae: Scydmaeninae)

The ant-like stone beetle tribe Glandulariini harbours approximately 3700 species, representing more than 70% of the extant diversity of the subfamily Scydmaeninae. Fossil records of this large tribe are rare, and are mostly confined to Eocene to Miocene amber, whereas only four genera were known from Mesozoic deposits. Here we report a new Mesozoic taxon, Pangusyndicus excavatus Yin, Zhou & Cai gen. et sp. nov., from Cenomanian Burmese amber. For the first time the ventral cephalic structures of an extinct Glandulariini are observable, a character system usually difficult to discern, especially in fossil scydmaenines. Pangusyndicus clearly lacks lateral sulci on the submentum, a critical character state that justifies the placement of Pangusyndicus in one of the two assumedly ‘natural groups’ of Glandulariini. Pangusyndicus displays strongly reduced antennomeres XI, similar to those found only in the extant Australo-Oriental genus Syndicus Motschulsky. To test the phylogenetic position of Pangusyndicus, cladistic analyses were performed based on morphological data. Both parsimonious and maximum likelihood cladograms recovered a close relationship between Pangusyndicus, Syndicus and Loeblites Franz, indicating the unique reduction of apical antennomeres in this group may have originated once, and by the mid-Cretaceous at the latest. More strikingly, Pangusyndicus possesses a large cavity on the mediobasal surface of the pronotal disc, a feature previously unknown among all scydmaenines. We interpret this as a specialized glandular structure, although its function remains uncertain. The presence of such a cavity in Pangusyndicus represents the earliest known presumably glandular structure in Staphylinidae. Moreover, Pangusyndicus and several recently described mid-Cretaceous scydmaenines belonging to various tribes exhibit notably longer legs than their extant relatives, indicating shifts in habitat and prey preferences may have occurred during the evolution of these groups.http://zoobank.org/urn:lsid:zoobank.org:pub:11529A5C-DAF3-4AD9-A074-10E937CE75CF