A new Eocene genus of ant-like stone beetles sheds new light on the evolution of Mastigini Pawel Jaloszyński Museum of Natural History, University of Wroclaw, Sienkiewicza 21, 50-335 Wroclaw, Poland 〈scydmaenus@yahoo.com〉 Abstract.—Fossil Scydmaeninae beetles are exceptionally poorly known and those described usually lack important details to reliably analyze their phylogenetic relationships with extant taxa. Baltostigus n. gen. is the first extinct ant-like stone beetle taxon unambiguously assigned to the tribe Mastigini. It includes B. antennatus n. sp. (the type species of Baltostigus) and B. horribilis n. sp., from the lower to middle Eocene amber of Poland and Lithuania, respectively. Results of a phylogenetic analysis comprising morphological characters of all extant and extinct genera of the supertribe Mastigitae strongly support the placement of Baltostigus as a sister group to all remaining Mastigini. The new genus shows character states not known in any extant Mastigini: fully developed hind wings, prominent humeral calli, deep elytral punctures partly arranged in longitudinal rows and symmetrical aedeagal parameres. These features suggest that Mastigini might have evolved from forms morphologically similar to small-bodied extant Clidicini of the ‘Leptochromus lineage.’ Introduction The beetle family Staphylinidae, or rove beetles, is the largest animal family, currently comprising over 55,000 nominal species (Grebennikov and Newton, 2009). Scydmaeninae, or ant-like stone beetles, is a large and diverse evolutionary lineage within staphylinids, with nearly 4,900 species (Grebennikov and Newton, 2009). These minute beetles have constituted a separate family of Staphylinoidea for nearly 200 years, and only recently Grebennikov and Newton (2009), based on a compre- hensive phylogenetic analysis, included scydmaenines in the megadiverse rove beetles. Scydmaeninae is currently divided into four supertribes, which are highly unequal in terms of species richness. The extinct Cretaceous Hapsomelitae is ill-defined and comprises only several enigmatic species known from Burmese amber (Poinar and Brown, 2004; Chatzimanolis et al., 2010); their phylogeny, relationships and systematic position remain unclear. Mastigitae is also a small group of less than a hundred species classified into nine genera, and internal relationships of this group were recently studied, with not entirely conclusive results (Jaloszyński, 2012a). Cephenniitae includes three tribes with 26 genera and less than 500 species; the phylogeny of this lineage and its well-supported monophyly was a subject of recent studies (Jaloszyński, 2012b, 2014). Scydmaenitae, after recent exclusion of tribes now forming the Cephenniitae, comprises five tribes with 65 genera and ca. 4400 species; this is the largest supertribe, but also the least studied and most problematic in terms of evolutionary relation- ships between genera. Many genera of Scydmaenitae, especially those of the largest tribe Glandulariini (= Cyrtoscydmini), require revision. The monophyly of Scydmaenitae was recently challenged, as the tribe Scydmaenini was placed as a sister group of Mastigitae (O’Keefe, 2005; Jaloszyński, 2012b, 2014). Mastigitae is an intriguing group because of its distribution, morphological structures and biology. It includes the largest known Scydmaeninae, reaching the body length of 10 mm, which is a gigantic size for a subfamily in which the majority of species do not exceed 2 mm and the smallest forms do not reach 0.5 mm. In addition, a diurnal and exposed mode of life is common among some tribes of Mastigitae, while nearly all other members of Scydmaeninae are cryptic beetles living in leaf litter, soil, rotten wood, and other moist substrates, or within ant and termite colonies (O’Keefe, 2005). Genera of Mastigitae share the following characters: antennal scape strongly elongate, much longer than pedicel; antennae distinctly geniculate between scape and pedicel (distal part of antenna bent ventrally); labial palpomere III pointed; maxillary palps longer than head with palpomere IV either larger than III or smaller, but then strongly asymmetrical and never dome-shaped; and eyes located in the anterior part of the head (O’Keefe et al., 1997; O’Keefe, 2005; Jaloszyński, 2012a). This supertribe is currently divided into three tribes. Leptomastacini includes small (2–3 mm), flattened and often lightly pigmented beetles associated with leaf litter or deeper soil layers, occurring mostly in northern and north-eastern Mediterranean subregion and in the Balkans. Clidicini comprises beetles of various lengths, from 1.57 to 10 mm, with three genera showing disjunctive distributions: Clidicus Laporte occurs in SE Asia and N Australia, Leptochromus Motschulsky in Central and South America, and Papusus Casey in SW United States and Mexico (Jaloszyński, 2012a). The placement of Papusus in Clidicini remains problematic (Jaloszyński, 2012a). This tribe is also most diverse in terms of the biology: small, light brown, short- legged and flattened species of Papusus live under stones on arid deserts; moderately large, light brown, long-legged and strongly convex species of Leptochromus are associated with Journal of Paleontology, 89(6), 2015, p. 1056–1067 Copyright © 2016, The Paleontological Society 0022-3360/16/0088-0906 doi: 10.1017/jpa.2015.75 1056