Basal polyphagan beetles in mid-Cretaceous amber from Myanmar: biogeographic implications and long-term morphological stasis

The origin and early evolutionary history of polyphagan beetles have been largely based on evidence from the derived and diverse ‘core Polyphaga’, whereas little is known about the species-poor basal polyphagan lineages, which include Scirtoidea (Clambidae, Decliniidae, Eucinetidae, and Scirtidae) and Derodontidae. Here, we report two new speciesAcalyptomerus thayerae sp. nov. andSphaerothorax uenoi sp. nov., both belonging to extant genera of Clambidae, from mid-Cretaceous Burmese amber.Acalyptomerus thayerae has a close affinity toA. herbertfranzi , a species currently occurring in Mesoamerica and northern South America.Sphaerothorax uenoi is closely related to extant species ofSphaerothorax , which are usually collected in forests ofNothofagus of Australia, Chile, and New Zealand. The discovery of two Cretaceous species from northern Myanmar indicates that both genera had lengthy evolutionary histories, originated at least by the earliest Cenomanian, and were probably more widespread than at present. Remarkable morphological similarities between fossil and living species suggest that both genera changed little over long periods of geological time. The long-term persistence of similar mesic microhabitats such as leaf litter may account for the 99 Myr morphological stasis inAcalyptomerus andSphaerothorax . Additionally, the extinct staphylinoid family Ptismidae is proposed as a new synonym of Clambidae, and its only included speciesPtisma zasukhae is placed as incertae sedis within Clambidae.