The mid-Cretaceous Lepiceratus gen. nov. and the evolution of the relict beetle family Lepiceridae (Insecta: Coleoptera: Myxophaga)

摘要

A newly discovered adult beetle †Lepiceratus ankylosaurus gen. & sp. nov. (mid-Cretaceous Myanmar amber) is the first extinct member of the suborder Myxophaga to reveal fine morphological structures, studied by synchrotron microtomography. A comprehensive phylogenetic analysis places it as a stem group in the extant relict family Lepiceridae. The fossil (along with previously known taxa) demonstrates that lepicerids with seven and five antennomeres co-existed during mid-Cretaceous, and only forms with extremely reduced, pentamerous antennae survived till present. The hitherto known fossil record of the enigmatic myxophagan beetles and phylogenetic reconstructions demonstrate that the reduction of antennomeres (i.e. antennal anarthrogenesis) from the ancestral 11 to seven took place as early as the Triassic, when the ancestral lineage of Myxophaga split into the 'lepicerid lineage' and an ancestor of all remaining families. Furthermore, the anarthrogenesis in the 'Lepicerus lineage' had been completed by the mid-Cretaceous. As all remaining morphological structures that define extant Lepicerus had differentiated by then, the extreme reduction of antennae to merely five antennomeres was the last major morphological transformation in their evolution. Morphological adaptations to protect body appendages (presumably against predators) also remained unchanged since the mid-Cretaceous, indicating a similar environmental pressures during this nearly 100 Ma-long period of lepicerid evolution.