The mid-Cretaceous †<i>Lepiceratus</i> Gen. nov. and the Evolution of the Relict Beetle Family Lepiceridae (Insecta: Coleoptera: Myxophaga)

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. https://zoobank.org:pub:28DCD51D-90EA-4516-810E-3B898DE5A729本次发现的产自白垩纪中期缅甸琥珀的成虫甲虫新属新种†甲龙细扁泥甲（Lepiceratus ankylosaurus gen. et sp. nov.），是扁泥甲亚目（Myxophaga）首个被揭示精细形态结构的灭绝类群，相关研究采用了同步辐射显微断层扫描（synchrotron microtomography）技术。综合系统发育分析结果显示，该物种隶属于现生孑遗类群细扁泥甲科（Lepiceridae）的干群。 该化石结合此前已知类群的研究数据表明，拥有7节触角与5节触角的细扁泥甲科类群在白垩纪中期曾共存，而仅触角极度简化的五节触角类群延续至今。 目前已发现的神秘扁泥甲亚目甲虫化石记录与系统发育重建结果证实，触角节从祖先的11节退化（antennal anarthrogenesis）至7节的过程最早可追溯至三叠纪，彼时扁泥甲亚目的祖先支系分化为"细扁泥甲支系"与其余所有现生科的共同祖先。 此外，"细扁泥甲支系"的触角节退化过程在白垩纪中期已完成。 由于定义现生细扁泥甲属（Lepicerus）的所有其余形态结构均已在该时期完成分化，触角简化至仅5节触角是该类群演化历程中最后一次关键的形态变革。 其用于保护身体附肢（推测用于抵御捕食者）的形态适应性特征自白垩纪中期以来便未发生改变，这表明在细扁泥甲科近1亿年（100 Ma）的演化历程中，环境选择压力始终保持相似。 https://zoobank.org:pub:28DCD51D-90EA-4516-810E-3B898DE5A729