| Summary: | Mackenziella psocoides Hammer, 1953 (Collembola: Mackenziellidae) is a widespread but uncommon springtail. Its unusual body shape (ovoid, with partial coalescence of abdominal segments) has puzzled the specialists for a long time, until the discovery of males allowed to relate the species to a family of globular springtails, the Sminthurididae. Yet, the precise phylogenetic position of M. psocoides, and hence of the Mackenziellidae, remained ambiguous. In this work, we report a new locality for M. psocoides in Germany. We provide the first DNA sequences (nuclear ribosomal DNA operon) for the species, as well as the first images using scanning electron microscopy. We investigate its phylogenetic position based on the molecular data and specify details on its morphology. Our results show that M. psocoides is nested inside of Sminthurididae, as the sister group of Sphaeridia Linnaniemi, 1912. Consequently, Mackenziellidae syn. nov. is here synonymized with Sminthurididae. We include Mackenziella and Sphaeridia in the Sphaeridiainae subfam. nov., a replacement name for Sphaeridiinae Richard, 1968 that is a junior homonym of Sphaeridiinae Latreille, 1802 (Coleoptera: Hydrophilidae). Corresponding to its phylogenetic position within Sminthurididae, the evolutionary origin of M. psocoides is younger than previously thought (79 mya +/- 35 my). The lineage accumulated an unusual amount of body modifications involving, among others, the loss of the globular body shape. This rapid rate of evolution is, to our knowledge, unique in springtails. It shows that globular body shape is not an evolutionary dead-end, and the secondary acquisition of a linear body shape and recovery of longitudinal flexibility is still possible.
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