Harvestmen (Arachnida: Opiliones) from Palaeogene European and mid-Cretaceous Burmese ambers as a model for evolution, palaeobiogeography and palaeoecology

Abstract

Harvestmen (Arachnida: Opiliones) represent the third most diverse order of arachnids following spiders (Araneae) and mites (Acari). Despite their modern species richness and importance for various ecosystems, fossils of this group are largely understudied. This also includes their evolutionary history, palaeobiogeography and palaeoecology. 156 harvestmen fossils were available for this thesis, which are mostly preserved as inclusions in Baltic (Late Eocene, Priabonian) and Burmese amber (mid-Cretaceous, Cenomanian-Albian). Besides the description of new species, the main focus was to answer two palaeobiogeographical hypotheses: Is the Burmese fauna of Laurasian or Gondwanan origin? Does the European amber complex sample a Holarctic fauna? The fossils were mainly examined with classical light microscopy but also with the help of micro-CT scans and then compared with their living relatives available in the literature and the collection of the Museum für Naturkunde, Berlin. 11 new fossil harvestmen species and 12 additional specimens from all four suborders (Laniatores, Eupnoi, Dyspnoi, Cyphophthalmi) are described in this thesis. Of particular interest is the discovery and description of Sirocellus iunctus (Cyphophthalmi) from Burmese amber, the first dwarf harvestmen which combines characters from the families Sironidae and Stylocellidae, and thus represents a “missing link” between these two families. Based on this fossil, previous hypotheses regarding a Gondwanan origin of the Burmese amber fauna could be reconstructed and confirmed. Tyrannobunus aculeus (Eupnoi) is the first fossil of its suborder from Burmese amber and stands out because of its unusual body characters. Large eyes, a small but strongly armed body, spined legs and pedipalps and an undifferentiated penis indicate an early origin in the Eupnoi tree of life. Its very thin cuticle shows additionally an adaptation to humid and warm tropical climates. Balticolasma wunderlichi represents the first fossil from the subfamily Ortholasmatinae (Dyspnoi) and was described as a male from the Baltic and as a female from the Ukrainian Rovno amber. These ortholasmatine harvestmen only occur in North-Central America and in parts of Asia these days. This record shows once again that some groups of harvestmen were once widespread throughout the Holarctic during the Eocene and subsequently became extinct due to decreasing temperatures, at least in Europe. The harvestmen fossils could be placed in the appropriate palaeoenvironmental context based on known reconstructions of the palaeoenvironment for the Burmese and European amber forests and on analogues to their extant relatives. The described fossil harvestmen species fit well in the proposed warm-temperate Baltic amber forests with a number of swamp and open habitats, and in the very humid and warm tropical Burmese amber forests.

All newly described fossils represent an important foundation for future phylogenetic studies, especially in the form of calibration points, which allows to calculate the origin of the individual groups more precisely. This in combination with the description of new fossils from other unexplored amber sources (e.g. Lebanese amber, African amber, Spanish amber) will further increase our knowledge of the evolution and palaeobiogeography of these animals.