The evolutionary history of body organisation in the lineage towards modern scorpions

Abstract

Euarthropoda is extremely successful in evolutionary terms. Part of this success has been attributed to one evolutionary “strategy”: the stem species of Euarthropoda had a body with numerous segments, each of these segments bearing a pair of appendages, all of these subsimilar (e.g. Maas & Waloszek 2001; Haug J.T. et al. 2013, fig. 2.3.b, and references therein). Different lineages of Euarthropoda varied this ancestrally uniform body. Several adjacent segments were modified in groups, forming then functional units for specific needs. Such functional units, tagmata, may perform sensory functions, locomotion, feeding, respiration or other tasks. With this background we should expect that tagmosis, the subdivision of the body into several functional units, evolves within the different lineages of Euarthropoda, leading to very different patterns of body organisation between the different lineages as well as within one lineage (partly this morphological diversity or disparity appears to be a result of developmental plasticity, see e.g. Moczek 2010, Moczek et al. 2011, Minelli 2016 and references therein). Yet, in many lineages the pattern appears to be fixed already quite early within a lineage. For example, Euchelicerata, the group including spiders, scorpions and all their relatives, is generally thought to have a stereotypic tagmosis pattern. The ocular segment and post-ocular segments 1–6 are supposed to form the so-called prosoma; post-ocular segments 7–19 supposedly form the opisthosoma (see Dunlop & Lamsdell 2017 for a recent discussion). However, when looking closely at many eucheliceratan ingroups this is not quite that obvious or at least not as simple as often stated (see e.g. discussion in Haug C. et al. 2012a). Here we want to consider the evolution of the tagmosis in modern scorpions. Generally, modern scorpions have been considered to be organised into three tagmata: the prosoma, the mesosoma and the metasoma, the latter two representing subdivisions of the opisthosoma. The prosoma supposedly includes, as mentioned above, the ocular segment and post-ocular segments 1–6. These segments are supposed to dorsally form the prosomal shield. Ventrally, (proximal portions of) appendages of post-ocular segments 1–4 form the feeding apparatus. Appendages of post-ocular segment 1, the chelicerae, are small pincers that can squash