The evolution of feeding within Euchelicerata: data from the fossil groups Eurypterida and Trigonotarbida illustrate possible evolutionary pathways

When the evolution of Euarthropoda is discussed, often the lineage of Chelicerata s. str. is assumed to be the more ‘primitive’ or ‘basal’ part of the tree, especially when compared to the other major lineage, Mandibulata. This claimed primitiveness is (at least partly) based on the assumption that different morphological structures are still in an ancestral state and did not evolve any further. One of these sets of structures is the feeding apparatus, which has been stated to be highly advanced in Mandibulata, but not ‘properly’ developed, or at least not to such a high degree, within Chelicerata s. str. In this study, I reinvestigate the feeding apparatus of different ingroups of Euchelicerata, with a focus on assumed ‘primitive’ groups such as Eurypterida and Trigonotarbida. The basis of this study is a large amount of material from different museum collections, with fossils with the entire feeding apparatuses being exceptionally well preserved. Based on high-resolution micro-photography and three-dimensional imaging, it is possible to resolve fine details of the feeding apparatuses. The results make clear that the feeding apparatuses of different ingroups of Euchelicerata are highly specialised and often possess morphological structures comparable to those of the feeding apparatuses of representatives of Mandibulata, apparently convergently evolved. Though the reconstruction of the evolution of the feeding apparatus within Euchelicerata is to a certain degree hampered by unclear phylogenetic relationships, there was clearly a shortening of the feeding apparatus from posterior (i.e. only the anterior appendages being involved in the feeding apparatus), probably linked to the colonisation of land in Arachnida.