Consensus and conflict in studies of chelicerate fossils and phylogeny

Recent arachnid phylogenies support an Arachnopulmonata clade including scorpions, pseudoscorpions and the tetrapulmonate arachnids (i.e. spiders and their closest relatives). The position of the other arachnids is less certain, with molecular data suggesting that Arachnida may be paraphyletic with respect to horseshoe crabs. Here we explore the potential role fossil data can play in this debate. We outline the beneficial aspects of including fossils in phylogenies - fossils calibrate trees to time - as well as the challenges integrating these data. We tabulate the oldest occurrences of all major groups and superimpose these on recent phylogenetic hypotheses. Given that a key question is when (and how often) arachnids moved from water onto land, we review the early plant fossil record as a framework for when arthropod life on land may have been viable. In light of the aquatic ecology of Horseshoe crabs, we then discuss the implications of placing this group within the arachnids, especially since some extinct lineages differ substantially from living species. In this context we re-assess a horseshoe crab is from a palaeontological perspective, and speculate that some traditional Xiphosura fossils may actually lie on other parts of the euchelicerate tree. The oldest unequivocal horseshoe crabs are Ordovician in age (ca. 480 Ma), and probably predate terrestrial ecosystems. We conclude that recent phylogenetic results are best reconciled with fossils by inferring multiple terrestrialization events, possibly involving quite different approaches to breathing air. The lung-bearing (arachnopulmonate) branch of the tree is well resolved. Future work should focus on the apulmonate arachnids, and integrate the various early horseshoe-crab-like fossils into chelicerate phylogeny.