The position of scorpions is particularly controversial. Some early fossils such as the Late Silurian "Proscorpius" have been classified by paleontologists as scorpions, but described as wholly aquatic as they had gills rather than book lungs or tracheae. Their mouths are also completely under their heads and almost between the first pair of legs, as in the extinct eurypterids and living horseshoe crabs. This presents a difficult choice: classify "Proscorpius" and other aquatic fossils as something other than scorpions, despite the similarities; accept that "scorpions" are not monophyletic but consist of separate aquatic and terrestrial groups; or treat scorpions as more closely related to eurypterids and possibly horseshoe crabs than to spiders and other arachnids, so that either scorpions are not arachnids or "arachnids" are not monophyletic. Cladistic analyses have recovered "Proscorpius" within the scorpions, based on reinterpretation of the species' breathing apparatus. This is reflected also in the reinterpretation of "Palaeoscorpius" as a terrestrial animal.