A fossil trigonotarbid arachnid with a ricinuleid-like pedipalpal claw

Abstract

A fossil trigonotarbid (Arachnida: Trigonotarbida) assigned to Palaeocharinus sp. from the Early Devonian (c. 410 Ma) Rhynie cherts of Aberdeenshire, Scotland, UK is described, specifically for a previously unrecognised feature of the distal end of the pedipalp. This exhibits a small chela formed from a movable, unpaired apotele articulating against a slightly shorter, fixed projection from the inferior surface of the tarsus. Among other arachnids, this morphology has only previously been observed in the rare and enigmatic Carboniferous-Recent taxon Ricinulei. This character offers explicit support for a monophylum (Trigonotarbida + Ricinulei); a hypothesis previously based on opisthosomal characters which (if correct) would draw Ricinulei within the so-called Pantetrapulmonata clade (i.e. spiders and their closest relatives). Our data is not consistent with Ricinulei's traditional position as sister-group of mites (Acari) and is an example of how considering extinct taxa may help to resolve the position of a 'difficult' living group. © Springer-Verlag 2009.