Mouthpart morphology and feeding structures in the palaeocharinid trigonotarbids of the Rhynie chert: insights from comparisons to modern arachnids

Trigonotarbida, an extinct order of spider‐like arachnids, were significant predators between the late Silurian and early Permian. Characterized by their segmented opisthosoma, clasp‐knife chelicera, and paired book lungs, they played a pivotal role in the formation of Early Devonian terrestrial ecosystems. However, the compression‐fossil or mould preservation of most trigonotarbids has been a limiting factor in understanding their fine morphology. Here, we re‐examine the mouthparts of Palaeocharinus, a trigonotarbid genus from the c. 408 Ma Rhynie chert of Scotland. Rhynie preserves the palaeocharinid trigonotarbids in three dimensions with extraordinary fidelity, offering detailed insights into their anatomy, feeding and hunting behaviours, and phylogenetic position. Here we present the first confocal laser scanning microscopy (CLSM) data for Palaeocharinus using three‐dimensional modelling to reconstruct the morphology of a tiered filtration apparatus, comprising a coarse outer mesh of interlacing plumose setae and a fine inner filter of pinnate setae. Together with the clasp‐knife action of the chelicera and mastication by the cheliceral teeth, Palaeocharinus emerges as a sophisticated terrestrial predator with a feeding mechanism resembling that of extant representatives of Pedipalpi (Amblypygi + Uropygi + Schizomida). Phylogenetic analyses with new and modified mouthpart characters affirm the placement of Trigonotarbida within Pantetrapulmonata (Aranaeae + Pedipalpi). This study provides insights into the evolutionary innovations that facilitated the diversification and proliferation of trigonotarbids in early terrestrial landscapes.