Cuticle ultrastructure of the Early Devonian trigonotarbid arachnid Palaeocharinus

The cuticle is a key evolutionary innovation that played a crucial role in arthropod terrestrialization. Extensive research has elucidated the chemical and structural composition of the cuticle in extant arthropods, while fossil studies have further informed our understanding of cuticle evolution. This study examines the three-dimensionally preserved cuticular structure of the Early Devonian trigonotarbid arachnid genus Palaeocharinus, from the Rhynie chert of Scotland (∼408 Ma). Trigonotarbids, an extinct group of tetrapulmonate arachnids, are among the earliest known unequivocally terrestrial arthropods, and thus may shed light on the evolution of terrestriality. Using high-resolution Confocal Laser Scanning Microscopy (CLSM), we reveal detailed morphological features at the nanometre level. The external cuticle surface of Palaeocharinus is characterized by polygonal scales, sensilla, and small pores identified as the openings of dermal glands and wax canals. Internally, the cuticle exhibits polygonal clusters of pore canals, through which wax was transported from the epidermis to the cuticular surface. The pore canals twist along their vertical axes, reflecting the "twisted plywood" or Bouligand arrangement of chitin-protein microfibril planes characteristic of modern arthropod cuticles. Overall, the cuticle of Palaeocharinus is characteristically thick relative to those of other extinct and extant chelicerates, such thickening being a possible adaptation to terrestrial life.