The Complex Structure of the Cuticle of Pseudagnostus (Agnostina, Trilobita?)

The cuticle of Pseudagnostus ex gr. cyclopyge, a member of Agnostina, which is currently conditionally considered as an arthropod stem lineage, was studied using SEM. It has been believed so far that the agnostoid cuticle is simple and single-layered, differing in its primitive structure from other arthropods, both modern and extinct. However, our results showed that at least in this species, the cuticle has a very complex structure. Their carapace consisted of three layers, and each layer was characterized by its own features. The outer layer, which was a combination of a thin upper prismatic unit and a thick principal unit composed of relatively loosely packed lamellae, overlies the transitional layer. The transitional layer consists of stacks of thin lamellae, directed differently in each stack. On the thin inner layer there were numerous pits with a central element (peg). The peg pits create a specific profile of the inner layer, although the functions of these peg pits are not clear. The inner layer is underlain by a basal matrix of rhomboid fibrillar mesh possibly of chitin-protein nature. Specific cuticular sensilla were found both on the outer and inner layers. On the glabellar culmination there is a V-shaped groove with two rows of digitiform sensilla in the V-branches; campaniform sensilla were located along the symmetry axis between the branches. This sensory field apparently performed a mechanosensory function responding to currents outside an enrolled animal. Given the complexity of the integument of this species, further comparison with other agnostoid species, as well as with the cuticle of other arthropods, is needed. Such a comparison would be promising for understanding the phylogenetic position of Agnostina, as well as the evolution of the arthropod exoskeleton.