The miniaturized arachnid order Palpigradi has ambiguous phylogenetic affinities owing to its odd combination of plesiomorphic and derived morphological traits. This lineage has never been sampled in phylogenomic datasets because of the small body size and fragility of most species, a sampling gap of immediate concern to recent disputes over arachnid monophyly. To redress this gap, we sampled a population of the cave-inhabiting species Eukoenenia spelaea from Slovakia and inferred its placement in the phylogeny of Chelicerata using dense phylogenomic matrices of up to 1450 loci, drawn from high-quality transcriptomic libraries and complete genomes. The complete matrix included exemplars of all extant orders of Chelicerata. Analyses of the complete matrix recovered palpigrades as the sister group of the long-branch order Parasitiformes (ticks) with high support. However, sequential deletion of long-branch taxa revealed that the position of palpigrades is prone to topological instability. Phylogenomic subsampling approaches that maximized taxon or dataset completeness recovered palpigrades as the sister group of camel spiders (Solifugae), with modest support. While this relationship is congruent with the location and architecture of the coxal glands, a long-forgotten character system that opens in the pedipalpal segments only in palpigrades and solifuges, we show that nodal support values in concatenated supermatrices can mask high levels of underlying topological conflict in the placement of the enigmatic Palpigradi.

Despite significant advances in invertebrate phylogenomics over the past decade, the higher-level phylogeny of Pycnogonida (sea spiders) remains elusive. This group of arthropods appeared early in the fossil record, with the oldest unambiguous fossils dating to the Silurian. Due to the inaccessibility of some small-bodied lineages, few phylogenetic studies have sampled all sea spider families. In addition, previous efforts based on a handful of genes have yielded unstable tree topologies from one analytical approach to the next. Here, we inferred the relationships of 89 sea spider species using targeted capture of the mitochondrial genome, 56 conserved exons, 101 ultraconserved elements, and three nuclear ribosomal genes. We inferred molecular divergence times by integrating morphological data for fossil species to calibrate 15 nodes in the arthropod tree of life. This integration of data classes resolved the basal topology of sea spiders with high support. The enigmatic family Austrodecidae was resolved as the sister group to the remaining Pycnogonida and the small-bodied family Rhynchothoracidae as the sister group of the Page 3 of 31 robust-bodied family Pycnogonidae. This stable, dated phylogeny of Pycnogonida enables confident polarization of of cephalic appendage loss across pycnogonid families, with the consistent lack of the adult chelifore in a grade of basally diverging lineages. Molecular divergence time estimation recovered a basal divergence of crown group sea spiders in the Ordovician. Comparison of diversification dynamics with other marine invertebrate taxa that originated in the Paleozoic suggests that sea spiders and some crustacean groups exhibit resilience to mass extinction episodes, relative to mollusk and echinoderm lineages.

The scorpion Diplocentrus zacatecamis Hoffmann (1931) was originally described as a subspecies of Diplocentnis keyserlingi Karsch 1880 on the basis of six syntypes and was later elevated to species level. We designate a male lectotype and redescribe the species, including illustrations of the hemispermatophore of a male collected near the type locality. In this genus, the hemispermatophore is poorly sclerotized and lacks elaborate capsular structures, which are taxonomically useful in other genera. We review the variability in the hemispermatophores of males from one population, including five comparisons of the right and left hemispermatophores of the same males. Our results showed asymmetry in the length of the right and left hemispermatophores of the same individual. We also observed the presence of "crenulations" or "spines" in two different hemispermatophores (not complementary ones). We conclude that caution should be used when describing the hemispermatophore of only one male and considering it as diagnostic for the species, because of the high levels of intraspecific variation.