Arachnidae

Arachnidae is a proposed, though not universally accepted, taxonomic rank within the arthropods. It is intended to encompass the class Arachnida and a related, extinct order of arthropods called Trigonotarbida. The primary characteristic that unites these groups is the presence of book lungs as a respiratory organ, or at least a derivation thereof. This distinguishes them from other chelicerate arthropods like horseshoe crabs, which possess book gills. The concept of Arachnidae was put forward to create a more unified classification based on shared respiratory anatomy, potentially reflecting a closer evolutionary relationship between arachnids and trigonotarbids than either group has with other chelicerates. However, the validity and utility of this higher-level grouping are still debated among paleontologists and zoologists. The fossil record for Trigonotarbida is limited, and their precise phylogenetic position remains uncertain, leading some to view Arachnidae as a provisional or speculative taxon. Further fossil discoveries and advancements in molecular phylogenetics may eventually clarify the relationships and inform the acceptance or rejection of Arachnidae as a formal taxonomic unit.