Chelicerategrade

Chelicerategrade is a hypothetical evolutionary grade proposed within the arthropod phylum. This term refers to a conceptual grouping of arthropods that share certain derived characteristics, primarily related to the structure and function of their anterior appendages. The defining feature of chelicerategrade is the presence of chelicerae, which are typically pincer-like or fang-like appendages located at the mouthparts. These are thought to have evolved from modified appendages in an ancestral arthropod lineage.

Organisms within the chelicerategrade are generally considered to be characterized by the absence of antennae, a feature present in many other arthropod groups like crustaceans and insects. Instead, the first pair of appendages in chelicerategrade animals are the aforementioned chelicerae. Following the chelicerae, other appendage pairs, such as pedipalps and walking legs, are also present.

This evolutionary grade is primarily associated with the subphylum Chelicerata, which includes well-known groups such as arachnids (spiders, scorpions, mites, ticks), horseshoe crabs, and sea spiders. The concept of chelicerategrade helps to unify these diverse but related groups under a common evolutionary framework, highlighting key morphological innovations that occurred early in their divergence from other arthropod lineages. While the term is descriptive of a shared set of traits, it is important to note that its precise phylogenetic standing and inclusion of all potential taxa continue to be subjects of research and discussion within the field of arthropod systematics.