Tactopoda

Tactopoda is a proposed clade within the arthropods. It is a hypothesis concerning the evolutionary relationships of chelicerates (spiders, scorpions, mites, and related organisms) and the extinct trilobites with the mandibulates (myriapods, crustaceans, hexapods (insects and their relatives)). The name "Tactopoda" refers to the sensory appendages proposed to be present in the last common ancestor of these groups.

The Tactopoda hypothesis suggests that chelicerates and mandibulates are more closely related to each other than either is to other arthropod groups, such as pycnogonids (sea spiders). Support for Tactopoda comes primarily from morphological similarities, particularly in the structure and function of the anterior-most appendages and nervous system.

However, the monophyly of Tactopoda is still debated. Molecular phylogenetic studies, which analyze DNA and RNA sequences, have produced conflicting results, with some supporting and others contradicting the Tactopoda hypothesis. Alternative hypotheses propose different arrangements of arthropod phylogeny, such as Mandibulata (grouping myriapods, crustaceans, and hexapods together) or Myriochelata (grouping chelicerates and myriapods).

The evolutionary relationships within Arthropoda remain an area of active research, and the validity of the Tactopoda hypothesis continues to be investigated using both morphological and molecular data. More data and more rigorous analyses are needed to resolve the phylogenetic relationships among these diverse and important groups of animals.