panarthropod

Panarthropoda is a major group of animal phyla that includes arthropods, tardigrades, and onychophorans. These phyla are united by several shared characteristics, although the exact phylogenetic relationships between them are still a subject of ongoing research. The name Panarthropoda literally means "all joints," referring to a key feature of many of its members.

Arthropods are the most diverse and widespread group within Panarthropoda, characterized by a segmented body, an exoskeleton made of chitin, and jointed appendages. Insects, arachnids, crustaceans, and myriapods are all examples of arthropods. Tardigrades, often called water bears or moss piglets, are microscopic invertebrates known for their remarkable resilience to extreme environmental conditions. Onychophorans, commonly known as velvet worms, are soft-bodied, segmented worms with unjointed legs.

Key synapomorphies, or shared derived characteristics, that define Panarthropoda are thought to include a segmented body, paired appendages (though these are reduced or absent in some groups), and a cuticle. The evolutionary origin of Panarthropoda is ancient, with fossil evidence suggesting their presence in the Cambrian period. Understanding the relationships within Panarthropoda is crucial for comprehending the evolution of bilaterian animals.