Arthropoda, one of the most diverse and abundant groups of animals on Earth, includes subphyla such as Trilobita and Merostomata that have faced extinction.
Trilobita, known commonly as trilobites, are perhaps the most famous of the extinct arthropods. These creatures flourished in the Paleozoic era, with an impressive variety of species, but they disappeared during the Permian-Triassic extinction event about 252 million years ago.
Another subphylum is Merostomata, which includes the horseshoe crabs. While modern horseshoe crabs are not extinct, the group has a long evolutionary history, and some of its ancient relatives, or extinct members, have gone entirely. The extinct Xiphosura is often discussed in this context.
Overall, while many subphyla of arthropods are still thriving, Trilobita is the most notable complete extinction, illustrating the rich tapestry of life and the eventual fate that some groups face in the evolutionary timeline.