Isopygidan

Isopygidan is a taxonomic term that refers to a group of trilobites. These ancient marine arthropods existed during the Paleozoic Era. The term "isopygid" specifically describes a pygidium, which is the posterior part of the trilobite exoskeleton, that is roughly the same size and shape as the cephalon, the anterior part. This symmetry between the head shield and tail shield is a defining characteristic of isopygid trilobites. While not a formal taxonomic rank itself, it is a descriptive term used by paleontologists to classify and understand the morphology of different trilobite species and superfamilies. Trilobites are a diverse and extinct group, and understanding variations in their pygidial structure, like that found in isopygids, is crucial for their classification and evolutionary studies. The presence of an isopygidial tail plate suggests certain developmental or functional adaptations within these ancient creatures.