Homalozoa

Homalozoa is a historical, informal grouping used in the study of early echinoderms to describe a collection of Paleozoic forms with body plans that differ markedly from the later, more recognizable echinoderm radiations. The name derives from Greek roots meaning “even” or “flat” animals, and the term was applied to organisms that lack the conspicuous pentaradial symmetry seen in crown-group echinoderms. Members of Homalozoa display a variety of shapes, many with bilateral or irregular forms, and they often possess simplified calcareous skeletons.

Historically, the group included stylophorans—such as cornutes and mitrates—as well as other enigmatic, often small-shelled echinoderms that occupied a stem or transitional position relative to the major echinoderm lineages. Membership varied among authors, reflecting a practical labeling rather than a tightly defined evolutionary clade. The usefulness of the concept lay more in highlighting deep, early experimentation in echinoderm body plans than in conveying a natural phylogenetic unit.

In contemporary systematics, Homalozoa is largely considered obsolete as a formal clade because it does not represent a monophyletic grouping. Modern analyses place the included forms in different echinoderm lineages, with stylophorans generally treated as echinoderms near the base of the crown group or as stem echinoderms. As a result, the term is rarely used in current classifications and is typically described as historical.

Despite being retired as a formal group, the concept of Homalozoa helped illuminate the diversity and evolutionary experimentation of early Paleozoic echinoderms, illustrating the range of morphologies that appeared during the Cambrian to Devonian.