trilobitomorphs

Trilobitomorpha is a term used in paleontology to refer to a clade or grade within Arthropoda that encompasses trilobites and a number of closely related Cambrian and Paleozoic arthropods. The exact circumscription has varied among authors, and in modern systematics the name is not universally adopted as a formal clade. In general, trilobitomorphs are characterized by a calcified, dorsoventrally flattened exoskeleton composed of a cephalon (head) and trunk segments, with a laterally expanded thorax and a pygidium in many lineages; many members display a tripartite segmentation of the head, and a mosaic of segment boundaries across the body. The group includes the iconic trilobites (class Trilobita) and a variety of related artiopodan lineages that share a trilobite-like body plan, especially from the early to middle Paleozoic.

The fossil record of trilobitomorphs is rich, beginning in the early Cambrian and continuing through much of the Paleozoic. They inhabited a range of marine environments, from shallow shelves to deeper waters, and displayed diverse life modes, including benthic detritivory, scavenging, and predation. The lineage is important for understanding early arthropod evolution and the Cambrian explosion.

Phylogenetically, trilobitomorphs are often treated as a stem group or paraphyletic assemblage relative to trilobites, with different authors placing Trilobita within a broader Artiopoda. Because of this variation, Trilobitomorpha is less often used as a strict, universally accepted clade in contemporary phylogenetic analyses, which typically prefer explicit cladistic definitions.