Lower Ordovician (Arenig) – Middle Devonian
Phacopida is an extinct order of fast-moving, low-level epifaunal carnivores.
Characteristics of the Order
- Glabella with deeply impressed grooves; frequently expands or tapers forward.
- Short to no preglabellar field.
- Can have rostral plates, but not all do.
- Thorax contains 8 to 19 segments.
- Pygidium is typically medium to large, except in early representatives.
- Variable facial sutures.
Treatise on Invertebrate Paleontology (1959):
- An order of post-Cambrian trilobites probably derived from the Ptychopariida. Cephalon with facial sutures typically proparian (most Phacopina and Cheirurina) or gonatoparian (most Calymenina, some Cheirurina), but may be opisthoparian (some Calymenina and Cheirurina) or even lacking (some Cheirurina); glabella variously shaped, commonly expanding forward (Phacopina, most Cheirurina) or tapering forward (Calymenina, some Cheirurina), lateral glabellar furrows (if present) variously developed; preglabellar field short (sag.) or lacking; rostral plate present (Calymenina, most Cheirurina) or lacking (Phacopina, some Cheirurina). Thorax with 8 to 19 segments. Pygidium mostly medium to large, but small in some exceptional early representatives. Known protaspides of similar type.