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Superregnum: Eukaryota
Cladus: Unikonta
Cladus: Opisthokonta
Cladus: Holozoa
Regnum: Animalia
Subregnum: Eumetazoa
Cladus: Bilateria
Cladus: Nephrozoa
Cladus: Protostomia
Cladus: Ecdysozoa
Cladus: Panarthropoda
Phylum: Arthropoda
Subphylum: †Trilobitomorpha
Classis: †Trilobita
Ordo: †Phacopida
Subordines: †Calymenina – †Cheirurina – †Phacopina

Phacopida Salter, 1853

Phacopida ("lens-face") is an order of trilobites that lived from the Late Cambrian to the Late Devonian. It is made up of a morphologically diverse assemblage of taxa in three related suborders.
Reconstruction of Phacops (Phacopidae, left) and Walliserops (Acastidae, right)

Phacopida had 8 to 19 thoracic segments and are distinguishable by the expanded glabella, short or absent preglabellar area, and schizochroal (Phacopina) or holochroal (Cheirurina and Calymenina) eyes. Schizochroal eyes are compound eyes with up to around 700 separate lenses. Each lens has an individual cornea which extended into a rather large sclera.

The development of schizochroal eyes in phacopid trilobites is an example of post-displacement paedomorphosis. The eyes of immature holochroal Cambrian trilobites were basically miniature schizochroal eyes. In Phacopida, these were retained, via delayed growth of these immature structures (post-displacement), into the adult form.

Eldredgeops rana (Phacopidae) and Dalmanites limulurus (Dalmanitidae) are two of the best-known members of this order. Other known phacopids include Cheirurus (Cheiruridae), Deiphon (Cheiruridae), Calymene (Calymenidae), Flexicalymene (Calymenidae) and Ceraurinella (Cheiruridae).

The origin of the order Phacopida is uncertain. It comprises three suborders (Phacopina, Calymenina, and Cheirurina) which share a distinctive protaspis (unsegmented larva ) with three pairs of spines on its body. The Cheirurina and Calymenina retain a rostral plate (an apomorphy) but in virtually all Phacopina the free cheeks are yoked as a single piece. This sort of similarity in development suggests phylogenetic unity. The suborder Calymenina is the most primitive of the Phacopida order and shares some characteristics with the order Ptychopariida, though it is not included in the subclass Librostoma.

Phacopida was one of only two trilobite orders (along with the Proetida) to survive the Kellwasser event (Late Devonian extinction) at the Frasnian-Famennian boundary. However, they would die out soon afterwards at the Hangenberg event (end-Devonian extinction) at the end of the Famennian.[2]

Suborder Calymenina
Family Bathycheilidae
Family Bavarillidae
Family Calymenidae
Family Homalonotidae
Family Pharostomatidae
Suborder Cheirurina
Family Cheiruridae
Family Encrinuridae
Family Pilekiidae
Family Pliomeridae
Suborder Phacopina
Superfamily Acastoidea
Family Acastidae
Family Calmoniidae
Superfamily Dalmanitoidea
Family Dalmanitidae
Family Diaphanometopidae
Family Prosopiscidae
Superfamily Phacopoidea
Family Phacopidae
Family Pterygometopidae

Stereo image
Left frame
Right frame
Parallel view ()
Cross-eye view ()
Image of phacopidan trilobite Calymene tristani in anodule.

J.M. Adrain (2014). "20. A synopsis of Ordovician trilobite distribution and diversity". In Harper, D.A.T.; Servais, T. (eds.). Early Palaeozoic Biogeography and Palaeogeography (PDF). Memoirs of the Geological Society of London. Vol. 38. Geological Society of London. p. 490. ISBN 978-1862393738. Archived from the original (PDF) on 2018-10-04. Retrieved 2016-03-27.
Bault, Valentin; Balseiro, Diego; Monnet, Claude; Crônier, Catherine (2022). "Post-Ordovician trilobite diversity and evolutionary faunas". Earth-Science Revie


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