Phacopida

Phacopida ("lens-face") is an order of trilobite that lived from the Late Cambrian to the Late Devonian. It is made up of a morphologically diverse group of related suborders.

Phacopida
Temporal range: Furongian – End Famennian, 497–359 Ma[1]
Dalmanites limulurus, a trilobite
of the suborder Phacopina
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: †Trilobita
Order: †Phacopida
Salter, 1864
Suborders

The origin of the Phacopida order is uncertain. It comprises three suborders (Phacopina, Calymenina, and Cheirurina) which share a distinctive protaspis (developmental stage lacking segments) type. 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.

Reconstruction of the phacopids Phacops and Walliserops

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.

Phacops rana and Dalmanites limulurus are two of the best-known members of this order. Other known phacopids include Cheirurus, Deiphon, Calymene, Flexicalymene and Ceraurinella.

Stereo image
Right frame 
Image of Phacopidan Trilobite Calymene tristani in nodule.

References
  1. 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. 38. Geological Society of London. p. 490. ISBN 1862393737.


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