Trilobite Order Phacopida

Phylum Arthropoda
Subphylum Chelicerata
Class Trilobita

    Order Agnostida
    Order Redlichiida
    Order Corynexochida
    Order Phacopida
    Order Lichida
    Order Proetida
    Order Harpetida
    Order Ptychopariida
    Order Asaphida

Trilobite Relatives & Ancestors
   

o Arthropoda
`--o Trilobita
   |?-Agnostida
   |?-Eodiscina
   `--+--o Redlichiida
      |  |--Olenellina
      |  `--Redlichiina
      |--Corynexochida
      |--Phacopida
      |--Lichida
      `--o Librostoma
         |--Proetida
         |--Harpetida
         |--Ptychopariida
         `--Asaphida

Trilobite Order Phacopida (pronounced Phah-co-pee-da) is large and diverse, comprising the related suborders Calymenina, Phacopina, and Cheirurina. Their grouping mainly derives from a shared and differentiating form during the early protaspid larval form period. The Phacopids likely appeared near the base of the Ordovician as Suborder Calymenina. The Calymenina share hypostomal characteristics with Order Ptychopariida, and Phacopida exhibits similar tuberculation with Lichida, confounding the determination of the Phacopids closest sister group.

Phacopids of Suborder Phacopina are the only trilobites to have evolved the incredible schizochroal form of eyes, an instance of appaent evolutionary improvement over the more common holochroal eyes of suborders Calymenina and Cheirurina of the Phacopids, most other trilobites, and generally, most extant arthropods. Holochroal eyes comprise up to 15,000 closely packed hexagonal lenses underneath a single cornea. In contrast, the Schizochroal eyes were far fewer in number (less than about 700), as in juvenile trilobites, in which each lens was a separate calcite spheroid with its own lens, each lens separated by an inter-lens sclera (cuticle), and covered by its own cornea. What survival benefit was accrued by this form of eye? -- correction of spherical aberration. Clarkson and Levi-Setti (1975) determined that the schizochroal eye was unique in that each lens is biconvex and is made of two calcite elements with different refractive indices that were separated by an aspherical surface with an overall affect that the lens corrected for spherical aberration. The development of the schizochroal eyes is but one example in nature of so called post-displacement paedomorphosis, whereby selective pressures, because survival is benefited, results in the retaining of traits previously only existing in juvenile members of a species. The Phacopina thus constitute a notable example that Life remembers its history, with a memory embedded in the genomes. This evolutionary innovation has never been repeated among the Arthropods, though modern arthropods exhibit a myriad of other remarkable and innovative visual systems.

Phylum Arthropoda Subphylum Chelicerata Class Trilobita Order Phacopida Taxonomy: Suborder Calymenina Superfamily Calymenoidea Family Calymenidae Family Pharostomatidae Family Homalonotidae Family Bavarillidae Family Bathycheilidae Suborder Phacopina Superfamily Phacopoidea Family Phacopidae Family Pterygometopidae Superfamily Dalmanitoidea Family Dalmanitidae Family Prosopiscidae Family Diaphanometopidae Superfamily Acastoidea Family Acastidae Family Calmoniidae Suborder Cheirurina Superfamily Cheiruroidea Family Cheiruridae Family Pliomeridae Family Pilekiidae Family Encrinuridae Also see potential Phacopida Phylogeny		Suborder Calymenina
		Flexicalymene retrorsa Family Calymenidae Ordovician Mount Orab, Ohio	Flexicalymene senaria Family Calymendae Ordovician Trenton Group, Quebec
		Flexicalymene meeki Family Calymenidae Ordovician Mount Orab, Ohio	Calymene clavicula Family Calymenidae Middle Silurian Henryhouse Formation Oklahoma
Suborder Phacopina
Huntoniatonia (Huntonia) huntonensis Superfamily Dalmanitoidea Family Dalmanitidae Lower Devonian Haragan Formation, Oklahoma			Eldredgeops rana crassituberculata Superfamily Phacopoidea Family Phacopidae Middle Devonian Silica Shale Formation, Ohio
Hollardops merocristata Superfamily Acastoidea Family Acastidae Devonian Tafilalt, Morocco	Phacops speculator Superfamily Phacopoidea Family Phacopidae Devonian Alnif, Morocco	Walliserops trifurcatus Superfamily Acastoidea Family Acastidae Devonian Foum Ziguid, Morocco	Coltraenia oufatenensis Superfamily Acastoidea Family Acastidae Middle Devonian Alnif, Morocco
Greenops boothi Trilobite Order Phacopida Superfamily Acastoidea Family Acastidae Subfamily Asteropyginae Widder Shale, Ontario, Canada	Dalmanites limulurus Family: Dalmanitidae Middle Silurian Rochester Shale Formation Middleport, New York	Paciphacops birdsongensis Superfamily Phacopoidea Family: Phacopidae Lower Devonian Birdsong Formation, Benton County, Tennessee	Chasmops praecurrens Superfamily Phacopoidea Family Pterygometopidae Middle Ordovician Wolchow river, Russia
Phacops rana norwoodensis Superfamily Phacopoidea Family Phacopidae Devonian Cedar Valley Formation, Johnson County, Iowa	Kainops raymondi Superfamily Phacopoidea Family Phacopidae Lower Devonian Haragan Formation Oklahoma	Acastocephala macrops Superfamily Acastoidea Family Acastidae Middle Silurian Wenlock Edge, Shropshire, England
Suborder Cheirurina
Acanthoparypha gibba Family Cheiruridae Middle Ordovician Wolchow river, Russia	Pliomera fisheri Suborder Cheirurina Family Pliomeridae Middle Ordovician Wolchow river, Russia	Encrinuroides capitonis Family Encrinuridae Ordovician Bromide Formation, Oklahoma	Anacheirurs (Lehua sp.) Family Cheiruridae Ordovician Tanssikhte, Zagora, Morocco
Leviceraurus mammilloides Family Cheiruridae Ordovician Cobourg Formation, Ontario, Canada	Ceraurus globulobatus Family Cheiruridae Ordovician Bobcaygeon Formation, Ontario, Cana	Pseudocybele nasuta Suborder Cheirurina Superfamily Pilekiidae Family Pliomeridae Ordovician Fillmore Formation Millard County, Utah	Cybele bellatula Suborder Cheirurrina Superfamily Cheiruroidea Family Encrinuridae Lower Ordovician Wolchow river, Russia
Ceraurus pleurexanthemus Family Cheiruridae Ordovician Trenton Group, Quebec, Canada	Ceraurus pleurexanthemus Family Cheiruridae Ordovician Trenton Group, Quebec, Canada	Lehua ponti Family Cheiruridae Ordovician Zagora, Morocco	Crotalocephalina (Crotalocephalus) gibbus Family Cheiruridae Devonian Alnif, Morocco

Phacopid Phylogeny
Trilobita
`--Phacopida
   |--Calymenina
   |  |--Pharostomatidae
   |  |--Homalonotidae
   |  |--Bavarillidae
   |  `--Bathycheilidae
   |--Phacopina
   |  |--Dalmanitoidea
   |  |  |--Dalmanitidae
   |  |  |--Prosopiscidae
   |  |  `--Diaphanometopidae
   |  `--Acastoidea
   |     |--Acastidae
   |     `--Calmoniidae
   `--Cheirurina
      |--Cheiruridae
      |--Pliomeridae
      |--Pilekiidae
      `--Encrinuridae

Clarkson, E. N. K., and Levi-Setti, R., Nature, 254, 663–667 (1975).