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The Phacopida comprises one of the most diverse and charismatic components of trilobite diversity: certain representatives display particularly intricate form including spinose projections, head ornaments, etc. For this reason it is not surprising that the greatest number of phylogenetic studies of trilobites have concentrated on this order. Phacopids were originally presumed to originate in the Ordovician, although their roots clearly extended back into the paraphyletic mass of “Ptychopariida” in the Cambrian (Jell and Adrain, 2003). They vanished during the Late Devonian biodiversity crisis. Members of the Phacopida are united by, among other traits, their shared possession of a schizochroal, large lensed eye. (Note that it was the evolution of those lenses that figured so prominently in the development of punctuated equilibria by Eldredge and Gould, 1972.)


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© Lieberman, Bruce

Source: Trilobites Online Database

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