Evolution and Systematics

Evolution

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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Molecular Biology and Genetics

Genetics

Phylogeny

Edgecombe (1992) used parsimony analysis to consider the phylogenetic relationships of the major groups within the Phacopida, and he recognized that some of these groups as presently defined may be paraphyletic. The close relationship between Homalonotidae and Calymenidae was also supported by a phylogenetic analysis based on larval characters conducted by Chatterton et al. (1990).
Creative Commons Attribution Non Commercial 3.0 (CC BY-NC 3.0)

© Lieberman, Bruce

Source: Trilobites Online Database

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