 Orders
Ptychopariida and Corynexochida trilobites both have related
taxa that could be placed in Redlichiida, making both orders
paraphyletic (Lieberman,
2010). In fact, Ptychopariida Superfamily
Ellipsocephaloidea was once placed within Order Redlichiida, but is now accepted
as an evolutionary step from the Redlichiids to the Ptychopariids.
Fortey (2001)
considered the highly diverse Family Olenidae to likely be
monophyletic. The
first Ptychopariid trilobites appeared toward the end of the
Lower Cambrian, and the entire order met extinction in the Ordovician
to Silurian extinction event some 440 to 450 mllion years
ago.
A
primary trait of the Ptychopariida is a hypostome that is conterminant
in the protaspid
stage,
but becomes natant early
in development and remains natant in the holaspid
stage.
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Subclass Librostoma*
Trilobite Order Ptychopariida
- Suborder
Olenina
- Suborder
Ptychopariina
- Superfamily
Ellipsocephaloidea
- Superfamily
Ptychoparioidea
*
Subclass Librostoma contains Proetida, Harpetida, Ptychopariida,
Asaphida
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