Trilobites

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Trilobites
The Nine Orders of Class TrilobitaTaxonomy and
Characteristics"Butterflies of the Paleozoic
Seas"
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• The Paleozoic is often called the age of the
  trilobite.Trilobites particularly flourished in
  the oceans of the Cambrian and Ordovician
  periods, beginning around 540 million years ago,
  with a diminishing number of families persisting
  until the Permian. The number of families
  actually peaked in the Late Cambrian when an
  extinction event removed many. The morphological
  diversity actually peaked in the Ordovician. Many
  more families were removed at the end of the
  Ordovician 440 million years ago during a great
  ice age where ice sheets advanced to the equator.

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• The diminished number of trilobite families that
  survived to the Silurian radiated into new and
  exotic forms, and still more exotic spiny and
  pustulose forms in the Devonian. The Devonian was
  punctuated by periods of rising seas that
  disrupted the reef systems where the trilobites
  flourished forcing selective adaptation. The end
  of the Devonian saw the Frasnian-Famennian event
  where only Proteus survived into the
  Carboniferous.

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Mass extinction

• Failing to adapt to deep-water habitats, their
  vulnerability to climatic change remained and led
  to their disappearance prior to yet another great
  mass extinction at the end of the Permian. The
  age of the trilobite yielded to the age of the
  insect.

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Agnostida

• Among the most primitive of trilobites
• Length of a few mm and smaller
• Similar cephalon and pygidium (isopygous)
• Lower Cambrian to Upper Ordovician

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Redlichiida

• Among the most primitive of trilobites
• Many thoracic segments
• Spinocity usually limited to pleurae tips
• Small pygidium
• Lower to Middle Cambrian

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Corynexochida

• Hypostomal attachment in common
• Normally spinous
• Lower to Middle Cambrian

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Lichida

• Often elaborate and often highly spinous (making
  them highly sought)
• Ordovician to Devonian

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Phacopida

• Particularly noted for detailed preservation of
  compound eyes
• Typical deep furrows between thoracic segments
• Typically not spinous
• Lower Ordovician to Upper Devonian

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Proetida

• Among the last survivors before Trilobita faded
  away, and disappeared in the Permian extinction
• Typically small with small spineless pygidium
• Ordovician to Permian

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Asaphida

• Ubiquitous trilobite sharing distinct suture
  structure
• Effacement of features common with typically
  large pygidium
• Middle Cambrian to Lower Silurian

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Harpetida

• Presence of the broad semicircular to ovate brim
• Lack of rostral plate
• Upper Cambrian to Upper Devonian

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Ptychopariida

• Appeared early and persisted long, yielding much
  variability in form
• Formerly included what is now Order Harpina
• Lower Cambrian to Devonian

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