Trilobite Orders


Listed below are the most basic phenotypic or diagnostic traits across the nine orders of Class Trilobita:

Refer to: Trilobite Terminology and Trilobite Morphology.

Trilobite Order Agnostida

Geological Time Periods: Early Cambrian to Late Ordovician
General Traits: Small, normally a few mm long; primitive; some species have small spines
Morphology
Cephalon: Hard to tell pygidium from cephalon
Sutures: Suborder Agnostina lack sutures on cephalon. Suborder Eodiscina have proparian sutures.
Hypostome: Natant and no rostral plate.
Thorax: Normally only 2-3 articulated segments; capable of enrollment, though fossils of adults seldom found enrolled
Pygidium: Isopygous condition
Eyes: Most were eyeless (blind)

Exoskeleton:

Taxonomy: Contains two suborders, Agnostina and Eodiscina

Lifestyle: Probably benthic, though some support planktonic

Also see: Agnostid Trilobite Gallery

Trilobite Order Agnostida

  • Suborder Agnostina
    • Superfamily Agnostoidea
    • Superfamily Condylopygoidea
  • Suborder Eodiscina
    • Superfamily Eodiscoidea

Also see: Agnostida Trilobites Classification

Order Asaphida

Geological Time Periods: Middle Cambrian to Silurian

General Traits: Significant morphological variation

Morphology:

Cephalon: Usually smooth surface due to adaptive effacement, glabellar furrows subdued, as well as a wide doublure.
Sutures: Typically with pronouced median ventral suture.
Hypostome: Conterminant or impendent, though natant in superfamily Anomocaroidea.
Thorax: Typically between five and twelve segments, but only two or three in Superfamily Trinucleioidea, 13 or more in some in superfamily Anomocaroidea, and up to 30 in an Alsataspidid (Trinucleioidea).
Pygidium: Isopygous to subisopygous to macropygous, with a wide doublure.

Eyes: Large, adaptive secondary blindness in some genera.

Exoskeleton:

Taxonomy: Contains six superfamilies, Anomocaroidea, Asaphoidea, Dikelokephaloidea, Remopleuridoidea, Cyclopygoidea, and Trinucleioidea.

Lifestyles:

Misc Facts:

Diverse and large order comprising some 20% of trilobite genera.

Effacement may be an evolutionary adaptation to allow burrowing, which is especially evident in Illaenina. Alternatively, effacement might have been an adaptation to streamline the trilobite in a pelagic lifestyle. Perhaps both hypotheses are true.

Also see: Asaphida Trilobite Gallery

  

Order Redlichiida

Geological Time Periods: Lower Cambrian to Middle Cambrian

General Traits: Said to look primitive – so now you know what primitive looks like, usually showing a large amount of thoracic segments ending in distinct pleural spines

Morphology:

Cephalon: Semicircular in shape, usually having prominent genal spines
Sutures: Suborder Olenellina lacked facial sutures. In contrast, They Suborder Redlichiina had facial sutures and their fossils are often found without their librigena, which is often called cheekless
Hypostome: Natant and without rostral plate
Thorax: Highly segmented and elongated thorax tapering back to a small pygidium. Numerous (up to more than 100 in rare cases). Thoracic segments of varyied length, and normally terminating in pleural spines.
Pygidium: Micropygous condition and often tiny

Eyes: Mostly large, long and distinctly crescent shape.

Exoskeleton: Normally spiny, sometimes quite ornately including genal, glabular, tail, segment spines. The Redlichid family Olenellidae typically have long spines on the third thoracic segment (Lieberman, 1999B).

Taxonomy: Two Suborders, Olenellina and Redlichiina.

Lifestyle: Probably benthic

Misc Facts:

Redlichiid trilobites are the first arthropods to appear in the fossil record; the earliest known trilobite seems to be the genus Fallotaspis. (See example Fallotaspis)
Cephalon: semicircular.

Suborder Redlichiina is associated with Cambrian the Laurentia Craton during the Cambrian.

Also see: Redlichiid Trilobite Gallery

Trilobite Order Redlichiida

  • Suborder Olenellina
    • Superfamily Olenelloidea
    • Superfamily Judomioidea
    • Superfamily Nevadioidea
    • Superfamily Fallotaspidoidea
  • Suborder Redlichiina
    • Superfamily Emuelloidea
    • Superfamily Redlichioidea
    • Superfamily Paradoxidoide
Also see: Redlichida Trilobites Classification

Order Corynexochida

Geological Time Periods: Lower Cambrian to Late Devonian

General Traits: Significant morphological variation

Morphology:

Cephalon: Glabella typically elongated, with furrows unless effaced.
Sutures: Opisthoparian sutures
Hypostome: Conterminant or impendent
Thorax: Typically with 7 to 8 segments, but full range is 2 to 12, often with spinose pleural tips.
Pygidium: Isopygous or subisopygous to macropygous, normally large.

Eyes: Normally well-developed

Exoskeleton:

Taxonomy: Three suborders, Corynexochina, Illaenina, Leisotegiina

Lifestyles:

Misc Facts:

Diverse and large order comprising some 20% of trilobite genera.

Effacement may be an evolutionary adaptation to allow burrowing, which is especially evident in Illaenina. Alternatively, effacement might have been an adaptation to streamline the trilobite in a pelagic lifestyle. Perhaps both hypotheses are true.

Also see: Corynexochida Trilobite Gallery

Trilobite Order Corynexochida

  • Suborder Corynexochina
    • Superfamily Corynexochoidea
  • Suborder Illaenina
    • Superfamily Illaenoidea
  • Suborder Leiostegiina
    • Superfamily Leiostegioidea

Also see: Corynexochida Trilobites Classification