Trilobites

Trilobites:
About the Nine Orders of Class
Trilobita

Taxonomy and Chracteristics

"Butterflies of the Paleozoic Seas"

Trilobite Taxonomy

Trilobite Phylogeny

Phylum Arthropoda
Subphylum Chelicerata
or Subphylum Trilobitamorph
Superclass Arachnomorpha *
Class Trilobita

    Order Agnostida
    Order Redlichiida
    Order Corynexochida
    Order Phacopida
    Order Lichida
    Order Proetida
    Order Harpetida
    Order Ptychopariida
    Order Asaphida
    Order Odontopleurida **
    Order Nectaspida

o Arthropoda
`--o Trilobita
   |?-Agnostida
   |?-Eodiscina
   `--+--o Redlichiida
      |  |--Olenellina
      |  `--Redlichiina
      |--Corynexochida
      |--Phacopida
      |--Lichida
      |--Odontopleurida
      `--o Librostoma
         |--Proetida
         |--Harpetida
         |--Ptychopariida
         `--Asaphida

* Several alternative taxonomies are found in past and current literature. For example, trilobites are sometimes found placed in arthropoda subphylum Schizoramia within Superclass Arachnomorpha.
** Some authors argue that a separate Order Odontopleurida should be separated from Order Lichida, but the concention is not followed here pending settlement of the science.

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. 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. Despite reduced ancestry, with decent with modification ruling, these trilobites filled the same ecological niches such that adaptation led to a repeating of many of the forms of their extinct cousins. While the genetic path was assuredly different, the newly evolved forms had recognizable morphological similitude with those long extinct. Regrettably, trilobites never truly recovered in the Carboniferous, with but a handful of genera from Order Proetida extant by the Permian. Failing to adapt to deep-water habitats, trilobite 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.

Trilobites comprise a complex and huge clade of arthropods with estimates of number of species exceeding 15,000. The extinct trilobite represents a problem for classification, a problem unlikely to benefit from modern genomic science. Darwin was confident in his conjecture that trilobites descended from one Pre-Cambrian crustacean ancestor. But, the trilobite's position in the universal tree of life remains a mystery today, with debate remaining whether their closest extant cousins are, for example, the horseshoe crabs, the spiders or the scorpions. Classification requires following the tree of life back to points of branching. This we cannot do for the trilobite whose first appearance in the fossil record is in the Cambrian. When they appear, they are already diverse in form, and dispersed in geography, clearly indicative of the incomplete nature of the fossil record. The highly diverse soft-bodied and jointed-legged animals that were ostensibly produced in the Cambrian explosion, but must also have Precambrian ancesters intensify the mystery. Among the Cambrian fauna, the crystal eyes of trilobites are unique. In the eyes is a strong clue, since the fossil record indicates pre-sight neural tissue existed in forms of worms that contain segmented morphology also retained in the trilobite forms. Mystery is allure for those with a scientific propensity, and the trilobite does not disappoint. Reading Richard Fortey's recent book "Trilobite! Eyewitness to Evolution", you can almost feel the author's sadness that these magnificent animals that once dominated life on Earth, declined near the end of the Paleozoic with but a single order surviving to the Carboniferous, before it too faded away.

The Nine Orders of Arthropod Class Trilobita and Its Relatives Fossil Record Appearance and Duration, Salient Distinguishing Characteristics and Examples

Trilobite Order Agnostida		Trilobite Order Redlichiida

Agnostid trilobites: Lower Cambrian to Upper Ordovician Among the most primitive of trilobites Length of a few mm and smaller Similar cephalon and pygidium (isopygous)		Redlichiid trilobites: Lower to Middle Cambrian Among the most primitive of trilobites from early Cambrian Many thoracic segments and small pygidium Spinocity usually limited to pleurae tip

Trilobite Order Corynexochida		Trilobite Order Lichida

Corynexochid trilobites: Middle Cambrian to Upper Devonian Allied by having hypostomal attachment in common Normally spinous		Lichid trilobites: Ordovician to Devonian Often elaborate and often highy spinou

Trilobite Order Phacopida		Trilobite Order Proetida

Phacopid trilobites: Particularly noted for detailed preservation of compound eyes Typical deep furrows between thoracic segments Typically not spinous		Proetid trilobites: Ordovician to Permian The last survivors before Trilobita faded away, and disappeared in the Permian extinction Typically small with small spineless pygidium

Trilobite Order Asaphida		Trilobite Order Harpetida

Asaphid trilobites: Middle Cambrian to Lower Silurian Ubiquitous trilobites sharing distinct suture structure Effacement of features common with typically large pygidium		Harpetid trilobites: Upper Cambrian to Upper Devonian Presence of the broad semicircular to ovate brim Lacking rostral plate

Trilobite Order Ptychopariida		Trilobite Relatives & Stem Group Order Nectaspida

Ptychopariid trilobites: Lower Cambrian to Devonian Appeared early and persisted long, yielding much variability in form Formerly included what is now Suborder Harpina Confounded phylogenics		Trilobite Relatives: Precambrian to Upper Cambrian Soft bodies Fossil record generally limited to Lagerstatten