Trilobite Order Agnostida
Trilobites
 

Agnostid Trilobites

Class Trilobita

 

 

Trilobite Taxonomy
Trilobite Phylogeny
Phylum Arthropoda
Subphylum Chelicerata
or Subphylum Schizoramia *
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
Trilobite Relatives
o Arthropoda
`--o Trilobita
   |?-Agnostida
   |?-Eodiscina
   `--+--o Redlichiida
      |  |--Olenellina
      |  `--Redlichiina
      |--Corynexochida
      |--Phacopida
      |--Lichida
      |--Odontopleurida
      `--o Librostoma
         |--Proetida
         |--Harpetida
         |--Ptychopariida
         `--Asaphida
* Different taxonomies are found, most recently with trilobites contained in Superclass Arachnomorpha in Subphylum Schizoramia.

The trilobites of Order Agnostida (pronounced Ag-nós-t-eh-da)appeared in the Lower Cambrian where they were diverse and ubiquitous, and declined to become rare in the Ordovician prior to their complete extinction by the end of the Ordovician. There is debate about whether agnostids were benthic or pelagic, and the systematic position of Order Agnostida remains unsettled, if not contentious. The agnostids were mostly blind and exhibit a strong symmetry of cephalon and pygidium size, making it hard to distinguish front from rear.

There are two Suborders. Members of Suborder Agnostina have two segments between the cephalon and pygidium, and lack sutures on the cephalon. In contrast, members of Suborder Eodiscina may have two or three body segments, and some have small eyes and proparian sutures ending forward of the genal angle. Among trilobites, the Agnostida are so relatively unusual that some researchers have suggested, based on meraspis phase larval limb structure differing markedly from what is prototypical for trilobites (Muller and Walossek, 1987), that one of the two suborders, Agnostina, should not be included in Class Trilobita; the Agnostina have been proposed as a stem group separately descended from Crustaceans. Such a conclusion would be premature, since fossils showing limbs from adults have yet to be discovered (Fortey, 2001). There has also been speculation that agnostids might be paedomorphic due to their superficial resemblance to the meraspis phase of some trilobites; this would suggest precocious sexual maturity, which could include retention of a larval limb structure. Jell (2003) proposes a progenesis from the Ellipsocephaloidea family of Order Ptychopariida as the origin of the members of Suborder Eodiscina, from which the Agnostina descended. Additionally, cladistic analysis (Cotton and Fortey, 2005) Trilobite Death Assemblage from House Rangestrongly supports the continued unification of Suborders Agnostina and Eodiscina as true trilobites based on exoskeleton morphology, despite an obvious specialized body plan.

A highly widespread paleo-geographic dispersion was long considered consistent with a pelagic, planktonic lifestyle. In contrast, a lack of eyes in most species suggests a benthic lifestyle in deep, cold water on the ocean floor in minimal or no light where they fed on detritus. The agnostids are often found in dense mass mortality assemblages and in association with other benthic trilobites, particularly those of Order Ptychopariida. Additionally, their diminutive size, low mass and general morphology are hydrodynamically inconsistent with a swimming pelagic mode of life. A weight of evidence tends to support that the Agnostida were benthic, but more data are needed; perhaps the little Agnostida made their living in different or multiple ways, depending on their age, or on the paleoenvironment where they lived.

 
Superfamily Agnostoidea

Subphylum Chelicerata
Class Trilobita
Order Agnostida

    Suborder Agnostina
          Superfamily Agnostoidea
                Family Agnostidae
                Family Ptychagnostidae
                Family Peronopsidae
                Family Doryagnostidae
                Family Diplagnostidae
                Family Clavagnostidae
                Family Metagnostidae 
          Superfamily Condylopygoidea
                Family Condylopygidae 
    Suborder Eodiscina
          Superfamily Eodiscoidea
                Family Tsunyidiscidae
                Family Hebediscidae
                Family Calodiscidae
                Family Weymouthiidae
                Family Yukoniidae
                Family Eodiscidae 
          Superfamily Incertae sedis
                Family Sphaeragnostidae
Peronopsis sp
Family Peronopsidae
Early Middle Cambrian
Kaili Formation
Guizhou Province, China
Ptychagnostus richmondensis
Family Ptychagnostidae
Middle Cambrian
Marjum Formation
Millard County, Utah
Grandagnostus
Ptychagnostus buckleyi
Family Peronopsidae
Middle Cambrian
Christmas Hills
Tasmania, Australia
Grandagnostus sp
Family Peronopsidae
Middle Cambrian
Christmas Hills
Tasmania, Australia
Ptychagnostus praecurrens
Family
Ptychagnostidae
Early Cambrian
Burgess Shale, Canada
Peronopsis segmenta
Family
Peronopsidae
Marjum Formation
Millard County, Utah
Ptychagnostus akanthodes
Family Ptychagnostidae
Marjum Formation
Millard County, Utah
Ptychagnostus michaeli
Family
Ptychagnostidae
Marjum Formation
Millard County, Utah
Ptychagnostus atavus
Family
Ptychagnostidae
Marjum Formation
Millard County, Utah
Ptychagnostus michaeli
Family
Ptychagnostidae
Marjum Formation
Millard County, Utah
Baltagnostus eurypyx
Family Diplagnostidae
Wheeler Formation
Millard County, Utah
Peronopsis interstricta
Family Peronopsidae
Middle Cambrian
Wheeler Formation
Millard County, Utah