Trilobite Order Agnostida
Agnostid Trilobites |
|
| 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
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) 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.
| |||||||||||||||||||||||||||||||||||||||||||
Fossil
Museum Navigation:
Home Geological Time Paleobiology Geological History Tree of Life Fossil Sites Fossils Evolution Fossil Record Museum Fossils |