Agnostida
Encyclopedia
Agnostida is an order
of arthropod
which first developed near the end of the Early Cambrian
period and thrived during the Middle Cambrian. They are present in the lower Cambrian fossil record along with trilobites from the Redlichiida
, Corynexochida
, and Ptychopariida
orders. The last agnostids went extinct in the Late Ordovician
.
. As a group, agnostids are isopygous, meaning that their pygidium
is similar in size and shape to their cephalon. Most agnostid species were eyeless.
The systematic position of the order Agnostida within the class Trilobita remains uncertain, and there has been continuing debate whether they are trilobites or a stem group. The challenge to the status has focused on the Agnostina partly because juveniles
of one genus have been found with legs greatly different from those of adult trilobites, suggesting they are not members of the lamellipedia
n clade
, of which trilobites are a part. Instead, the limbs of agnostids closely resemble those of stem group crustaceans, although they lack the proximal endite, which defines that group. They are likely the sister taxon to the crustacean stem lineage, and, as such, part of the clade Crustaceomorpha
. Other researchers have suggested, based on a cladistic analyses of dorsal exoskeletal
features, that Eodiscina and Agnostida are closely united, and that the Eodiscina descended from the trilobite order Ptychopariida
.
that descended from upper layers of the sea to the bottom. In contrast, their wide geographic dispersion in the fossil record is uncharacteristic of benthic animals, suggesting a pelagic existence. The thoracic segment appears to form a hinge between the head and pygidium allowing for a bivalved ostracod
an-type lifestyle. Furthermore, the orientation of the thoracic appendages appears ill suited for benthic living. However, recent work suggests that some agnostids were benthic predators, engaging in cannibalism and possibly manifesting pack-hunting behavior.
They are sometimes preserved within the voids of other organisms, for instance within empty hyolith conchs, within sponges, worm tubes
and under the carapaces of bivalved arthropods
, presumably in order to hide from predators or strong storm currents; or maybe whilst scavenging for food. In the case of the tapering worm tubes Selkirkia
, trilobites are always found with their heads directed towards the opening of the tube, suggesting that they reversed in; the absence of any moulted carapaces suggests that moulting was not their primary reason for seeking shelter.
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
of arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
which first developed near the end of the Early Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
period and thrived during the Middle Cambrian. They are present in the lower Cambrian fossil record along with trilobites from the Redlichiida
Redlichiida
Redlichiida is an order within the major extinct arthropod class Trilobita. The Redlichids are one of the four older classes of trilobites that originated in the Lower Cambrian....
, Corynexochida
Corynexochida
Corynexochida is an order of trilobite that lived from the Lower Cambrian to the Middle Devonian. Like many of the other trilobite orders, Corynexochida contains many species with widespread characteristics....
, and Ptychopariida
Ptychopariida
Ptychopariida is a large, heterogeneous order of trilobite containing some of the most primitive species known. Many date to the Early Cambrian Period, but the order was extant through the Late Ordovician...
orders. The last agnostids went extinct in the Late Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
.
Systematics
The Agnostida are divided into two suborders — Agnostina and Eodiscina — that are then divided into a number of familiesFamily (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
. As a group, agnostids are isopygous, meaning that their pygidium
Pygidium
The pygidium is the posterior body part or shield of crustaceans and some other arthropods, such as insects and the extinct trilobites. It contains the anus and, in females, the ovipositor...
is similar in size and shape to their cephalon. Most agnostid species were eyeless.
The systematic position of the order Agnostida within the class Trilobita remains uncertain, and there has been continuing debate whether they are trilobites or a stem group. The challenge to the status has focused on the Agnostina partly because juveniles
Juvenile (organism)
A juvenile is an individual organism that has not yet reached its adult form, sexual maturity or size. Juveniles sometimes look very different from the adult form, particularly in terms of their colour...
of one genus have been found with legs greatly different from those of adult trilobites, suggesting they are not members of the lamellipedia
Lamellipedia
Lamellipedia is a proposed clade of arthropods that includes most trilobites and their close relatives. Distinctive of the clade are the flat setae aligned in a comb-like structure on the exopod....
n clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
, of which trilobites are a part. Instead, the limbs of agnostids closely resemble those of stem group crustaceans, although they lack the proximal endite, which defines that group. They are likely the sister taxon to the crustacean stem lineage, and, as such, part of the clade Crustaceomorpha
Crustaceomorpha
Crustaceomorpha is a proposed clade of arthropods that includes the agnostids and the crustaceans. Synapomorphies for the clade are that the larval antenna is a feeding or locomotory organ, and there are six endopodal podomeres in post-antennal limbs....
. Other researchers have suggested, based on a cladistic analyses of dorsal exoskeletal
Exoskeleton
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...
features, that Eodiscina and Agnostida are closely united, and that the Eodiscina descended from the trilobite order Ptychopariida
Ptychopariida
Ptychopariida is a large, heterogeneous order of trilobite containing some of the most primitive species known. Many date to the Early Cambrian Period, but the order was extant through the Late Ordovician...
.
Ecology
Scientists have long debated whether the agnostids lived a pelagic or a benthic lifestyle. Their lack of eyes, a morphology not well-suited for swimming, and their fossils found in association with other benthic trilobites all suggest a benthic (bottom-dwelling) mode of life. They are likely to have lived on areas of the ocean floor that received little or no light and fed on detritusDetritus
Detritus is a biological term used to describe dead or waste organic material.Detritus may also refer to:* Detritus , a geological term used to describe the particles of rock produced by weathering...
that descended from upper layers of the sea to the bottom. In contrast, their wide geographic dispersion in the fossil record is uncharacteristic of benthic animals, suggesting a pelagic existence. The thoracic segment appears to form a hinge between the head and pygidium allowing for a bivalved ostracod
Ostracod
Ostracoda is a class of the Crustacea, sometimes known as the seed shrimp because of their appearance. Some 65,000 species have been identified, grouped into several orders....
an-type lifestyle. Furthermore, the orientation of the thoracic appendages appears ill suited for benthic living. However, recent work suggests that some agnostids were benthic predators, engaging in cannibalism and possibly manifesting pack-hunting behavior.
They are sometimes preserved within the voids of other organisms, for instance within empty hyolith conchs, within sponges, worm tubes
Selkirkia
Selkirkia is a genus of predatory priapulid worm known from the Middle Cambrian Burgess Shale. In the Burgess Shale, 20% of the tapering, organic-walled tubes are preserved with the worm inside them, whereas the other 80% are empty...
and under the carapaces of bivalved arthropods
Sidneyia
Sidneyia is an extinct arthropod known from fossils found in the Cambrian-age Burgess Shale formation of British Columbia. -General description:...
, presumably in order to hide from predators or strong storm currents; or maybe whilst scavenging for food. In the case of the tapering worm tubes Selkirkia
Selkirkia
Selkirkia is a genus of predatory priapulid worm known from the Middle Cambrian Burgess Shale. In the Burgess Shale, 20% of the tapering, organic-walled tubes are preserved with the worm inside them, whereas the other 80% are empty...
, trilobites are always found with their heads directed towards the opening of the tube, suggesting that they reversed in; the absence of any moulted carapaces suggests that moulting was not their primary reason for seeking shelter.
External links
- Order Agnostida by Sam Gon III.
- The Virtual Fossil Museum – Trilobite Order Agnostida
- Agnostida fact sheet by Sam Gon III.
- http://news.discovery.com/animals/early-animals-cannibals.html by Larry O'Hanlon