Redlichiida is an order of

trilobite Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the At ...

s, a group of extinct

marine Marine is an adjective meaning of or pertaining to the sea or ocean. Marine or marines may refer to: Ocean * Maritime (disambiguation) * Marine art * Marine biology * Marine debris * Marine habitats * Marine life * Marine pollution Military * ...

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...

s. Species assigned to the order Redlichiida are among the first trilobites to appear in the

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

record, about halfway during the Lower Cambrian. Due to the difficulty to relate sediments in different areas, there remains some discussion, but among the earliest are '' Fallotaspis'' (suborder Olenellina), and ''

Lemdadella ''Lemdadella'' is an extinct genus of redlichiid trilobites that lived during the late Atdabanian stage, which lasted from 521 to 514 million years ago during the early part of the Cambrian Period. Distribution * ''Lemdadella'' sp. occurs in ...

'' (suborder Redlichiina), both belonging to this order. The first representatives of the orders

Corynexochida Corynexochida is an order of trilobite that lived from the Lower Cambrian to the Late Devonian. Like many of the other trilobite orders, Corynexochida contains many species with widespread characteristics. The middle region of the cephalon (the ...

and

Ptychopariida Ptychopariida is a large, heterogeneous order of trilobite containing some of the most primitive species known. The earliest species occurred in the second half of the Lower Cambrian, and the last species did not survive the Ordovician–Silurian ...

also appear very early on and may prove to be even earlier than any redlichiid species. In terms of anatomical comparison, the earliest redlichiid species are probably ancestral to all other trilobite orders and share many primitive characters. The last redlichiid trilobites died out before the end of the Middle Cambrian.

Description

Most redlichiids are rather flat (or have low dorso-ventral convexity) and their

exoskeleton An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton (endoskeleton) in for example, a human. In usage, some of the ...

typically has an oval outline, about 1½× longer than wide. Each back edge of the headshield (or

cephalon Cephalon, Inc. was an American biopharmaceutical company co-founded in 1987 by pharmacologist Frank Baldino, Jr., neuroscientist Michael Lewis, and organic chemist James C. Kauer—all three former scientists with the DuPont Company. Baldino s ...

) very often carries a spine, termed a genal spine. The eye lobes are sickle-shaped, long and extend from the frontal lobe of the central raised area of the cephalon (or

glabella The glabella, in humans, is the area of skin between the eyebrows and above the nose. The term also refers to the underlying bone that is slightly depressed, and joins the two brow ridges. It is a cephalometric landmark that is just superior to ...

) curving outward and increasingly backwards and sometimes eventually inwards again. The visual surface, that contains the calcite lenses is surrounded by fracture lines (or circumocular sutures), so that it has most often broken away from the rest of the cephalon. The glabella tapers forward and is relatively long, with the frontal lobe boss-like or pointy, followed by three rings or pairs of lobes (defined by furrows that may or may not cross over the midline), and finally at the back of the cephalon the so-called occipital ring. On the ventral side of the cephalon, the palate (or

hypostome In zoology, the hypostome can refer to structures in distinct animal groups: *Hypostome (trilobite), the ventral mouthpart plate in trilobites *Hypostome (tick) The hypostome (also called the maxilla, radula, or labium) is a calcified harpoo ...

) is attached to the part of the calcified exoskeleton that defines the contour at the ventral side (the so-called doublure), a state called conterminant. The hypostomes of redlichiids have narrow borders, are not split into backward pointing forks, and have only small muscle attachment areas (or maculae). The articulate middle part of the exoskeleton (or

thorax The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the cre ...

) is composed of many of segments that often end in a spine at the side of the animal. To each side of the central axis, the third segment from the front may have an exta large and wide rib (a state called macropleural). The tailshield (or

pygidium The pygidium (plural pygidia) is the posterior body part or shield of crustaceans and some other arthropods, such as insects and the extinct trilobites. In groups other than insects, it contains the anus and, in females, the ovipositor. It is compo ...

) is small but appears to be composed of more than one segment. In Olenellina the earliest larval stage (called protaspis) has not been found, so it is presumed it was not calcified. The development of ''Redlichia'' from protaspis to adult was gradual without a metamorphosis at any stage. Two major

Lagerstätte A Lagerstätte (, from ''Lager'' 'storage, lair' '' Stätte'' 'place'; plural ''Lagerstätten'') is a sedimentary deposit that exhibits extraordinary fossils with exceptional preservation—sometimes including preserved soft tissues. These for ...

n where redlichiids are found are the

Emu Bay shales The Emu Bay Shale is a geological formation in Emu Bay, South Australia, containing a major Konservat-Lagerstätte (fossil beds with soft tissue preservation). It is one of two in the world containing Redlichiidan trilobites. The Emu Bay Shale i ...

of Southern Australia and the

Maotianshan shales The Maotianshan Shales are a series of Cambrian, Early Cambrian deposits in the Chiungchussu Formation, famous for their ''Lagerstätte, Konservat Lagerstätten'', deposits known for the exceptional preservation of fossilized organisms or traces. ...

near Chengjiang in China. Other regions include Morocco, Labrador, Canada, the western

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...

, and

Bohemia Bohemia ( ; cs, Čechy ; ; hsb, Čěska; szl, Czechy) is the westernmost and largest historical region of the Czech Republic. Bohemia can also refer to a wider area consisting of the historical Lands of the Bohemian Crown ruled by the Bohem ...

Czech Republic The Czech Republic, or simply Czechia, is a landlocked country in Central Europe. Historically known as Bohemia, it is bordered by Austria to the south, Germany to the west, Poland to the northeast, and Slovakia to the southeast. The ...

. The appendages have been preserved in a few specimens. They follow typical trilobite patterns in terms of the number, placement, and types of legs, antennae,

gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...

s, etc.

Suborders

The order Redlichiida is divided into two

suborder Order ( la, ordo) is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between family and class. In biological classification, the order is a taxonomic rank used in the classification of organisms and ...

Olenellina Olenellina is a suborder of the order Redlichiida of trilobites that occurs about halfway during the Lower Cambrian, at the start of the stage called the Atdabanian. The earliest trilobites in the fossil record are arguably Olenellina, although ...

and

Redlichiina Redlichiina is a suborder of the order Redlichiida of Trilobites. The suborder contains three superfamilies: Emuelloidea, Redlichioidea and Paradoxidoidea. These trilobites are some of the oldest trilobites known. They originated at the begin ...

. The main difference between the two groups is the lack of facial sutures in the Olenellina. This absence of the facial sutures is regarded as primitive by most scholars. Opisthoparian facial sutures are a shared character of all Redlichiina. In other trilobites, dorsal sutures may be opisthoparian, gonatoparian, proparian or they may be lost secondarily. The absence in the fossil record of the earliest larval stage, the protaspid, suggests that it may have been uncalcified, which would be a second unique character that distinguishes the Olenellina from all other trilobites. Fossils of olenellinid trilobites are found in North America, and other associated areas that comprised the Cambrian continent of

Laurentia Laurentia or the North American Craton is a large continental craton that forms the ancient geological core of North America. Many times in its past, Laurentia has been a separate continent, as it is now in the form of North America, although ...

. They are very common and are used to define the extent of Laurentia. Their abrupt disappearance marks the boundary between the Lower and the Middle Cambrian. Fossils of redlichiinid are associated with Cambrian regions other than Laurentia.

References

{{Taxonbar, from=Q134807 Trilobite orders Cambrian trilobites Terreneuvian first appearances Miaolingian extinctions