Temporal range: Early – Middle Cambrian, 525–500Ma
|Eoredlichia intermedia, 15mm|
Redlichiida is an order of trilobites, a group of extinct marine arthropods. Species assigned to the Redlichiida are among the first trilobites to appear in the fossil 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 (Olenellina), and Lemdadella (Redlichiina), both belonging to this order. However, the first representatives of the Corynexochida and Ptychopariida orders also appear very early on and may prove to be even earlier than any Redlichiida species. However, the earliest Redlichiida are probably ancestrial to all other trilobite orders and share many primitive characters. The Redlichiida died out before the end of the Middle Cambrian.
Most Redlichiida are rather flat (or have low dorso-ventral convexity) and their exoskeleton typically has an oval outline, about 1½× longer than wide. Each back edge of the headshield (or cephalon) very often carries a spine (that science calls 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) 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) 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 Redlichiida 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) 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) 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.
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, gills, etc.
The Redlichiida are divided into two suborders: Olenellina and Redlichiina. The main difference between the 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 Olenellina are found in North America, and associated areas that comprised the Cambrian continent of Laurentia. 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.
Members of suborder Redlichiina are associated with Cambrian regions other than Laurentia.