Arthropoda Redlichiida

Trilobita : Redlichiida

Members of trilobite Order Redlichiida (pronouced Red-li-chee-da) are the oldest trilobites known from the fossil record. The Redlichiids appear in the Cambrian Epoch 2 and disappear from the fossil record prior to the Furongian at the end of the Cambrian. The order is believed to contain the progenitors of members of Order Corynexochida, and Order Ptychopariida.
A number of morphological features in the Redlichiids are considered to be the characteristics of the earlier and more primitive trilobites. These include: 1) numerous thoracic segments having spinose tips; 2) large and semicircular cephalon; 3) large crescent-shaped eyes; 4) and a diminutive (micropygous) pygidium having one to a few segments

Order Redlichiida is divided into two suborders, Olenellina and Redlichiina. Of the two, the Olenellina are considered to be the earliest trilobites. Primarily due to their lack of facial sutures, some scientists have argued unsuccessfully for their separation from Trilobita. In all likelihood, Olenellina is most closely related to the last arthropod common ancestor of the trilobites.

The Olenellids are restricted to what was Laurentia in the Lower Cambrian, which now includes part of North America. In contrast, the Redlichiina are found in numerous Lower Cambrian locations that were not part of Laurentia. The different stratigraphical ranges are important as they form the basis for the phylenogy of Redlichiida. In fact, Lieberman (2002) has argued that cladistic analysis together with the biogeographic data supports the notion that early trilobite cladogenesis (i.e., the evolutionary splitting) occurred about coincident with the breakup of Pannotia sometime between 600–550 million years ago. Lieberman also conducted cladistic analyses among a group of basal trilobites within the Redlichiina, and the paraphyletic Fallotaspidoids. The group had primitive characteristics, such as the absence of facial sutures allying them with the Olenellina, and other characteristics allying them with the Redlichiina. Shared characteristics supported a phylogenetic position of the fallotaspids as transitional to all or almost all other trilobites except the Olenellina.

The Olenellid Fallotaspis from Morocco at about 540 mya has been cited by Fortey (2000) as the oldest trilobite in the fossil record. This Fallotaspis possessed relatively large holochroal eyes. Redlichids are found in two of the world's famous Cambrian Lagerstätten, the Maotianshan Shales near Chengjiang in China, as well as Emu Bay in Southern Australia. They are also commonly found in many sites in the western part of the United States and Canada. Many examples from these and other various fossil sites are shown below.

The members of Family Olenellidae have been proposed as chemoautotrophic symbionts (Fortey, 2005). Their wide thoraces, large numbers of thoracic segments, remarkably thin exoskeletons and, in some species, degenerate hypostome, and the occasional development of brood pouches are all consistent with this hypothesis. The Olenids appear well adapted to anoxic. Their extended pleural areas could have provided area for the cultivation of sulfur bacteria. It is feasible that the bacteria were grown on the ventral membrane beneath the extended pleurae and/or on the appendages. Among living animals, the gills of bivalves or the appendages of carideans are modified in chemoautotrophic symbionts for bacterial growth.

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