Redlichiid 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 phylogegy 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, 2000). 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 conditions. 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 cnidarians (jellyfish) are modified in chemoautotrophic symbionts for bacterial growth.