Trilobite Relatives & Ancestors
Relatives and Ancestors of Trilobites - Trilobitomorpha |
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| The phylogenetically placement of the trilobites within Phylum Arthropoda remains problemmatic, as indeed are the relationships among all the early arthropods. Based on some cladistic analyses, the Helmetiidae and Naraoiidae are considered by some to be the early arthropods known from fossils that are phylogenetically the closest to Trilobita; sometimes a particular Naraoiidae is placed in another group, the Tegopeltidae. The Naraoiidae is a family of early arthropods resembling trilobites in appearance. Because Naraoids had an entirely non-mineralized exoskeletons, they are often referred to as soft-bodied trilobites. These Naraoids are known only from the early and middle Cambrian, and mainly from the Maotianshan Shales of Chengjiang in China, the Burgess Shale of British Columbia, Canada, and, though rare, the House Range of Utah (including at least the Marjum and Weeks Formations). Trilobitomorpha is a term that decreasingly used as a subphylum of Arthropoda that encompasses trilobita together with other arthropods that have at least some trilobite characteristics. Originally a variety of peculiar forms, mostly from the lower Cambrian, were included as the Class Trilobitoidea. However, the many species that are known only from fossils do not appear to be closely related to the trilobites or often even to each other either, and are now generally placed in separate subphyla or superclass when classified at all. The phylogeny sensu Hou & Bergström (1997) below shows one possible arrangement of various trilobite relatives.
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