The phylogenetically placement of the trilobites within Phylum
Arthropoda remains problemmatic, as indeed are the relationshipd
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.
The
phylogeny sensu Hou & Bergström (1997) below
shows one possible arrangement of various trilobite
relatives.
|
|
< LAMELLIPEDIA sensu Hou & Bergstr?m, 1997
|-- ? MARRELLOMORPHA
| |-- ? MARRELLIDA
| | `--? Marrellidae
| `-- ? MIMETASTERIDA
| `-- ? Mimetasteridae?
`-- ARTIOPODA
|-- ? NECTOPLEURA
| |-- ? NECTASPIDA
| | |-- ? Orientellidae
| | `--+-- ? Liwiidae
| | `-- ? Naraoiidae
| |-- ? VACHONISIIDA
| | `-- ? Vachonisia rogeri
| |-- ? ACERCOSTRACA
| |-- ? HALICYNA
| `-- ? RETIFACIIDA
| `-- ? Retifaciidae
`--+-- ? CONCILITERGA
| `-- ? HELMATIIDA
| |-- ? Helmitiidae
| `--+-- ? Skioldiidae
| `--+-- ? Tegopeltidae
| `-- ? Saperiidae
|-- ? PETALOPLEURA
| |-- ? XANDARELLIDA
| | |-- ? Xandarellidae
| | `-- ? Almeniidae
| |-- ? CHELONEILLIDA
| | `-- ? Cheloniellidae
| `-- ? SINOBURIIDA
| `-- ? Sinoburiidae
|-- TRILOBITA
`--+-- ? XENOPODA
| |-- ? EMERALDELLIDA
| | `-- ? Emeraldellidae
| `-- ? LIMULAVIDA
| `-- ? Sidneyiidae
|-- ? AGLASPIDA (AGLASPIDIDA)
| |-- ? AGLASPIDIDA
| | |-- ? Aglaspididae
| | `-- ? Lemoneitidae
| `-- ? STRABOPIDA
| `-- ? Strabopidae [Paleomeridae]
`-- CHELICERATA
-
Hou
& Bergström, 1997: Arthropods of the Lower
Cambrian Chengjiang fauna, southwest China. --Fossils
& Strata, Number 45, 22nd December 1997, pp.
1-116
-
Hou,
X. G.; Bergstrom J, (1991) The arthropods of the
Lower Cambrian Chengjiang fauna, with relationship
and evolutionary significance, p. 179-187. In A.
M. Simonetta and S. Conway Morris (eds.), The Early
Evolution of Metazoa and the Significance of Problematic
Taxa. Cambridge University Press, Cambridge.
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