Naraoiidae
Encyclopedia
The Naraoiidae is a family
of soft-shelled trilobite
-like arthropod
s that are known only from the early and middle Cambrian
-- primarily from the Burgess Shale
of British Columbia
and the Maotianshan Shale of China
.
The taxonomic placement of the Naraoiidae is uncertain; Naraoiids are similar to trilobites in many ways but have several peculiar features unlike those of other trilobite groups, primarily their unmineralized exoskeleton
and unsegmented shields. Some specialists consider the Naraoiidae true trilobites of the order Nectaspida
, others consider them trilobites of uncertain ordinal status, and still others consider them a separate group of non-trilobite arthropods.
There are several known genera
, of which Naraoia was the first to be described and is the most widely known. The species Naraoia naraoia has an entirely unmineralized body covering and unsegmented head and body shields. Its maximum size is about 4 cm. A second softbodied trilobite - Tegopelte
- is known from the same formations and is sometimes placed in the Naraoiidae, or is sometimes in its own family, the Tegopeltidae.
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
of soft-shelled trilobite
Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...
-like arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s that are known only from the early and middle Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
-- primarily from the Burgess Shale
Burgess Shale
The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...
of British Columbia
British Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...
and the Maotianshan Shale of China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
.
The taxonomic placement of the Naraoiidae is uncertain; Naraoiids are similar to trilobites in many ways but have several peculiar features unlike those of other trilobite groups, primarily their unmineralized exoskeleton
Exoskeleton
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...
and unsegmented shields. Some specialists consider the Naraoiidae true trilobites of the order Nectaspida
Nectaspida
The Nectaspida is an extinct order of soft-bodied arthropods proposed by Raymond in 1920; its taxonomic status is uncertain. Specimens are known from the early Cambrian to the upper Silurian periods. Whittington placed the order in the Trilobita...
, others consider them trilobites of uncertain ordinal status, and still others consider them a separate group of non-trilobite arthropods.
There are several known genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
, of which Naraoia was the first to be described and is the most widely known. The species Naraoia naraoia has an entirely unmineralized body covering and unsegmented head and body shields. Its maximum size is about 4 cm. A second softbodied trilobite - Tegopelte
Tegopelte
Tegopelte is a genus of soft-bodied arthropod known from two specimens from the Walcott Quarry. Trackways that may have been produced by this organism or a close relative are known from the Kicking Horse Shale, stratigraphically below its body fossil occurrences....
- is known from the same formations and is sometimes placed in the Naraoiidae, or is sometimes in its own family, the Tegopeltidae.
External links
- Are Naraoids trilobites? Photographs of specimens and discussion of classification controversy