''Naraoia'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of small-to-average-size (about long) marine

arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...

s within the family

Naraoiidae Naraoiidae is a family, of extinct, soft-shelled trilobite-like arthropods, that belongs to the order Nektaspida. Species included in the Naraoiidae are known from the second half of the Lower Cambrian to the end of the Upper Silurian. The total ...

, that lived from the early

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

to the late

Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...

period. The species are characterized by a large alimentary system and sideways oriented antennas.

Etymology

The name is derived from Narao, the name of a group of small lakes in Cataract Brook canyon, above Hector on the Canadian Pacific Railway,

British Columbia British Columbia is the westernmost Provinces and territories of Canada, province of Canada. Situated in the Pacific Northwest between the Pacific Ocean and the Rocky Mountains, the province has a diverse geography, with rugged landscapes that ...

, Canada.Walcott, C.D. Middle Cambrian Branchiopoda, Malacostraca, Trilobita, and Merostomata. In: Cambrian Geology and Paleontology II. Smithsonian. 1914. s:Cambrian Geology and Paleontology/Volume 2/Middle Cambrian Branchiopoda, Malacostraca, Trilobita, and Merostomata

History of the classification

When the fossil was first discovered in

Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...

Burgess Shale The Burgess Shale is a fossil-bearing deposit exposed in the Canadian Rockies of British Columbia, Canada. It is famous for the exceptional preservation of the soft parts of its fossils. At old (middle Cambrian), it is one of the earliest fos ...

, it was believed to be a

crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...

, such was the difference between this and other trilobites. Its continuous shield hid most of its structure, interfering with proper classification. When Harry B. Whittington began dissecting some specimens (''Naraoia'' was among the most populous of the Burgess Shale animals), he discovered that the legs (and gills) of the beasts were very similar, if not identical to those of trilobites, thus the current placement of ''Naraoia'' in

class Class, Classes, or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used d ...

Trilobita Trilobites (; meaning "three-lobed entities") are extinction, extinct marine arthropods that form the class (biology), class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most succ ...

. ''Misszhouia longicaudata'' was formerly considered a member of the genus ''Naraoia'', originally known as ''N. longicaudata'', until separated in 1997.

Description

USNM PAL 57687 Naraoia compacta Image 01.jpg, Fossil lectotype of ''N. compacta'' Misszhouia longicaudata 2.jpg, Fossil of ''N. taijiangensis'' Naraoia_size_comparison.svg, Size comparison of all species File:Naraoia_magna_reconstruction.png, Naraoia magna reconstruction ''Naraoia'' is almost flat (dorso-ventrally). The upper (or dorsal) side of the body consists of a non-calcified transversely oval or semi-circular headshield (

cephalon Cephalon, Inc. was an American biopharmaceutical company co-founded in 1987 by pharmacologist Frank Baldino Jr., Frank Baldino, Jr., neuroscientist Michael Lewis, and organic chemist James C. Kauer—all three former scientists with the DuPont ...

), and a tailshield (

pygidium The pygidium (: pygidia) is the posterior body part or shield of crustaceans and some other arthropods, such as insects and the extinct trilobites. In groups other than insects, it contains the anus and, in females, the ovipositor. It is compos ...

) longer than the cephalon, without any body segments in between. The body is narrowed at the articulation between cephalon and pygidium. The long many-segmented antennas are directed sideways. There are no eyes. The gut has a relatively large diameter (14-18% of the width of the body), and next to four pairs of large digestive sacs (or caeca). The cephalon has branched diverticula occupying most of the cephalon (unlike in '' Misszhouia''). Naraoia had appendages with two branches on a common basis, like ''Misszhouia'' and trilobites. At least the anterior trunk limbs have exopods with large, paddle-shaped distal lobes and short flattened side branches (setae) on the shaft. The endopod (known only in ''N. compacta'') is composed of six podomeres.Chen, J.-Y., G.D. Edgecombe and L. Ramskjöld. Morphological and ecological disparity in naraoiids (Arthropoda) from the Early Cambrian Chengjiang Fauna, China. Records of the Australian Museum 49(1), pp. 1-24. 199

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Distribution

Species of ''Naraoia'' are known from Canada, the United States, South China, and Australia, occurring in deposits ranging from the Lower Cambrian (

Atdabanian Cambrian Stage 3 is the still unnamed third stage of the Cambrian. It succeeds Cambrian Stage 2 and precedes Cambrian Stage 4, although neither its base nor top have been formally defined. The plan is for its lower boundary to correspond approxi ...

) to the late Upper Silurian ( Pridoli).

Ecology

Sediments present in the gut of ''Naraoia'' suggest that it may have been a deposit feeder, eating large amounts of soil, like an

earthworm An earthworm is a soil-dwelling terrestrial invertebrate that belongs to the phylum Annelida. The term is the common name for the largest members of the class (or subclass, depending on the author) Oligochaeta. In classical systems, they we ...

. A very large, complex system of gut diverticula and a gut with great holding capacity indicate that the diet of ''Naraoia spinosa'' was low in

nutrition Nutrition is the biochemistry, biochemical and physiology, physiological process by which an organism uses food and water to support its life. The intake of these substances provides organisms with nutrients (divided into Macronutrient, macro- ...

. On the other hand, the morphology of the digestive system has also been interpreted as representing a predatory habit. The large, paddle-shaped distal lobes and short lamellar setae on the exopods, the implanting of the antennas to the side, and the angle of the cephalon with the pygidium of up to 90° with which many specimen are found, all agree with a life of burrowing.

Habitat

All naraoiids were probably marine bottom dwellers.

Key to the species

References

External links

*
The Smithsonian Intstitution's ''Naraoia'' page

Genus †Naraoia - Hierarchy - The Taxonomicon
{{Taxonbar, from=Q283508 Nektaspida Burgess Shale animals Maotianshan shales fossils Cambrian first appearances Silurian extinctions Wheeler Shale Cambrian genus extinctions