Naraoia
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Naraoia Temporal range: Cambrian Stage 3–Late Silurian |
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| Naraoia spinosa | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Stem group: | Chelicerata |
| Order: | †Nektaspida |
| Family: | †Naraoiidae |
| Genus: | †Naraoia |
| Type species | |
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Naraoia compacta Walcott, 1912 |
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| Species | |
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Naraoia is a genus of small to average size (about 2-4½ cm long) marine arthropods within the family Naraoiidae, that lived from the early Cambrian to the late Silurian period. The species are characterized by a large alimentary system and sideways oriented antennas.
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, Canada.
When the fossil was first discovered in Canada's Burgess Shale, it was believed to be a crustacean, 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 Trilobita. Misszhouia longicaudata was formerly considered a member of the genus Naraoia, originally known as N. longicaudata, until separated in 1997.
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), and a tailshield (pygidium) 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). 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.
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