Temporal range: Cambrian Stage 3–Late Silurian
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.
History of the classification
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.
Sediments present in the gut of Naraoia suggest that it may have been a deposit feeder, eating large amounts of soil, like an earthworm; 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. 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.
All naraoiids were probably marine bottom dwellers.
Key to the species
|1||Pygidium less than 1½× as long (along the axis) as the cephalon; If the digestive system is visual, gut more than ⅛ of the width of the body, with branched diverticula filling most of the cephalon; Antennas implanted laterally. → 2|
|-||Pygidium more than 1¾× as long as the cephalon; If the digestive system is visual, gut less than ⅛ of the width of the body, with four pairs of small bifurcating sacs of equal size reaching at most ⅓ of the width of the cephalon; Antennas implanted anteriorly. 25 pairs of biramous legs. Up to 6 cm in length.
→ Misszhouia longicaudata (Zhang & Hou 1985), jr. syn. Naraoia longicaudata
|2||Pygidium with more or less prominent spines; cephalon at least 1⅓× as wide as long, with genal spines. → 3|
|-||Pygidium without spines, termination rounded or slightly pointed; cephalon at most 1¼× as wide as long, genal spines present or not. → 4|
|3||Dominant spines are located at or slightly in front of the widest point of the pygidium and at the termination. ±17 pairs of biramous legs. Up to 4 cm in length. Known from the Middle Cambrian Burgess Shale, Stephen Formation, British Columbia, Canada.
→ Naraoia spinifer Walcott 1931
|-||Dominant spines are located at the pygidial angle, the posterior border slightly concave. ±19 pairs of biramous legs. Up to 4½ cm in length. Known from the Lower Cambrian Chengjiang biota of China.
→ Naraoia spinosa Zhang & Hou 1985
|4||Doublure in the cephalon narrow; Pygidium with rounded termination. → 5|
|-||Anterior doublure broad, almost ¼× the length of the cephalon at midline; Pygidium terminates slightly pointed. Known from the late Upper Silurian (Pridolian) Williamsville Member of the Bertie Formation of Southern Ontario, Canada.
→ Naraoia bertiensis Caron, Rudkin & Milliken 2004
|5||Genal angle rounded, spines absent. Known from the Balang Formation, near Geyi, Taijang County, Guizhou, China.
→ Naraoia taijiangensis Peng, Zhao & Sun 2012
|-||Genal angle blunt, spines may be present. 19 pairs of biramous legs. Up to 4 cm in length. Know from the Middle Cambrian Burgess Shale, the Early to Middle Cambrian of Idaho and Utah, and from the Early Cambrian Emu Bay Shale in Australia.
→ Naraoia compacta Walcott 1912, jr. syn. N. halia, N. pammon
- 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
- 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. 1997 
- Bergström, J.; Hou, X.G.; Hålenius, U. (2007). "Gut contents and feeding in the Cambrian arthropod Naraoia". GFF 129 (2): 71. doi:10.1080/11035890701292071.
- Vannier, J.; Chen, J. N. Y. (2007). "Digestive system and feeding mode in Cambrian naraoiid arthropods". Lethaia 35 (2): 107. doi:10.1111/j.1502-3931.2002.tb00072.x.
- L. Ramskold, J.-Y. Chen, G.D. Edgecombe, and G.-Q. Zhou ( 1996). Preservational folds simulating tergite junctions in tegopeltid and naraoiid arthropods. Lethaia 29:15-20. ISSN 0024-1 164.
- Whittington, H.B. The Middle Cambrian trilobite Naraoia, Burgess Shale, British Columbia. Philosophical Transactions of the Royal Society of London, series B, 280, pp.409-443. 1977.
- Zhang, W.T. and X.G. Hou. Preliminary notes on the occurrence of the unusual trilobite Naraoia in Asia. Acta Paleontologica Sinica, 24, pp. 591-595. 1985.
- Caron, J.-B., D.M. Rudkin and S. Milliken. A new Silurian (Pridolian) naraoiid (Euarthropoda: Nektaspida) from the Bertie Formation of Southern Ontario, Canada - Delayed fallout from the Cambrian explosion. Journal of Paleontology 78(6), pp. 1138-1145. 2004.
- Peng, J., Y.L. Zhao and H.J. Sun. Discovery and significance of Naraoia from the Qiandongian (lower Cambrian) Balang Formation, Eastern Guizhou, South China. Bulletin of Geosciences 87(2). 2012 
- Robison, R.A. New occurrences of the unusual trilobite Naraoia from the Cambrian of Idaho and Utah. University of Kansas Paleontological Contributions 112, pp.1-8. 1984.
- Nedin, C. Anomalocaris predation on nonmineralized and mineralized trilobites. Geology 27, pp. 987-990. 1999.