Unranked clade (stem group arthropods)
Naraoia is usually compared to the trilobites, but its exact relationships are uncertain (Whittington, 1977). The naraoiids and other trilobite-like arthropods, sometimes referred to as Trilobitoidea, can be grouped together with the trilobites to form the Lamellipedians (Hou and Bergström, 1997; Wills et al. 1998; Edgecombe and Ramsköld, 1999). This group has been variously placed in the upper stem lineage of the arthropods (Budd, 2002), or in the stem lineage of either the mandibulates (Scholtz and Edgecombe, 2006) or the chelicerates (Cotton and Braddy, 2004).
Naraoia – from Narao Lakes, near Kicking Horse Pass in Yoho Park, British Columbia. From the Stoney First Nation Nakoda word Narao, meaning “hit in the stomach,” which likely refers to James Hector, who was kicked by a horse while travelling up the Kicking Horse River in 1858.
compacta – from the Latin compactus, “joined together.”
Lectotype –USNM57687 (N. compacta) and holotypesUSNM83946 (N. spinifer) andUSNM189210 (N. halia) in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.
Burgess Shale and vicinity: N. spinifer (Walcott, 1931); N. halia (Simonetta and Delle Cave, 1975) from the Walcott Quarry, Burgess Shale.
Other deposits: N. longicaudata and spinosa (Zhang and Hou, 1985) from the Early Cambrian Chengjiang biota of South China, of which N. longicaudata was later placed in its own genus, Misszhouia (Chen et al., 1997); Possible specimens of Naraoia have been found at the Lower Cambrian Emu Bay Shale in Australia (Nedin, 1999). Unlike most Burgess Shale arthropods, Naraoia has also been found in rocks younger than the Cambrian, in the Late Silurian Bertie Formation of Southern Ontario (Caron et al., 2004).
Middle Cambrian, Bathyuriscus-Elrathina Zone (approximately 505 million years ago).
The Walcott and Raymond Quarries on Fossil Ridge. The Trilobite Beds on Mt. Stephen, Tulip Beds (S7) and Collins Quarry as well as other smaller localities on Mount Stephen.
Brief history of research:
The first description of Naraoia was N. compacta by Walcott (1912), who later described a second specimen, N. spinifer (1931). Simonetta and Delle Cave (1975) re-examined the specimens and designated the new species N. halia and N. pammon. A major redescription of all Burgess Shale material was undertaken by Whittington (1977), and N. compacta specimens from the Marjum Formation in Utah and the Gibson Formation in Idaho were described by Robison (1984), both of whom synonymized N. halia and N. pammon with N. compacta. However, a major restudy of the naraoiids by Zhang et al. (2007) concluded that N. halia is actually a valid species.
Naraoia consists of two dorsal shields with a convex axial region, including a roughly square head shield and an elongated body shield. A pair of long, multi-jointed antennae emerges from beneath the head shield. Behind the antennae are four pairs of cephalic appendages and 14 pairs of trunk appendages. All these appendages are segmented and branch into two (biramous), with a spiny walking limb made up of seven segments, and a filamentous branch consisting of a thin shaft bearing many lamellae (flexible and elongated plate-like elements). The basal segment of the biramous appendage is composed of a large, spiny plate.
Internal structures of Naraoia are well preserved, with the most conspicuous feature being the complexly branched gut glands visible on the cephalic shield. The gut passes along the whole length of the body, with paired gut glands visible in the anterior half.
Hundreds of specimens of Naraoia are known from the Walcott Quarry, where they make up about 0.74% of the community (Caron and Jackson, 2008). Naraoia is rare in all the other known localities.
Naraoia likely spent much of its time walking on the sea floor, since the rigidity of its appendages would only allow for limited periods of swimming. It would have sensed its environment, including food items, using its antennae. Naraoia used the segmented walking limbs of its biramous appendages for walking and for manipulating food items, which were crushed and moved towards the mouth using the spiny basal plate. The filamentous branches of the biramous limb were used for gas exchange and to propel the animal through the water during short burst of swimming. The large gut glands and spiny appendages suggest that Naraoia was a predator or scavenger. Specimens with healed injuries suggest that Naraoia was also a prey item for other larger predators.