| Nektaspida | |
|---|---|
 | |
| Naraoia compacta fossils with preserved appendages | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Clade: | †Artiopoda |
| Subphylum: | †Trilobitomorpha |
| Order: | †Nektaspida Raymond, 1920 |
| Families[2] | |
Nektaspida (also calledNaraoiida,Nektaspia andNectaspida) is an extinctorder of non-mineralisedartiopodanarthropods. They are known from the lower-Cambrian to the upperSilurian. Originally classified astrilobites, which they superficially resemble, they are now placed as close relatives as members of theTrilobitomorpha within Artiopoda. The order is divided into three major families;Emucarididae,Liwiidae, andNaraoiidae.
The order was originally proposed by Raymond in 1920 as Nektaspia.[3] Størmer corrected it to Nectaspida for the 1959Treatise on Invertebrate Paleontology to conform with the names of the other trilobite orders. Whittington described it in 1985 with the spelling Nektaspida; the revised 1997 Treatise by Raymond and Fortey uses this spelling, as do other modern works.[4]Whittington (1985) placed the order in theTrilobita. Cotton & Braddy (2000) place it in a new "Trilobite clade" containing the Trilobita, recognizing the close affinities of the nektaspids to trilobites. However this necessitates the inclusion of genera that look very little like trilobites.,[4] it was formerly placed in the stem-group to thechelicerata subdivision of the Arthropoda phylum.[5] However, it currently considered part ofArtiopoda, the clade that contains trilobites and their close relatives.[6]
The group is united by several morphological characters, including reduced or absent lateral eyes, ahypostome with anatant attachment, extensive articulation overlap between unfusedtrunktergites, and fusedpleurae that do not formlateral spines.[7] Preserved soft tissue of the group indicates that the cephalic (head) shield of nektaspids and liwiids contained branched digestive glands.[2] The Naraoiidae have their exoskeletons mostly composed of only two major shields, which have a single articulation point.[8]
The naraoiids have been interpreted asbenthic organisms that were opportunistic scavengers and predators of soft-bodied prey, with the spine-likeendites of the limbs possibly allowing soft-bodied prey to be shredded before ingestion. The differences in gut morphology between some species of naraoiids suggests that some species fed only intermittently, while others fed regularly.[9]
The group first appeared and was most diverse in theCambrian, especially during theCambrian Series 2.Emucarididae are only known from the Cambrian Series 2 of EastGondwana (now Australia) and the then nearbySouth China.Naraoiidae were diverse in low latitudes during Cambrian Series 2 and the followingMiaolingian inLaurentia and in South China.Liwiidae first appeared inBaltica during the Cambrian, and are absent from major Cambrian deposits elsewhere during this period, but are widespread in the southern high-latitude margin of Gondwana during theOrdovician.[2] The youngest known member of the group is the Laurentian naraoiidNaraoia bertensis from theBertie Formation in Ontario, Canada, dating to theUpper Silurian.[10]
