Nektaspida (also called Naraoiida, Nektaspia and Nectaspida) is an extinct
order of non-
mineralisedartiopodanarthropods. They are known from the mid-
Cambrian to the upper
Silurian. Originally classified as
trilobites, which they superficially resemble, they are now placed as close relatives as members of the
Trilobitomorpha within Artiopoda. The order is divided into three major families;
Emucarididae,
Liwiidae, and
Naraoiidae.
Naming history and taxonomic placement
The order was originally proposed by Raymond in 1920 as Nektaspia.[1] Størmer corrected it to Nectaspida for the 1959 Treatise 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.[2]Whittington (1985) placed the order in the
Trilobita. 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.,[2] it was formerly placed in the stem-group to the
chelicerata subdivision of the Arthropoda phylum.[3] However, it currently considered part of
Artiopoda, the clade that contains trilobites and their close relatives.[4]
Anatomy
The group is united by several morphological characters, including reduced or absent lateral eyes, a
hypostome with a
natant attachment, extensive articulation overlap between unfused
trunktergites, and fused
pleurae that do not form
lateral spines.[5] Preserved soft tissue of the group indicates that the cephalic (head) shield of nektaspids and liwiids contained branched digestive glands.[6] The Naraoiidae have their exoskeletons mostly composed of only two major shields, which have a single articulation point.[7]
Ecology
The naraoiids have been interpreted as
benthic organisms that were opportunistic scavengers and predators of soft-bodied prey, with the spine-like
endites 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 only intermittently fed, while others regularly fed.[8]
Evolutionary history
The group first appeared and was most diverse in the
Cambrian, especially during the
Cambrian Series 2.
Emucarididae are only known from the Cambrian Series 2 of East
Gondwana (now Australia) and the then nearby
South China.
Naraoiidae were diverse in low latitudes during Cambrian Series 2 and the following
Miaolingian in
Laurentia and in South China.
Liwiidae first appeared in
Baltica 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 the
Ordovician.[6] The youngest known member of the group is the Laurentian naraoiid Naraoia bertensis from the
Bertie Formation in Ontario, Canada, dating to the
Upper Silurian.[9]