Nektaspida (also called Naraoiida, Nektaspia and Nectaspida) is an extinct

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...

of non- mineralised artiopodan

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...

s. They are known from the mid-

Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...

to the upper

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleoz ...

. Originally classified as

trilobite Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the ...

s, which they superficially resemble, they are now placed as close relatives as members of the

Trilobitomorpha The Artiopoda is a grouping of extinct arthropods that includes trilobites and their close relatives. It was erected by Hou and Bergström in 1997 to encompass a wide diversity of arthropods that would traditionally have been assigned to the Trilo ...

within Artiopoda. The order is divided into three major families; Emucarididae,

Liwiidae Liwiidae is a family of arthropods in the order Nektaspida. Members are known from the Cambrian and Ordovician periods. Taxonomy *''Liwia'' Dzik and Lendzion, 1988 :*'' Liwia plana'' Lendzion, 1975 :*'' Liwia convexa'' Lendzion, 1975 *''Soomas ...

, and Naraoiidae.

Naming history and taxonomic placement

The order was originally proposed by Raymond in 1920 as Nektaspia. Størmer corrected it to Nectaspida for the 1959 ''

Treatise on Invertebrate Paleontology The ''Treatise on Invertebrate Paleontology'' (or ''TIP'') published by the Geological Society of America and the University of Kansas Press, is a definitive multi-authored work of some 50 volumes, written by more than 300 paleontologists, and ...

'' 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. Whittington (1985) placed the order in the

Trilobita Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the At ...

. 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., it was formerly placed in the stem-group to the

chelicerata The subphylum Chelicerata (from New Latin, , ) constitutes one of the major subdivisions of the phylum Arthropoda. It contains the sea spiders, horseshoe crabs, and arachnids (including harvestmen, scorpions, spiders, solifuges, ticks, and mite ...

subdivision of the Arthropoda phylum. However, it currently considered part of

Artiopoda The Artiopoda is a grouping of extinct arthropods that includes trilobites and their close relatives. It was erected by Hou and Bergström in 1997 to encompass a wide diversity of arthropods that would traditionally have been assigned to the Tril ...

, the clade that contains trilobites and their close relatives.

Anatomy

The group is united by several morphological characters, including reduced or absent lateral eyes, a

hypostome In zoology, the hypostome can refer to structures in distinct animal groups: * Hypostome (trilobite), the ventral mouthpart plate in trilobites * Hypostome (tick), the barbed attachment structure associated with the mouthparts of parasitic arachni ...

with a natant attachment, extensive articulation overlap between unfused

trunk Trunk may refer to: Biology * Trunk (anatomy), synonym for torso * Trunk (botany), a tree's central superstructure * Trunk of corpus callosum, in neuroanatomy * Elephant trunk, the proboscis of an elephant Computing * Trunk (software), in rev ...

tergites, and fused pleurae that do not form

lateral Lateral is a geometric term of location which may refer to: Healthcare *Lateral (anatomy), an anatomical direction * Lateral cricoarytenoid muscle * Lateral release (surgery), a surgical procedure on the side of a kneecap Phonetics *Lateral co ...

spines. Preserved soft tissue of the group indicates that the cephalic (head) shield of nektaspids and liwiids contained branched digestive glands. The Naraoiidae have their exoskeletons mostly composed of only two major shields, which have a single articulation point.

Ecology

The naraoiids have been interpreted as

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning " ...

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.

Evolutionary history

The group first appeared and was most diverse in the

, especially during the

Cambrian Series 2 Cambrian Series 2 is the unnamed 2nd series of the Cambrian. It lies above the Terreneuvian series and below the Miaolingian. Series 2 has not been formally defined by the International Commission on Stratigraphy, lacking a precise lower boundar ...

. Emucarididae are only known from the Cambrian Series 2 of East

Gondwana Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final sta ...

(now Australia) and the then nearby

South China South China () is a geographical and cultural region that covers the southernmost part of China. Its precise meaning varies with context. A notable feature of South China in comparison to the rest of China is that most of its citizens are not n ...

. Naraoiidae were diverse in low latitudes during Cambrian Series 2 and the following Miaolingian in

Laurentia Laurentia or the North American Craton is a large continental craton that forms the ancient geological core of North America. Many times in its past, Laurentia has been a separate continent, as it is now in the form of North America, althoug ...

and in South China.

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 The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. T ...

. The youngest known member of the group is the Laurentian naraoiid ''Naraoia bertensis'' from the

Bertie Formation The Bertie Group or Bertie Limestone, also referred to as the Bertie Dolomite and the Bertie Formation, is an upper Silurian ( Pridoli, or Cayugan and Ulsterian age in the local chronologies) geologic group and Lagerstätte in southern Ontario, ...

in Ontario, Canada, dating to the

Upper Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleo ...

Gallery

References

External links

Are Naraoids trilobites?
��photographs of specimens and discussion of classification controversy.

��more technical overview with references and descriptions of known genera. {{Taxonbar, from=Q3337678 Prehistoric chelicerates Prehistoric arthropod orders Cambrian first appearances Silurian extinctions