Nektaspida (also called Naraoiida, Nektaspia and Nectaspida) is an extinct order of non- mineralised artiopodan

arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...

s. They are known from the lower-

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

to the upper

Silurian The Silurian ( ) is a geologic period and system spanning 23.5 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 third and shortest period of t ...

. 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. 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. 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., it was formerly placed in the stem-group to the chelicerata subdivision of the Arthropoda phylum. However, it currently considered part of Artiopoda, 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 with a natant attachment, extensive articulation overlap between unfused trunk tergites, and fused pleurae that do not form

lateral Lateral is a geometric term of location which may also refer to: Biology and healthcare * Lateral (anatomy), a term of location meaning "towards the side" * Lateral cricoarytenoid muscle, an intrinsic muscle of the larynx * Lateral release ( ...

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 the Ancient Greek word (), meaning "the depths". ...

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

Gondwana Gondwana ( ; ) was a large landmass, sometimes referred to as a supercontinent. The remnants of Gondwana make up around two-thirds of today's continental area, including South America, Africa, Antarctica, Australia (continent), Australia, Zea ...

(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 The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

. 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.

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 arthropod orders Cambrian first appearances Silurian extinctions