''Pseudonaraoia'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

of small (about long) marine

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

s within the family

Naraoiidae Naraoiidae is a family, of extinct, soft-shelled trilobite-like arthropods, that belongs to the order Nectaspida. Species included in the Naraoiidae are known from the second half of the Lower Cambrian to the end of the Upper Silurian. The tota ...

, that lived during the late

Middle 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. Th ...

period (or Darriwillian epoch). The only species presently known is ''Pseudonaraoia hammanni'' (the genus is

monotypic In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispec ...

Etymology

The genus name indicates the likeness to the related genus

Naraoia ''Naraoia'' is a genus of small to average size (about 2-4½ cm long) marine arthropods within the family Naraoiidae, that lived from the early Cambrian to the late Silurian period. The species are characterized by a large alimentary system and ...

. The species was named in honor of Dr. Wolfgang Hammann (†2002), the outstanding German palaeontologist who studied both trilobites and naraoids.

Description

''P. hammanni'' is almost flat (dorso-ventrally). The upper (or dorsal) side of the body consists of a non-calcified headshield ( cephalon) and tailshield (

pygidium The pygidium (plural pygidia) is the posterior body part or shield of crustaceans and some other arthropods, such as insects and the extinct trilobites. In groups other than insects, it contains the anus and, in females, the ovipositor. It is compo ...

) of about equal length, without any body segments in between. The body is strongly narrowed at the articulation between cephalon and pygidium. Both shields have a narrow border, reminiscent of some effaced

agnostid Agnostidae is a family of Agnostida trilobites. Like all Agnostina, they were eyeless and bore only two thoracic segments. They ranged in benthic waters across the globe from 508 to 461 million years ago, containing the following genera, among o ...

trilobites 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 ...

. The axis is not defined by furrows. There are no eyes. Appendages and internal organs are not yet known.

Differences with other Naraoiidae

''Naraoia'' and ''Misszhouia'' are narrowed, less pronounced at the articulation between cephalon and pygidium. Only ''Naraoia bertiensis'' shares a marginal border with ''P. hammanni'', but it can be distinguished by its slightly pointed pygidial termination.

Distribution

''Pseudonaraoia'' has been collected from lower part of the Šárka Formation, Middle Ordovician (Darriwillian), at a temporary exposure of the Červený vrch Hill, in Prague, representing the Barrandian Basin, Czech Republic.

Ecology

All naraoids were probably marine bottom dwellers. ''P. hammanni'' lived in an environment that was most probably poor in oxygen.

Taxonomic history

Only one entire exoskeleton and one pygidium have been found. The authors that described ''Pseudonaraoia'', note its resemblance to effaced agnostids such as '' Leiopyge'', that are contemporary. They cannot rule out that ''Pseudonaraoia'' might actually represent a freak composite of the cephalon and pygidium of such an agnostid, without its two thorax segments. However, no agnostids are known as large as about 1¾cm from the Middle Ordovician. ''Leiopyge'' also has a prominent marginal pygidial furrow. Later scholars seem to accept the validity of the genus.

References

{{Taxonbar, from=Q4798189 Nektaspida Ordovician arthropods Fossils of the Czech Republic