''Sphaeragnostus'' is an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

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 ...

from a well-known class of

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

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, the

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 At ...

s. It can be recognized by having two thorax segments, a totally effaced headshield (or cephalon), while the tailshield (or pygidium) although effaced, has a clear furrow parallel to its border, and a short, convex, subcircular axis. It lived during the Ordovician (upper Tremadoc to

Ashgill Ashgill is a village in South Lanarkshire, Scotland near Larkhall. It is part of the Dalserf parish. The village church dates back to 1889 The village has a shop The local school was first in Scotland to have an automated bell, installed in 1 ...

).Whittington, H. B. et al. Part O, Treatise on Invertebrate Paleontology. Revised, Volume 1 – Trilobita – Introduction, Order Agnostida, Order Redlichiida. 1997

Taxonomy

It is difficult to relate ''Sphaeragnostus'' to other agnostids, and it is undecided whether this genus should be assigned to the Agnostoidea, the Condylopygoidea, or a new monotypic superfamily.

Distribution

* ''S. similaris'' has been found in the Middle Ordovian of the Czech Republic (Llandeilo of Bohemia, Dobrotivå Formation). * ''S. cingulatus'' is known from the Upper Ordovician of Sweden (Koången, Lindegård near Lund, and Scania, Jerrestad Mudstone Formation), Denmark (Vasegård, Læså, Bornholm, Jerrestad Mudstone), Poland (Brzezinki) and China (Katian of Jiangxi, western Zhejiang and Subei, Gansu). * ''S. gaspensis'' was collected from the Upper Ordovician of Canada (Grande Coupe beds, Percé, Quebec) * ''S. nudatus'' has been excavated from the Upper Ordovician of Kazakhstan (lower Majlisor horizon, Caradoc).

Description

Like all Agnostida, ''Sphaeragnostus'' is diminutive and the

cephalon Cephalon, Inc. was an American biopharmaceutical company co-founded in 1987 by pharmacologist Frank Baldino, Jr., neuroscientist Michael Lewis, and organic chemist James C. Kauer—all three former scientists with the DuPont Company. Baldino s ...

and

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 ...

are of approximately the same size and outline (or isopygious). Like all Agnostina, Sphaeragnostus has only two

thorax The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the cre ...

segments. The cephalon is externally totally effaced and lacks border furrows, axial furrows, and the glabellar node. The pleural regions of thorax segments are very small, while the axis is very wide (perpendicular to the midline) and remarkably short (along the axis). The pygidium has well-defined border furrows and narrow borders. Axial furrows define a short, broad, convex, subcircular, and unfurrowed axis, that bears both axial and terminal nodes. It lacks a median postaxial furrow. If there are spines on the ”rear corners” of the pygidium, these are minute.

Differences with Leiagnostus

Cephalons of ''Sphaeragnostus'' are extremely effaced and look much like '' Leiagnostus'' species. Pygidia of ''Sphaeragnostus'' however, differ from other highly effaced genera in a smooth but easily recognizable subcircular axis.

References

Agnostida genera Agnostoidea Ordovician trilobites Fossils of Canada Fossils of the Czech Republic Fossils of Poland Fossils of Denmark Fossils of Sweden Fossils of Kazakhstan Fossils of China Paleontology in Quebec Paleozoic life of Quebec {{agnostida-stub