''Sphaeragnostus'' is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

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 preserve ...

marine

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, the

trilobite Trilobites (; meaning "three-lobed entities") are extinction, extinct marine arthropods that form the class (biology), class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most succ ...

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. It is twinned with Saint-Cirq-Lapopie in France. The village has a shop, a chip shop and a hairdress ...

).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., Frank Baldino, Jr., neuroscientist Michael Lewis, and organic chemist James C. Kauer—all three former scientists with the DuPont ...

and

pygidium The pygidium (: 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 compos ...

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

thorax The thorax (: thoraces or thoraxes) or chest is a part of the anatomy of 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 di ...

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