Sphaeragnostus

Sphaeragnostus is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. 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).
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.
Description.
Like all Agnostida, "Sphaeragnostus" is diminutive and the cephalon and pygidium are of approximately the same size and outline (or isopygious). Like all Agnostina, Sphaeragnostus has only two thorax 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.