Agnostus is from the Ancient Greek άγνωστος "unknown", and reflects the fact that early paleontologists did not know what type of animal with which they were confronted. The species epithet has this origin:
pisiformis derives from the Latin pīsum which means "pea", and forma meaning "shape", for its globe-like appearance when rolled.
Description
Like all Agnostida, Agnostus is diminutive and the headshield and tailshield are around the same size and outline. As in all Agnostina, Agnostus has two thoraxsegments. When Agnostina are enrolled, a tiny opening between the cephalon and the thorax becomes visible that uniquely distinguishes this group from the Eodiscina. Two features shared with all Agnostoidea are that the basal glabellar lobes are anteriorly expanded, and that the frontal lobe of the central raised area of the cephalon is not laterally expanded compared to the backward lobe. This is unlike in the Condylopygoidea. Like in all Agnostidae, the cephalic border is narrow. Like in all Agnostinae, the pygidial axis is simple, the posterior lobe not wider than the anterior, and usually long but not reaching border furrow. In Agnostus the glabella is connected to the border by a median preglabellar furrow. The anterior glabellar lobe is ogival or rounded at its front. The posterior lobe is tapering forward. The glabella carries an axial node level with or slightly behind a variably developed second furrow. The pygidium has a long axis, although this usually does not reach the border furrow. The pygidial axis is not connected to the border by a median furrow. The pygidium carries backward directed spines on its margins, where it curves back towards the midline. In the subgenus Agnostus , the axis of the pygidium is relatively narrow, ending pointed, or narrowly rounded, and furrows crossing the pygidial axis are weak at best. The subgenus Agnostus has a broadly rounded termination of the axis extending nearly to posterior border furrow. The axis is narrower halfway along its length and crossing furrows are well developed.
Appendages
The appendages of meraspid developmental stages of Agnostus pisiformis are the best preserved of any trilobite found so far. Agnostus pisiformis has nine pairs of appendages, the antennules, and eight pairs of "legs", that differ in size and composition. Four pairs of appendages originate from the cephalon, two from the thorax, and three from the pygidium. The frontal antennules are short and stout. The most forward pair of "legs" has a small coxa, a seven-podomere exopoditecarrying long, bristle-like setae and gill-like extensions, and no endopodite. The second pair has a five-podomere exopodite and a six-podomere endopodite. The third has a three-podomere exopodite and a six-podomere endopodite. The fourth has a two-podomere exopodite and a seven-podomere endopodite. The fifth has a single-podomere exopodite and a six-podomere endopodite. Further appendages are more and more reduced in size and complexity.
Distribution
Agnostus is said to have had a cosmopolitan distribution. The nominate subgenus occurs from the late Middle Cambrian to the early Upper Cambrian, while the subgenus Agnostus is restricted to the early Upper Cambrian.
A. artilimbatus occurs in the Middle Cambrian of Kazakhstan.
A. pisiformis is known from the earliest Upper Cambrian of Sweden.
Taxonomy
A. pisiformis was the first nominal species to be described and illustrated in the literature. Magnus von Bromell in 1729 described disarticulated heads and tails in matrix as "minimorum vermiculorum vaginipennium". The earlier concepts of the genusAgnostus were very wide, initially including almost all of the Agnostida, but this has gradually been narrowed down. This is the reason why a large number of species are now assigned to other genera, often in other families or even to the Eodiscina.
Species
classified under the genus include:
Species previously assigned to ''Agnostus''
As the genus Agnostus was erected early on, many species have since been reassigned.
