micrograph of a fossil of
Agnostus pisiformis and reconstruction of 2-4th cephalic appendages Like all Agnostida,
Agnostus is diminutive and the headshield (or
cephalon) and tailshield (or
pygidium) are around the same size (or
isopygous) and outline. As in all Agnostina,
Agnostus has two
thorax segments. When Agnostina are enrolled, a tiny opening between the cephalon and the thorax becomes visible (the cephalothoracic aperture) 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 (or glabella) is not laterally expanded compared to the backward lobe. This is unlike in the
Condylopygoidea. In all Agnostidae the cephalic border is narrow and the pygidial axis is simple, the posterior lobe not wider than the anterior (or axiolobate), 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 tapers 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 (or rhachis) is not connected to the border by a median (postaxial ) furrow. The pygidium carries backward directed spines on its margins, where it curves back towards the midline. In the subgenus
Agnostus (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 (Homagnostus) 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 leg of
Agnostus pisiformis Agnostid appendages are known only from exceptionally preserved specimens of
Agnostus pisiformis from the Upper Cambrian Orsten deposits of Sweden (Müller & Walossek 1987) and the appendages of
meraspid developmental stages of
A. pisiformis (½-1 mm total body length) are the best preserved of any trilobite found so far. The species 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
antennule are short and stout. The most forward pair of "legs" has a small coxa, a seven-
podomere exopodite carrying 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 (Müller & Walossek 1987). == Distribution ==