Agnostus

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Agnostus is a genus of very small, ancient trilobites that lived about 506 to 485 million years ago during the middle to late Cambrian. It is the main example of the family Agnostidae and is split into two subgenera: Agnostus and Homagnostus.

The name Agnostus comes from Greek meaning “unknown,” since early scientists didn’t know what kind of animal it was. Like other agnostids, Agnostus has a headshield (cephalon) and a tailshield (pygidium that are roughly the same size, and only two thoracic segments. When the animal curled up, a small gap appeared between the head and thorax, which helps distinguish this group from related ones.

Key features include a narrow border around the head, a simple long pygidial axis, and a glabella (the raised area on the head) that is connected to the border by a median groove. The front part of the glabella is rounded, and the back part tapers forward. The pygidium’s axis is long but usually does not reach the border, and there are backward-facing spines along the margins.

Differences between the subgenera are subtle: Agnostus (Agnostus) has a narrow, pointed or narrowly rounded pygidial end, with weak cross-lines; Agnostus (Homagnostus) has a broadly rounded end near the rear border and a mid-length axis with stronger cross-lines.

Agnostus is known from fossils worldwide, with the subgenus Agnostus (Agnostus) ranging from the late middle to early upper Cambrian, and Agnostus (Homagnostus) mainly in the early upper Cambrian. The species A. pisiformis is the best-known and was the first to be described; over time, many fossils once placed in Agnostus have been reassigned to other genera.

Appendages of Agnostus are known mainly from exceptionally preserved specimens of A. pisiformis found in Sweden. These show nine pairs of limbs: a pair of antennules near the head and eight leg-like pairs that vary in size and structure, with the front limbs being simpler and the later ones more complex.