Agnostus
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Agnostus Temporal range: upper Middle Cambrian–lower Upper Cambrian |
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| disarticulate cephalons and pygidia of Agnostus pisiformis, from Kinnekulle, Sweden | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | †Trilobita |
| Order: | †Agnostida |
| Family: | †Agnostidae |
| Genus: |
†Agnostus Brongniart, 1822 |
| subgenera | |
and see text |
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and see text
Agnostus is a genus of agnostid trilobites that lived during the upper Middle Cambrian–lower Upper Cambrian (approximately 506 to 492 million years ago). It is the type genus of the family Agnostidae. It is subdivided into two subgenera, Agnostus and Homagnostus.
Agnostus is from the Ancient Greek άγνωστος (ágnostos) "unknown", and reflects the fact that early paleontologists did not know what type of animal they were confronted with. The species epithets have the following origins:
Like all Agnostida, Agnostus is diminutive and the headshield (or cephalon) and tailshield (or pygidium) are of approximately 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. 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 (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 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 (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.
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