''Acadagnostus'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

trilobite Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the At ...

from the

Middle Cambrian Middle or The Middle may refer to: * Centre (geometry), the point equally distant from the outer limits. Places * Middle (sheading), a subdivision of the Isle of Man * Middle Bay (disambiguation) * Middle Brook (disambiguation) * Middle Creek (di ...

, with 7 species currently recognized. The type species ''A. acadicus'' has the widest distribution known from any peronopsid (with '' Peronopsis integer'' and ''P. scutalis'') and has been found in North America, Greenland, England, Western Europe, Eastern Europe, Central Asia, the Altai Mountains, the Siberian shield, China, and Australia.

Etymology

* ''ancisa'' is Latin meaning 'cut round', referring to the shape of the rear pygidial margin. * ''australis'' is Latin for 'southern'. * ''certus'' is Latin meaning 'definite' or 'sure'. * ''syrma'' is

Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic peri ...

meaning 'robe with a train', indicating the wide pygidial border in the rear.

Description

Like all Agnostida, ''Acadagnostus'' is diminutive, with the headshield (or

cephalon Cephalon, Inc. was an American biopharmaceutical company co-founded in 1987 by pharmacologist Frank Baldino, Jr., neuroscientist Michael Lewis, and organic chemist James C. Kauer—all three former scientists with the DuPont Company. Baldino s ...

) and tailshield (or pygidium) of approximately the same size (or isopygous) and outline. Like all Agnostina, ''Acadagnostus'' has only two thorax segments. Like all Peronopsidae, the cephalon and pygidium have a complete set of furrows, and the preglabellar furrow - between the front of the central raised area of the cephalon (or

glabella The glabella, in humans, is the area of skin between the eyebrows and above the nose. The term also refers to the underlying bone that is slightly depressed, and joins the two brow ridges. It is a cephalometric landmark that is just superior to ...

) - is lacking or incomplete. ''Acadagnostus'' has a round cephalon without spines. The cephalic border is narrow, while the border furrow is wide. The lateral furrows of posterior lobe of glabella can be seen as sets of pits or a narrow straight or slightly curved backward furrow. It carries a node at or slightly behind the center of the rear lobe. The pygidium is subquadrate, has a flat border, that is sometimes posteriorly widened and has a weak bifurcation. A pair of small spines at the outer rear corners is present. The border furrow is wide. The rhachis has parallel sides or is conical, but the second segment is not narrower than its neighbours. Furrows crossing the rhachis are absent or weak. The segments are shown by differences in relief of the rhachis. There is a large median node, and a postaxial furrow is present, but usually very short.

Taxonomy

The ancestor of ''Acadagnostus'' is probably close to ''Peronopsis normata''. It is believed that ''Acadagnostus'' gave rise to '' Pseudoperonopsis'', the earliest representative of the Diplagnostidae family.

References

{{Taxonbar, from=Q15198212 Peronopsidae Agnostida genera Cambrian trilobites of North America Fossils of China Fossils of Great Britain Fossils of England Fossils of Greenland Fossils of Russia Fossils of Australia Cambrian trilobites of Asia Cambrian trilobites of Australia Cambrian trilobites of Europe Cambrian genus extinctions