''Placoparia'' is a genus of trilobites of average size (up to 6 cm) that lived during the late Lower to the early Upper Ordovician on the

paleocontinent A paleocontinent or palaeocontinent is a distinct area of continental crust that existed as a major landmass in the geological past. There have been many different landmasses throughout Earth's time. They range in sizes, some are just a collection ...

Gondwana Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final stages ...

Avalonia Avalonia was a microcontinent in the Paleozoic era. Crustal fragments of this former microcontinent underlie south-west Great Britain, southern Ireland, and the eastern coast of North America. It is the source of many of the older rocks of Wester ...

and Laurentia, now the Czech Republic, France, Germany, Morocco, Portugal, Spain and Wales. Its 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 ...

) is semi-circular to rectangular with rounded frontal corners. It lacks eyes, but eye ridges are present. The fact that the facial sutures are

opisthoparian 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 ...

(with sutures in ''

Pliomera ''Pliomera'' is a genus of trilobites that lived during the Middle Ordovician on the paleocontinent Baltica, now Norway, Sweden, Estonia and the Russian Federation, and in Argentina. It can be recognized for its pentagonal glabella widest bet ...

gonatoparian 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 ...

) is an exception in the otherwise proparian

Cheirurina Cheirurina is a suborder of the trilobite order Phacopida. Known representatives range from the uppermost Cambrian (upper Furongian) to the end of the Middle Devonian (Givetian The Givetian is one of two faunal stages in the Middle Devonian Pe ...

. The thorax has 11 or 12 segments, with the axis slightly wider than the ribs (or pleurae) to its sides. The tips of the pleurae are free, which resembles an old-fashion central heating radiator. The axis in the small tailshield (or pygidium) consists of four rings and a minute endpiece. The four pleurae end in spatulate spines that fit to corresponding indentations in the cephalon.

Distribution

* ''P. borni'' has been found in the Middle Ordovician of the Czech Republic, France, Spain and Portugal (Llandeilo, Dobrotivian). * ''P. cambriensis'' is known from the Lower Ordovician of Morocco (Arenig), Wales (Upper Arenig and lower Llarvirn), and the Lower and Middle Ordovician of Spain, Portugal, France and the Czech Republic (Llarvirn). * ''P. tournemini'' is present in the Middle Ordovician of Spain, Portugal and France (Llandeilo, Dobrotivian). * ''P. zippei'' occurs in the Middle Ordovician of the Czech Republic (Llandeilo, Dobrotivian) and Morocco (Llandeilo), and a comparable form in Germany (Llarvirn).

Description

The lateral borders of the free cheeks (or librigenae) of bear indentations that fit the tips of the pleurae of the thorax and the first pair of pygidial pleurae and these could effectively lock the uncalcified underside of the animal during enrollment. This mechanism is comparable the situation in ''Pliomera''. Remains of the alimentary tract containing coarse sediment, and of a multisegmented antenna have been found in ''Placoparia''.

References

{{Taxonbar, from=Q20716808 Pliomeridae Fossils of the Czech Republic Letná Formation Fossils of France Fossils of Germany Fossils of Morocco Fossils of Portugal Fossils of Spain Fossils of Wales Early Ordovician first appearances Late Ordovician extinctions Floian Sandbian Ordovician Wales Fezouata Formation fossils