''Pliomera'' is a
genus of
trilobites that lived during the
Middle 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 ...
Baltica, now Norway, Sweden, Estonia and the Russian Federation, and in Argentina. It can be recognized for its pentagonal glabella widest between the frontal corners, with an inverted V-shaped occipital ring. In front of the occipital furrow that crosses the entire glabella, two pairs of dead-ending furrows create three side lobes left and right. The front of the glabella also has three dead-ending furrows, a very short one on the midline and left and right a longer one, directed inward and slightly backward. The eyes are small and are not connected to the glabella by an eye ridge. The thorax and pygidium are very regularly divided into up to 23 rather narrow segments, without a furrow within each of the pleurae. The pleurae are clearly wider than the axis. The pygidium ends in downward pointing toothlike spines.
Taxonomy
Species previously assigned to ''Pliomera'' are:
* ''P. insangensis'' = ''
Encrinurella insangensis''
* ''P. insolita'' = ''
Kanoshia insolita''
* ''P. linnarssoni'' = ''
Pliomerops linnarssoni''
* ''P. martelli'' = ''
Pliomerina martelli''
* ''P. sulcifrons'' = ''
Pliomerina sulcifrons''
Distribution
* ''P. brevicapitata'' occurs in the Middle Ordovician of the Russian Federation (Volkov Stage, Darriwillian, St. Petersburg region).
* ''P. fischeri'' is known from the Middle Ordovician of the Russian Federation (Kunda Stage Darriwillian, St. Petersburg region).
[http://museum-21.ru/files/57/shemi_raspr/Shema%20rasprostraneniy.pdf ]
Description
''Pliomera'' has a pentagonal
glabella (the central area of the head shield) that is widest between the frontal corners, and has an inverted V-shaped occipital ring. In front of the occipital furrow that crosses the entire glabella, two pairs of dead-ending furrows, create three side lobes left and right. The front of the glabella also has three dead-ending furrows, a very short one on the midline and left and right a longer one, directed inward and slightly backward. A very efficient fastening device is unique in ''Pliomera''. It consists of a row of 7 to 9 downward directed denticles along the front of the glabella, which interlocked with corresponding teeth on the pygidium, when the animal protected itself by enrolling. The eyes are small and raised above the cheeks but not connected to the glabella by an eye ridge. The facial sutures in genus ''Pliomera'' itself are
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 ...
, while in the genus ''
Placoparia'' these 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 ...
, and this is unlike all other pliomerids where facial sutures are
proparian. The
thorax may have between 12 and 18 segments. The
pygidium has 4 or 5 axial rings and small termination. The corresponding pleural segments are very well defined and end in tooth-like points, the last pair of which is completely embracing the small terminal axial segment.
References
{{Taxonbar, from=Q16988268
Pliomeridae
Ordovician trilobites
Fossils of Argentina
Fossils of Estonia
Fossils of Latvia
Fossils of Norway
Fossils of Russia
Fossils of Sweden