Ptychopariida
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 Elrathia kingi
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Ptychopariida is a large, heterogeneous order of trilobite containing some of the most primitive species known. Many date to the Lower Cambrian period, but the order was extant through the Upper Ordovician period. Many of the offshoot species are difficult to classify in this single order.
of the superfamily Ptychoparioidea
These trilobites have facial sutures that run along the margin of the glabella (central portion of the head) to the shoulder point where the cephalon meets the thorax. These sutures outline the glabella, or the main, central part of the head that doesn't include the free cheeks. The eyes are medial along the glabella on the suture line (however, some species have no eyes). Genial spines stretching back from either side of the cephalon are often present. The moults of these trilobites can often be told from the actual fossils by whether or not the free cheeks are present, as they are not attached to the moults. Bladelike genial spines are often present.
The thorax is large, and is typically made up of eight or more segments. The thorax is usually much longer than the tail section (pygidium), which is usually small. In some species, the pygidia are outlined with a flat border.
The subclass Librostoma was recently created to encompass several highly related orders (Ptychopariida, Asaphida, Proetida, Harpetida, and possibly the Phacopida). They are now known as the "Librostome Orders". The Proetida and Harpetida were originally included in order Ptychopariida.
[edit] References
- Order Ptychopariida
- A Guide to the Orders of Trilobites By Sam Gon III
