Proetida
From Wikipedia, the free encyclopedia
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 Devonian Cyphaspis tafilalet trilobites
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Proetida is an order of trilobite that lived from the Ordovician to the Permian. It was the last order of trilobite to go extinct, finally dying out in the Permian extinction.
These typically small trilobites resemble those of the order Ptychopariida, from which the new order Proetida was only recently separated in 1975 by Fortey and Owens. Like the order Phacopida, the proetids have exoskeletons that sometime have pits or small tubercles, especially on the glabella (middle portion of the head). Because of their resemblance to the Ptychopariida in some features, the proetids are included in the subclass Librostoma.
Unlike some trilobites of the order Phacopida, whose eyes are schizochroal, the proetids have the more common holochroal eyes. These eyes are characterized by close packing of biconvex lenses beneath a single corneal layer that covers all of the lenses. Each lens is generally hexagonal in outline and in direct contact with the others. They range in number from one to more than 15,000 per eye. Eyes are usually large, and because the individual lenses are hard to make out, they look smooth and sometimes bead-like.
The thorax of proetids was made up of 8-22 segments, but more commonly there were 10. Many also have genial spines coming off either side of the head, though in some they are blunted. These two features can aid in distinguishing proetids from some trilobites in the order Phacopida, to which they can be very similar.
[edit] References
- Pictorial guide to the order Proetida, by Sam Gon III
- Order Proetida, also by Sam Gon III
