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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.〔(【引用サイトリンク】title=PBDB: Proetida )〕 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 the trilobites of the Phacopid suborder Phacopina, 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 anywhere between 8–22 segments, but most commonly 10. Many also extend the backcorners of the headshield into so-called genal spines. These two features can aid in distinguishing proetids from some Phacopid trilobites in the suborder Phacopina, to which they can be very similar. == Taxonomy == Opinions about the composition of and the affinities within the Proetida, and to other trilobites have been very divergent over time. Recently, it has been suggested to retain in the Proetida only the Proetidae and Tropidocoryphidae families. The remainder of the families should be combined in a new order Aulacopleurida Adrian, 2011 that would consist of the Aulacopleuridae, Brachymetopidae, Dimeropygidae, Rorringtoniidae, Scharyiidae, Bathyuridae, Telephinidae, Holotrachelidae and Hystricuridae (considered Proetida before), combined with Alokistocaridae, Crepicephalidae, Ehmaniellidae, Marjumiidae, Solenopleuridae and Tricrepicephalidae (considered Ptychopariida before). The argumentation for this split relies on differences in early larval stages. While the remaining Proetida have globular larvae very unlike the adult form, the Aulacopleurida have adultlike larvae with paired spines. Others observe that globular non-adult larvae also occur in some taxa within the proposed Aulacopleurida. More recently phylogenetic analysis of both larval and adult characters suggests the Proetida as earlier understood probably are monophyletic. Two larval characters are unique to all Proetida. The first is that the eye develops on the side of the headshield, not at the front. The second is a forwardly tapering glabella that is distanced from the rim of the headshield. The analysis identifies as sistergroup the Asapida, Olenina and Phacopida (including the Holotrachelidae). The earliest Proetida branch is the Hystricuridae. This is followed by a branch that consists of the Dimeropygidae and Toernquistiidae. At the third node the Aulacopleuroidea (Aulacopleuridae, Brachymetopidae) split off. Forth branch is the Scharyiidae. The fifth branche consists of the Roringtoniidae and Tropidocoryphidae. The sixth node splits a restricted Bathyuridae from ''Bathyurella'' and the Proetidae (including the Phillipsiinae). The following superfamilies and families are recognised: *Aulacopleuroidea : *Aulacopleuridae : *Brachymetopidae : *Rorringtoniidae *Bathyuroidea : *Bathyuridae : *Dimeropygidae : *Holotrachelidae : *Hystricuridae : *Raymondinidae : *Telephinidae : *Toernquistiidae *Proetoidea : *Proetidae : *Phillipsiidae : *Tropidocoryphidae 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Proetida」の詳細全文を読む スポンサード リンク
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