Librostoma

File:Cummingella belisama dorsal light2 CRF.jpg|Cummingella belisama, a Phillipsiid Proetid from the Carboniferous. File:Harpes perradiatus fossil trilobite (Devonian, Morocco).jpg|Harpes perridatus, a Harpetid from the Devonian of Morocco. File:ElrathiaKingi.jpg| Elrathia kingii, a ptychopariid from the Middle cambrian Librostomes make up a large component of the Trilobita. Traditionally, four orders are placed in the Librostoma; they are the: Asaphida, Harpetida, Proetida, and Ptychopariida; however, two primitive phacopids (Pharostomina and Bavarilla) have natant hypostomes, indicating that the phacopids are also librostomes. In more recent publications, the asaphid superfamily Trinucleioidea is placed in its own order, Trinucleida, however, even some more recent studies recover them as asaphids. Some librostomes may not even be within the group, with Whittington (2003) placing some genera (Hemibarrandia, Lakaspis, Peraspis and Symphysurina) of the asaphid family Nileidae in the order Corynexochida. ==Evolution ==

Origins The first librostomes were ellipsocephaloid ptychopariids in the family Bigotinidae from the early Cambrian. This group was once thought to be in the order Redlichiida, but are now considered primitive Ptychopariida. Evolutionary history File:Asaphus kowalewskii 3.jpg|Asaphus kowalewskii from the Ordovician of Russia. File:Opipeuterella swimming.png|Opipueterella is a genus of swimming trilobite in the family Telephinidae; order Proetida. File:Ampyx cetsarum - specimen number MGL 96718 - white (cropped).jpg|Ampyx, An Ordovician trinucleioid. these trilobites may have gone in single-file lines from deeper water to shallow water at night, mirroring modern Spiny lobsters. During the Middle Cambrian, the Ptychopariida became the dominant trilobites. Around that time, the Asaphida and Trinucleida would first appear. The first Harpetida would appear in the Late Cambrian with the primitive genera Baikadamaspis and Entomaspis of the family Entomaspididae. The first Proetida appeared during the Cambrian-Ordovician transition. Since the earliest Phacopida (which are most likely librostomes) appeared during the early ordovician, it is assumed that a late cambrian sister taxon was present. which included the swimming Proetida family Telephinidae, and all asaphids & trinucleids except for the raphiophorid Raphiophorus, which would go extinct shortly after in the Silurian. ==Diversity==

Librostomes occupied many niches, sizes, and ecologies. Both some of the largest (Ordovician asaphids Isotelus rex, and Hungioides) and smallest trilobites (Ordovician ptychopariid Acanthopleurella stipulae) are librostomes. Many trilobites in the group had some ability to stay above the sea floor, such as asaphids in the superfamilies Cyclopygoidea and Remopleuroidea, proetids in the family Telephinidae, and perhaps some Cheirurid phacopids such as Deiphon and Crotalocephalus. The Asaphida, Ptychopariida (and Phacopida if included) are some of the most morphologically and taxonomically diverse orders of Trilobites. Meanwhile, the Proetida and Harpetida had a more modest morphological diversity; the former, sometimes referred to as "garden variety trilobites", containing the most diverse family of trilobites in terms of genera, and the latter being more modest in both terms of diversity. == See also ==