Librostoma is a subclass of trilobites defined by having a natant hypostome, which is a hypostome (mineralized trilobite mouthpart) that is free from the anterior doublure and aligned with the anterior of the glabella,[4] this is unlike a conterminant hypostome, which is attached to the exoskeleton.[5]
They are a large and long lasting group, with some of the candidates for the first trilobites being librostomes in the order ptychopariida; and all post-Devonian[b]
trilobites being in the order proetida, whom are librostomes.
The monophyly of the group has been questioned, with Whittington (2003) placing some genera (Hemibarrandia, Lakaspis, Peraspis and Symphysurina) of the asaphid family Nileidae in the order Corynexochida.[6]
^Ontogeny, hypostome attachment and trilobite classification. RA Fortey, Palaeontology, 1990
^Cite error: The named reference Gon was invoked but never defined (see the help page).
^Bignon, Arnaud; Waisfield, Beatriz G.; Vaccari, Emilio; Chatterton, Brian D. E. (2020). "Reassessment of the Order Trinucleida (Trilobita)". Journal of Systematic Palaeontology. 18 (13): 1061–1077. Bibcode:2020JSPal..18.1061B. doi:10.1080/14772019.2020.1720324. S2CID 212995185.
^"The Hypostome".
^Whittington, H. B. "Reflections on the Classification of the Trilobita". University of Cambridge.
^Whittington, Harry B. (2003). "The trilobite family Nileidae: morphology and classification". Palaeontology. 46 (4): 635–646. Bibcode:2003Palgy..46..635W. doi:10.1111/1475-4983.00313.
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Librostoma is a subclass of trilobites defined by having a natant hypostome, which is a hypostome (mineralized trilobite mouthpart) that is free from the...
some species the pygidium is outlined with a flat border. The Subclass Librostoma was recently erected to encompass several related orders, including Ptychopariida...
and Harpetida were grouped together as order Ptychopariida; subclass Librostoma was erected in 1990 to encompass all of these orders, based on their shared...
with the order Ptychopariida, though it is not included in the subclass Librostoma. Phacopida was one of only two trilobite orders (along with the Proetida)...
recently given their own order (Ebach & McNamara 2002). The subclass Librostoma was erected in 1990 by Richard Fortey to cover the various orders originally...
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