Librostoma
| Librostoma Temporal range: | |
|---|---|
| Asaphus lepidurus | |
| Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | †Trilobita |
| Subclass: | †Librostoma Fortey, 1990 |
| Orders | |
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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 anterior doublure and aligned with the anterior of the glabella, this is unlike a conterminant hypostome, which is attached to the exoskeleton.
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 trilobites being in the order proetida, who are librostomes.
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.