Eodiscoidea
| Eodiscina Temporal range: Lower and Middle Cambrian |
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| Eodiscus punctatus, collected from the Plutonites hicksi-zone, Nine Wells Shale, Wales, Middle Cambrian (Menevia), 8mm, head disarticulated from the body | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | Trilobita |
| Order: | Agnostida |
| Suborder: |
Eodiscina Kobayashi, 1939 |
| Superfamily: |
Eodiscoidea Raymond, 1913 |
| Families | |
Eodiscina is a suborder of trilobites, a well known group of marine arthropods. The Eodiscina first developed near the end of the Lower Cambrian period (late Atdabanian) and became extinct at the end of the Middle Cambrian. They are small or very small, and have a thorax of two or three segments. Eodiscina includes six families classified under one superfamily, Eodiscoidea.
The Eodiscina are mostly considered the more primitive suborder of the Agnostida, and the Agnostina the more advanced. Some scholars do not consider the Agnostina true trilobites, and consequently rejected the idea that they were related to the Eodiscina. Consequently, these scientists have proposed to elevate the group to ordinal level, which would thus be called Eodiscida Kobayashi, 1939.
The oldest known eodiscoid is Tsunyidiscus. The glabella of Tsunyidiscus is extremely similar to that of Dipharus clarki, and distinct from all other eodiscoids. D. clarki is thought to represent an immature stage of the redlichioid Bulaiaspis rather than an eodiscoid. This is because of the dominant palpebroocular ridges, extremely long librigenae, and free pleural tips on the pygidium of variable numbers of segments. In short: Tsunyidiscus has probably developed through paedomorphosis from Bulaiaspis. Alternatively Sinodiscus changyangensis has been considered the most primitive of the Eodiscina.
Many characters that are divergent from other trilobites may be explained by paedomorphosis, such as the small number of thorax segments and proparian facial sutures, that occur in all trilobites during larval development, while the Eodiscoid abatochroal eye is not unlike early development stages of the holochoal eye, where lenses are also separated.
Three lineages are thought to have evolved from the monotypical family Tsunyidiscidae. First the Calodiscidae. Second the Yukoniidae, who sprouted the Eodiscidae. And finally the Hebediscidae, that themselves gave rise to the Weymouthiidae, which contain Tannudiscus, the probable ancestor of the peronopsid genus Archaeagnostus, the earliest of the Agnostina.
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