The Calodiscidae Kobayashi, 1943 'nom. transl.'' Öpik, 1975 ''ex'' Calodiscinae Kobayashi, 1943are a family of

trilobite Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the ...

s belonging to the order Agnostida that lived during the Lower Cambrian (Botomian and Toyonian). They are small or very small, and have a thorax of two or three segments. The Calodiscidae includes five genera (see box).

Taxonomy

The probable ancestors of the Calodiscidae are among the

Tsunydiscidae ''Tsunyidiscus'' is a trilobite belonging to the Suborder Eodiscina. ''Tsunyidiscus'' appeared near the end of the Lower Cambrian, during the late Atdabanian stage of geologic time and some collections suggest it may have survived into the Botom ...

. The Calodiscidae had no descendants. cited ''in''

Description

Like all Agnostida, the Calodiscidae are diminutive and the headshield (or cephalon) and tailshield (or

pygidium The pygidium (plural pygidia) is the posterior body part or shield of crustaceans and some other arthropods, such as insects and the extinct trilobites. In groups other than insects, it contains the anus and, in females, the ovipositor. It is compo ...

) are of approximately the same size (or isopygous) and outline. The central raised area of the cephalon (or

glabella The glabella, in humans, is the area of skin between the eyebrows and above the nose. The term also refers to the underlying bone that is slightly depressed, and joins the two brow ridges. It is a cephalometric landmark that is just superior to ...

) has parallel sides or tapers forward, the front being rounded and expanded, and may be divided by transverse glabellar furrows. The occipital ring is defined by a complete transverse furrow and is neither spinose nor expanded. When present, the eye lobes are short and prominent. The thorax is only known in ''Chelediscus'' (two segments) and ''Sinodiscus'' (three segments). The pygidium has a wide, tapering axis of less than six segments.

References

Eodiscina Trilobite families Cambrian trilobites Cambrian first appearances Cambrian Series 2 extinctions {{agnostida-stub