The Weymouthiidae Kobayashi 1943 are an extinct

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

of eodiscinid agnostid

trilobites 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 At ...

. They lived during the late Lower Cambrian and earliest Middle Cambrian (Botomian to Delamaran) in the so-called ''Olenellus''- and ''Eokochaspis''-zones in the former

paleocontinent A paleocontinent or palaeocontinent is a distinct area of continental crust that existed as a major landmass in the geological past. There have been many different landmasses throughout Earth's time. They range in sizes, some are just a collection ...

s of

Laurentia Laurentia or the North American Craton is a large continental craton that forms the ancient geological core of North America. Many times in its past, Laurentia has been a separate continent, as it is now in the form of North America, althoug ...

, Avalonia, Gondwana. The Weymouthiidae are all blind and lack free cheeks.

Taxonomy

Ancestors

The Weymouthiidae have developed from a stock within the

Hebediscidae The Hebediscidae Kobayashi, 1944, are a family of trilobites belonging to the order Agnostida that lived during the Lower Cambrian (Atdabanian to Toyonian). They are small or very small, and have a thorax of two or three segments. The Hebediscida ...

Descendants

The Weymouthiidae are a paraphyletic family because the Agnostina suborder is nested within it, particularly in the clade that further consists of the genera ''Mallagnostus'', ''Chelediscus'', ''Tannudiscus'' and ''Jinghediscus''. The trend in the Weymouthiidae to reduce the occipital ring is carried furthest in ''Chelediscus'', ''Tannudiscus'' and the Agnostina with the occipital ring divided into basal lobes.

Description

Most taxa in this family are small (1–2 cm or 0.4-0.8 inches long), but this is large for the Eodiscina. The glabella is wide at its base, normally parallel sided but may taper gently or be at its widest at half length. Glabellar furrows are mainly absent, but incomplete furrows may be present in some species and deeply impressed transglabellar furrows also occur. Free cheeks (or fixigenae) are normally confluent in front of glabella. The occipital ring may be simple or split into two lateral lobes (e. g. in ''Chelediscus'' and ''Tannudiscus''). All Weymouthiidae lack eyes. The thorax consists of three rings when known (''Mallagnostus'', ''Marocconus'', ''Serrodiscus'', ''Tannudiscus'' and ''Weymouthia''). The pygidial axis is long and strongly tapered with 10 or more axial rings. The segmentation of the axis, however, is often effaced. The Weymouthiidae include forms with the rear of the glabella roundly expanded over the occipital ring or with a vertical spine, forms in which the occipital ring cannot be discerned because the glabella is expanded, and forms with a primitive occipital structure but with a greatly increased numbers of axial segments. WHITTINGTON, H. B. ''et al''., Part O, Treatise on Invertebrate Paleontology. Revised, Volume 1 – Trilobita – Introduction, Order Agnostida, Order Redlichiida. 1997

References

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