Weymouthiidae
| Weymouthiidae Temporal range: Botomian/Delamaran (Olenellus- and Eokochaspis-zones) | |
|---|---|
 | |
| Marocconus, from Morocco | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | Trilobita |
| Order: | Agnostida |
| Suborder: | Eodiscina |
| Superfamily: | Eodiscoidea |
| Family: | Weymouthiidae Kobayashi, 1943 |
| Genera | |
| Synonyms | |
|
Ladadiscinae | |
The Weymouthiidae are an extinct family of eodiscinid agnostid trilobites. 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 paleocontinents of Laurentia, Avalonia, Gondwana. The Weymouthiidae are all blind and lack free cheeks.
Taxonomy
Ancestors
The Weymouthiidae have developed from a stock within the Hebediscidae.
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.[1] 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 central raised area of the cephalon, that is called glabella is wide at its base, normally parallel sided but may taper gently or are widest at half length. Mostly furrows incising the glabella are absent, but incomplete furrows in some species and deeply impressed transglabellar furrows also occur. Free cheeks (or fixigenae) normally confluent in front of glabella. The occipital ring may be simple or split into two lateral lobes (in Chelediscus and Tannudiscus) rounded L1. Without eyes. The tailshield (or pygidium) has a long, strongly tapering axis (or rhachis) with upto 10 or more rings. Segmentation however is commonly effaced. The Weymouthiidae include forms with the rear of the glabella roundly expanded over the occipital ring or with a vertical spine, forms in chich 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.[2]
References
- ↑ Sam Gon III. "The Relationship of Agnostina to Eodiscina". Retrieved 18 December 2012.
- ↑ Whittington, H.B. et al. Part O, Treatise on Invertebrate Paleontology. Revised, Volume 1 – Trilobita – Introduction, Order Agnostida, Order Redlichiida. 1997