Acimetopus

Acimetopus Temporal range: Botomian (Olenellus-zone) PreꞒ Ꞓ O S D C P T J K Pg N
Acimetopus bilobatus
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Trilobita
Order:	Agnostida
Family:	Weymouthiidae
Genus:	Acimetopus Rasetti, 1966
Type species
Acimetopus bilobatus Rasetti, 1966 [1]

Acimetopus Rasetti, 1966,[2] is a genus of Eodiscinid trilobite belonging to the family Weymouthiidae Kobayashi (1943),[3] Order Agnostida Salter (1864).[4] It lived during the Botomian stage.[1] = late Lower Cambrian Stage 4 (upper of two stages subdividing the un-named Series 2); the upper Botomian boundary corresponds to base of the Middle Cambrian, Miaolingian Series and Wuliuan stage.

Type species

Acimetopus bilobatus Rasetti, 1966, from his locality cs-4 in limestone beds which form several outcrops on a hillside at Griswold Farm, about 1 mile southeast of North Chatham, Columbia County, New York State, USA; the associated faunule is referred to as the Acimetopus bilobatus faunule due to the nominate species being one of the most common and characteristic trilobites.

Distribution

Acimetopus is known from the Lower Cambrian of the United States (unnamed formation, East Chatham Quad, Columbia County, New York State 42°24′N 73°30′W),[5] and England.[6]

Description

Like in all Agnostida, the body of Acimetopus is diminutive, the headshield (or cephalon) and tailshield (or pygidium) are approximately the same size (or isopygous). The thorax is unknown but all weymouthiids where it is known have 3 thoracic segments. Acimetopus has no eyes. Its cephalic border is strongly convex, with a pair of lateral marginal spines. The entire exoskeleton of Acimetopus is covered in granules. The central raised area of the cephalon, called the glabella, is strongly divided by two deep furrows that merge when crossing the midline. The large and inflated anterior lobe is isolated from border furrow. Behind it is a pair of small, separated, subtriangular lobes. The posterior part of the glabella is large and carries an inflated spine that is directed upwards and backwards. Genal spines are present. The axis of the pygidium (or rhachis) has nine rings that are strongly inflated, with the second ring carrying a vertical spine. The border surrounding the pygidium is defined by a deep furrow, is narrow, and lacks spines.[6]

Ecology

Acimetopus bilobatus occurs in association with other Weymouthiidae including (Acidiscus, Analox, Bathydiscus, Bolboparia, Leptochilodiscus, Serrodiscus), Calodiscus, and several species of Olenellus Billings, 1861, and Bonnia Walcott, 1916 (Family Dorypygidae).[5]

References

1. SEPKOSKI, J. (2002). "A compendium of fossil marine animal genera (Trilobita entry)". Bulletins of American Paleontology. 364: 560. Archived from the original on 5 September 2006. Retrieved 12 January 2008.
2. RASETTI, F., 1966. New Lower Cambrian trilobite faunule from the Taconic sequence of New York. Smithsonian Miscellaneous Collections 148, 1-52.
3. KOBAYASHI T. 1943. Brief notes on the Eodiscids 1, their classification with a description of a new species and a new variety. Proceedings of the Imperial Academy, Tokyo, Volume 19, pp. 37-42.
4. SALTER, J. W. 1864: On some new fossils from the Lingula-flags of Wales. Quarterly Journal of the Geological Society of London, 20, 233–241.
5. Paleobiology Database. "East Chatham Quad, unnamed fm., NY (Cambrian of the United States) (trilobite)". Retrieved 19 December 2012.
6. WHITTINGTON, H. B. (1997). Treatise on Invertebrate Paleontology. Part O, Revised, Volume 1 – Trilobita – Introduction, Order Agnostida, Order Redlichiida.