Acidiscus

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Acidiscus
Temporal range: Botomian (Olenellus-zone)
Acidiscus linedrawing.jpg
Acidiscus birdi
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Class: Trilobita (?)
Order: Agnostida
Family: Weymouthiidae
Genus: Acidiscus
Rasetti, 1966
Type species
Acidiscus birdi
Species
  • A. birdiRasetti, 1966 [1]
  • A. hexacanthusRasetti, 1966
  • A. theristesRushton, 1966

Acidiscus [2] is a genus of eodiscinid trilobite belonging to the family Weymouthiidae Kobayashi T. (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 unnamed Series 2); the upper Botomian boundary corresponds to base of the Middle Cambrian, Miaolingian Series and Wuliuan stage.

Contents

Etymology

Taxonomy

Stigmadiscus cladogram.png

A cladogram showing the relationship between several species of the genera Acidiscus, Bolboparia and Stigmadiscus [6] [7] Acidiscus is most closely related to Bolboparia and slightly more distantly to Stigmadiscus. Synapomorphies for Acidiscus-Bolboparia include anterolateral cephalic spines and a large preoccipital glabellar tubercle or spine. Both of these features have been considered as being diagnostic of Acidiscus (e.g., Rushton, 1966; Rasetti, 1966;) but they are shared with Bolboparia. Synapomorphies of Bolboparia include a sharply tapered and pointed glabella, no occipital spine, tubercles confined to posterior part of the cephalic border, strongly inflated genae, and a sculpture of tightly packed coarse granules. Stigmadiscus is characterised by an inflated posterior part of the glabella, the loss of anterolateral cephalic spines and a preoccipital glabellar node, and in having a very narrow cephalic border.

Among genera currently assigned to Weymouthiidae Kobayashi, 1943 (Jell in Whittington et al. 1997), Rushton (1966, p. 12) noted that, “Acidiscus Rasetti differs from Serrodiscus but slightly”, although several of the characteristics in the former genus that he listed, include the preoccipital glabellar and anterolateral cephalic spines, and the configuration of the lateral glabellar furrows.

Distribution

Ecology

A. birdi occurs in association with other Weymouthiidae ( Acimetopus , Analox , Bathydiscus , Bolboparia , Leptochilodiscus , Serrodiscus ), Calodiscus, several species of Olenellus , and Bonnia (Dorypygidae). [8]

Description

Like all Weymouthiidae, Acidiscus lacks eyes and facial sutures. The cephalic border carries one or two pairs of marginal spines. The glabella does not reach the border furrow and has two short pit-like pairs of lateral furrows. Occipital and genal spines are present. Thorax unknown, but all weymouthiids in which it is known have three segments. The pygidium is much like that in Serrodiscus , with an axis composed of ten rings. Pleural fields unfurrowed. [9]

References

  1. 1 2 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 Collections148, 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. RASETTI, F. (1966). "New Lower Cambrian trilobite faunule from the Taconic sequence of New York". Smithsonian Miscellaneous Collections. 148 (9): 1–52.
  6. WESTROP, S. R.; LANDING, E. (2011). "Lower Cambrian (Branchian) eodiscoid trilobites from the lower Brigus Formation, Avalon Peninsula, Newfoundland, Canada". Memoirs of the Association of Australasian Palaeontologists. 42: 209–262.
  7. COTTON, T. J.; FORTEY, R. A. (2005). "5. Comparative morphology and relationships of the Agnostida". In Koenemann, S.; Jenner, R. (eds.). Crustacean Issues 16, Crustacea and Arthropod Relationships. Boca Raton: CRC Press.
  8. 1 2 Paleobiology Database. "East Chatham Quad, unnamed fm., NY (Cambrian of the United States) (trilobite)" . Retrieved 19 December 2012.
  9. 1 2 WHITTINGTON, H. B. et al., 1997. Part O, Treatise on Invertebrate Paleontology. Revised, Volume 1 – Trilobita – Introduction, Order Agnostida, Order Redlichiida.
  10. RUSHTON, A.W.A. 1966. The Cambrian Trilobites from the Purley Shales of Warwickshire. Palaeontographical Society Monographs (1): p. 16, pl. 1, figs. 20 a, b.