Anarcestida

From Palaeos.org

The Anarcestida are primitive ammonoids that lived during the Devonian Period, and from which subsequent ammonoid orders are ultimately derived.

Anarcestids are characterized by a ventral siphuncle that is retrosiphonate (reflective of their nautiloid ancestry) and by gonititic sutures, some with only three or four lobes.

The Anarcestida have their origin in the Bacrititida, beginning with Lobobactrites and by evolving a tighter and tighter exogastric curvature went from the cyrtoconic Cyrtobactrites though forms such as the gyroconic Anetoceras and finally to forms like the evolutely coiled Agoniatites, characteristic of the order. (Klug and Korn 2004)

Classification

The Anarcestida have been divided into four suborders (Korn 2001), the Agoniatitina, Gephuroceratina, Anarcestina and Pharciceratina, which can be thought of as superfamilies, based on shell geometry and stratigraphic sequence. The Agoniatitina evolved more slowly and gave rive to the Gephyriceratina in the early Late Devonian (Fresnian). The Anarcestina (sensu Korn 2001) evolved more rapidly, giving rise to the Pharciceratina in the Middle Devonian (late Givetian). This follows closely on the work of Saunders et al (1999) wherein the Anarcestida are divided into five suborders, the Agoniatitina, Gephuroceratina , Anarcestina, Augurina, and Timanoceratina. The latter two are very small, containing one family each and all told three genera. The pharciceratids are included in the Gephuroceratina.

References

• Klug, C. and Korn, D. 2004.The origin of ammonoid locomotion. Acta Palaeontologica Polonica 49 (2): 235–242.
• Korn D 2001, Morphometric evolution and phylogeny of Palaeozoic ammonoids. Early and Middle Devonian, Acta Geologica Polonica, Vol 51, No 3, 2001.
• Miller, Furnish, and Schindewolf (MF&S), 1957; Paleozoic Ammonoidea, in the Treatise on Invertebrate Paleontology Part L, Ammonoidea; Geolgical Society of America and University of Kansas press, (L29-L36)

• Saunders et al 1999 [1]