Agoniatitida

Agoniatitida, also known as the Anarcestida, is the ancestral order within the cephalopod Subclass Ammonoidea, that lived in what would become Africa, Asia, Australia, Europe, and North America during the Devonian from about 410—385 mya, existing for approximately 25 million years.

Taxonomic nomenclature

The Order Agoniatitida, named by Ruzhencev, 1957,^[1] is a subjective synonym for the Order Anarcestida, named by Miller and Furnish, 1954.^[2] Accordingly, the name Anarcerstida is based on the family Anarcestidae (ex Anarcestinae) of Steinmann 1890. That of Agoniatiida is based on the family Agnoniatidae of Holzapfel, 1899.

Saunders, Work, and Nikolaeva, 1999,^[3] refer to the Anarcestida, with Agoniatina a suborder, maintaining the priority as found in the Treatise. Shevyrev, 2006 ^[4] on the other hand follows Ruzhencev, 1957 and used Agonititida.

An image showing the siphuncle, the tube which connects the current living shell to the previous ones.

Morphology

Agoniatitida are primitive ammonoids with a ventral retrochoanitic siphuncle (septal necks point to the rear) reflective of their nautiloid ancestors.and goniatitic sutures with a variable number of lobes. Shells vary from discoidal to globular. Coiling may be loose with whorls barely touching or tight with a dorsal impression.^[2]

Fossil distribution

Fossils are restricted to strata of the Zlichovian (Pragian/Early Emsian) to Famennian epochs.

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