Anarcestida - The First True Ammonite Order

Early Devonian through the End Devonian

412 to 216 million years ago

This order of Ammonites is thought to be the first true order of ammonites, springing from the Bactritids, the straight-shelled nautiloids.  Another name for this group is Agoniatitida.  For statistical data on this order, click here.

Diversity, Geographic Range and Habitat

• Anarcestida lacked a wide range of species and therefore it's diversity was quite sparse
• Abundant in shallow waters with plentiful oxygen found in coastal waters of continental shelves

Characteristics

Shell Structure

• Disc-shaped and globular in shape
• Coiled
• Involute

Siphuncle

• Ventral

Septal Neck

• Retrochoanitic (points backwards, towards the older chambers)

Suture Lines

• These are simple with only a few lobes (one large external lobe and a small internal lobe)
• Some species had an additional lobe

Ornamentation

• Fine sinuous ribs
• No spines

Identification

• Genera difficult to distinguish from one another

Ancestry and Descendants

• Ancestor:  Bactritid nautiloid
  • Retained some features such as simple septa and the retrochoanitic septal neck
• Descendants:  All other paleoammonoid orders:  Clymeniida, Prolecanitida, Goniatitida

Other

• Had a hyponomic sinus for jet propulsion

Ammonitella

• Large - usually < 1 mm in diameter
• Ornamentation considted of transverse ribs (lirae) (what's another word for transverse?)