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Temporal range: Tithonian[1]
Aulacostephanidae - Aulacostephanus yo.JPG
Fossil shell of Aulacostephanus yo from France, on display at Galerie de paléontologie et d'anatomie comparée in Paris
Scientific classification
Kingdom: Animalia
Phylum: Mollusca
Class: Cephalopoda
Subclass: Ammonoidea
Order: Ammonitida
Family: Aulacostephanidae
Genus: Aulacostephanus

Aulacostephanus is an extinct ammonoid cephalopod genus from the Upper Jurassic Kimmeridgian belonging to the perisphinctacean family Aulacostephanidae.

Aulacostephanus produced a discoidal, strongly ribbed,evolute shell of moderate size, reaching diameters of 16 cm (~6.5in.), or so. Ribs diverge in pairs and threes from nodes on the umbilical shoulder and extend radially outward onto the ventro-lateral shoulder, but do not cross the venter, (the outer rim). Sides are flat, the venter steeply rounded, with an open bend or median groove running down the middle.

Clutches of eggs attributed to this genus have been discovered in the Kimmeridge Clay.[2]


  1. ^ Sepkoski, Jack (2002). "Sepkoski's Online Genus Database". Retrieved 2014-05-28. 
  2. ^ Etches, S.; Clarke, J.; Callomon, J. (2009). "Ammonite eggs and ammonitellae from the Kimmeridge Clay Formation (Upper Jurassic) of Dorset, England". Lethaia. 42 (2): 204–217. doi:10.1111/j.1502-3931.2008.00133.x.