Aulacocerida is an order of primitive coleoid cephalopods, possibly derived from michelinoceraitids (
Orthocerida) early in the Devonian, which in turn gave rise to the Belemnites.
Aulococerids are characterized by a straight, slim, conical shell with a large living chamber, simple aperture. The protoconch (initial, embryonic chamber) is egg-shaped and separated from the phragnocone by a constriction. The siphucle is narrow and ventral, constricted at the septa and expanded in the middle of the chamber; necks are retrochoantiic, project backwards toward the apex.
Nacre recovered from aulococerids as well as from belemnites resembles that found in recent ''
Sepia
Sepia may refer to:
Biology
* ''Sepia'' (genus), a genus of cuttlefish
Color
* Sepia (color), a reddish-brown color
* Sepia tone, a photography technique
Music
* ''Sepia'', a 2001 album by Coco Mbassi
* ''Sepia'' (album) by Yu Takahashi
* " ...
'' and ''
Spirula
''Spirula spirula'' is a species of deep-water squid-like cephalopod mollusk. It is the only extant member of the genus ''Spirula'', the family Spirulidae, and the order Spirulida. Because of the shape of its internal shell, it is commonly known ...
'' but differs from that of recent ''
Nautilus
The nautilus (, ) is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina.
It comprises six living species in ...
''.
Opinions vary as to whether aulococerids were endocochleate,
having internal shells as with modern
squid
True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fitting t ...
, or were ectocochleate, having external shells as with
nautilods and
ammonites
Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefish) ...
. Naef (in Bandel 1985) thought of aulacocerids as slender squids that looked similar to recent ones; morphologically similar to belemnites on one hand and to orthocerids on the other, but with a mantle that completely covered the shell; making them endocochliate. Jeletzky (in Bandel 1985) considered that aulocerids were essentially ectocochleate and that they lacked a muscular mantle, the rostrum having been secreted by mantle flaps that could be folded over the chambered shell, but were not fused around it, and which could be withdrawn into the long living chamber.
See also
*
List of belemnites
This list of belemnite genera is an attempt to create a comprehensive listing of all genera that have ever been included in the extinct subclass Belemnoidea, excluding purely vernacular terms. The list includes all commonly accepted genera, as we ...
References
*Klaus Bandel, J. Reitner, and W. Sturmer, 1893. Coleoids from the Lower Devonian Black Slate (Hunstruck-Shiefer) of the Hunsruck (West Germany). N Jb Geol Paliont Abh 165(3) 397-414 Stuttgart, Apr 1983.
*Klaus Bandel, 1985. Composition and ontogeny of Dictyoconites (Aulacocerida, Cephalopoda). Palaont Z 59(3/4),223-244. Stuttgart, Dec 1985.
{{Taxonbar, from=Q1983748
Prehistoric cephalopod orders
Belemnoidea