Aulacocerida is an order of primitive coleoid cephalopods, possibly derived from michelinoceraitids (
Orthocerida
Orthocerida, from Ancient Greek ὀρθός (''orthós''), meaning "straight", and κέρας (''kéras''), meaning "horn", also known as the Michelinocerida, is an order of extinct Orthoceratoidea, orthoceratoid cephalopods that lived from the ...
) 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 phragmocone by a constriction. The siphuncle 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
Nacre ( , ), also known as mother-of-pearl, is an organicinorganic composite material produced by some molluscs as an inner shell layer. It is also the material of which pearls are composed. It is strong, resilient, and iridescent.
Nacre is ...
recovered from aulococerids as well as from belemnites resembles that found in recent ''
Sepia'' and ''
Spirula'' but differs from that of recent ''
Nautilus
A nautilus (; ) is any of the various species within the cephalopod family Nautilidae. This is the sole extant family of the superfamily Nautilaceae and the suborder Nautilina.
It comprises nine living species in two genera, the type genus, ty ...
''.
Opinions vary as to whether aulococerids were endocochleate,
having internal shells as with modern
squid
A squid (: squid) is a mollusc with an elongated soft body, large eyes, eight cephalopod limb, arms, and two tentacles in the orders Myopsida, Oegopsida, and Bathyteuthida (though many other molluscs within the broader Neocoleoidea are also ...
, or were ectocochleate, having external shells as with
nautiloid
Nautiloids are a group of cephalopods (Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and ''Allonautilus''. Fossil nautiloids are diverse and species rich, with over 2,500 recorded species. Th ...
s and
ammonite
Ammonoids are extinct, (typically) coiled-shelled cephalopods comprising the subclass Ammonoidea. They are more closely related to living octopuses, squid, and cuttlefish (which comprise the clade Coleoidea) than they are to nautiluses (family N ...
s. 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
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