Cyrtocerinida is an
order
Order, ORDER or Orders may refer to:
* A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica
* Categorization, the process in which ideas and objects are recognized, differentiated, and understood
...
of
Ordovician
The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...
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 ...
cephalopods
A cephalopod is any member of the molluscan Taxonomic rank, class Cephalopoda (Greek language, Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral symm ...
.
The order includes the families
Cyrtocerinidae and
Eothinoceratidae
Eothinoceratidae is a family of Lower Ordovician Nautiloidea, nautiloid cephalopods included in the Cyrtocerinida. The family was originally established for the genus ''Eothinoceras''.
Morphology
The Eothinoceratidae as typified by ''Eothinocera ...
, as well as the genera ''
Bathmoceras'' and ''
Rummoceras''.
Cyrtocerinids can be characterized by a broad
siphuncle
The siphuncle is a strand of biological tissue, tissue passing longitudinally through the mollusc shell, shell of a cephalopod mollusc. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the li ...
encased by very thick
connecting rings with concave outer surfaces. Despite their thickness, the connecting rings are nautilosiphonate, meaning that their inner layer is a poorly-mineralized organic sheath (similar to modern
nautiluses) rather than a porous calcified structure. There are various calcified
endosiphuncular deposits such as collars and longitudinal ridges on the inner layer of the connecting ring.
These deposits are discontinuous, reforming after each chamber. This differentiates them from the continuous endosiphuncular ridges found in other nautiloids, such as the related
oncocerids. The overall shell shape is typically orthoconic (straight, pointed) or breviconic (stout).
The
body chamber is oncomyarian, meaning that it is ringed by numerous small, undifferentiated muscle scars.
Cyrtocerinida was previously known under the name Cyrtocerinina, which was considered a suborder of the order
Ellesmerocerida
The Ellesmerocerida is an extinct order of primitive cephalopods belonging to the subclass Nautiloidea with a widespread distribution that lived during the Late Cambrian and Ordovician.
Morphology
The Ellesmerocerida are characterized by shell ...
.
Current studies generally consider Cyrtocerinida to be the earliest-branching
monophyletic
In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria:
# the grouping contains its own most recent co ...
clade
In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
within the subclass (or superorder)
Multiceratoidea. This is justified by the combination of oncomyarian muscle scars, nautilosiphonate connecting rings, and endosiphuncular deposits.
References
{{Taxonbar, from=Q112426913
Prehistoric nautiloids
Prehistoric cephalopod orders
Middle Ordovician first appearances