14-516: Bathmoceras is a primitive cephalopod genus from the Middle and Upper Ordovician. It is a member of the order Cyrtocerinida and is the only genus in the family Bathmoceratidae . At one time Bathmoceras was thought to have possibly given rise to Polydesmia , once thought to have been the ancestral actinocerid. Since then revision of actinocerid phylogeny and of Ordovician stratigraphy in east Asia have cast doubts on this hypothesis. Bathmoceras
28-457: A member of Multiceratoidea. A 2022 bayesian phylogenetic analysis supports the existence of Multiceratoidea as a valid monophyletic clade (discounting the question of nautilus origins), with a membership that includes Tarphycerida and excludes most ellesmerocerids. This is because Ellesmerocerida is a paraphyletic group, an assemblage of early nautiloids ancestral to various later groups, more than just Multiceratoidea. The order Oncocerida
42-415: A normal horizontal orientation with the aperture facing forward. They were apparently active predators with some degree of mobility. As for the peculiar internal structure of the siphuncle, beyond providing ballast, no reason has been given. The shells of Bathmoceras are rather large, straight or faintly exogastric. Sutures are simple except for a sharp mid ventral saddle. The siphuncle is large and near
56-529: A ventromyarian condition (with muscle scar area concentrated at the bottom of the body chamber). Multiceratoid shells are generally short and curled, with a relatively small aperture (opening). Cameral deposits are never found among the multiceratoids, though several orders are known to bear endosiphuncular deposits within their siphuncles . When originally defined in 2013, Multiceratoidea included four nautiloid orders : Ellesmerocerida , Oncocerida , Discosorida , and Ascocerida . The order Tarphycerida
70-450: Is a major subclass or superorder of Paleozoic nautiloid cephalopods . Members of this group can be characterized by nautilosiphonate connecting rings , with an organic inner layer and outer layer of calcitic spherules and blades, similar to the modern nautilus . The earliest-diverging multiceratoids have oncomyarian muscle scars (with numerous small muscle attachments ringing the body chamber ), though several orders trend towards
84-519: Is also apparently more slender. Furnish and Glenister also included Eothinceras in the family Bathmoceratidae . R.H. Flower, 1964, separates the two genera into their respective families within the ellesmerocerid suborder Cyrtocerinina, now known as the order Cyrtocerinida. Bathmoceras is of Middle and Late Ordovician age and has been found in northern and central Europe and in the Macdonnell Ranges in central Australia. The genus
98-570: Is found in Europe and Australia, mostly as internal molds in sandy sediments. One species B. linnarsoni was found in limestone in Sweden which allowed Holms to conduct thin section studies. The common occurrence in sandy sediments indicates that Bathmoceras lived over sandy bottom, although some species obviously did spend time over carbonate shelves. The straight or slightly upwardly curved, exogastric shells with weighted ventral siphuncles indicate
112-503: Is most likely derived from Eothinoceras through lengthening of the septal necks and of the inward projections of the connecting rings in a forward direction. Eothinoceras differs in that the septal necks are vestigial rather than being somewhat long as in Bathmoceras and that the inward projections of the connecting rings point straight in rather than projecting forward. Both are straight shelled or slightly exogastric. Eothinoceras
126-503: The basis of morphological traits, Tarphycerida and Nautilida appear to be well-nested within Multiceratoidea. Multiceratoidea would be an equivalent term to Nautiloidea sensu stricto if nautilids are confirmed to be within the group, though molecular divergence timing disagrees with this assessment. The recently-named order Bisonocerida shares traits with ellesmerocerids and endocerids , and some studies have placed it as
140-489: The genera Bathmoceras and Rummoceras . Cyrtocerinids can be characterized by a broad siphuncle 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
154-455: 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
SECTION 10
#1732791249275168-440: The venter, in which the segments have a slightly sinuous outline. Septal necks are rather long. Most diagnostically, the connecting rings are thickened into forward projecting lobes that may extend internally two or three segments within the siphuncle. Cyrtocerinida Cyrtocerinida is an order of Ordovician nautiloid cephalopods . The order includes the families Cyrtocerinidae and Eothinoceratidae , as well as
182-476: Was considered a potential member of the subclass, though their larger body chamber and specialized muscle attachments lent uncertainty to this idea. A later analysis added the early-diverging order Cyrtocerinida , which was previously considered a suborder of Ellesmerocerida. Nautilida (the order containing the modern nautilus) was allied with multiceratoids in a broader group termed "Nautilosiphonata", defined by its namesake connecting ring structure. Solely on
196-450: Was previously known under the name Cyrtocerinina , which was considered a suborder of the order Ellesmerocerida . Current studies generally consider Cyrtocerinida to be the earliest-branching monophyletic clade within the subclass (or superorder) Multiceratoidea . This is justified by the combination of oncomyarian muscle scars, nautilosiphonate connecting rings, and endosiphuncular deposits. Multiceratoidea Multiceratoidea
#274725