Bisonocerida is an order of

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...

nautiloid Nautiloids are a group of marine cephalopods ( Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and ''Allonautilus''. Fossil nautiloids are diverse and speciose, with over 2,500 recorded species ...

cephalopods A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, an ...

. Members of this order were originally placed in the order

Endocerida Endocerida is an extinct nautiloid order, a group of cephalopods from the Lower Paleozoic with cone-like deposits in their siphuncle. Endocerida was a diverse group of cephalopods that lived from the Early Ordovician possibly to the Late Silu ...

, but later investigation argued that this broad usage of Endocerida was a

polyphyletic A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of converg ...

assemblage encompassing two different groups of independent origin. Bisonocerida was differentiated from Endocerida in 2012 in order to resolve this issue. Bisonocerids are similar to endocerids in many respects. The

siphuncle The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, and ...

was broad and positioned ventrally in the shell, which ranged in shape from cyrtoconic brevicones (curved and short) to rare orthoconic longicones (straight and long). The inner surface of the siphuncle contains endosiphuncular deposits, which help distinguish the two orders. In both bisonocerids and endocerids, the endosiphuncular deposits are conical in shape ("endocones"), concreted from the rim of the siphuncle and tapering towards the apex of the shell. Both orders possess simple perforate endocones, with a hole at the tip of each cone for siphuncle tissue to pass through. Structures similar to simple endocones are also apparently present in some members of the earlier cephalopod orders

Yanhecerida Yanhecerida is a small order of Late Cambrian (mid- Stage 10) nautiloid cephalopods. They were similar to the more diverse Plectronocerida and Ellesmerocerida, with short shells, closely-spaced septa, and diaphragms (partitions) within the sip ...

and

Ellesmerocerida The Ellesmerocerida is an 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 shells that ...

. Unlike true endocerids, the simple endocones of bisonocerids are interlaced with a more complex form of endocone. “Complex” endocones are imperforate, without a hole at the tip. In cross-section, they are composed of several bulbous

calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines. In zoology ''Calcareous'' is used as an adje ...

crests extending into the siphuncle. This only leaves an irregular and discontinuous pinched space, known as an infula, for the remaining siphuncle tissue. The infula may be compressed to the point that it resembles radiating splinters in cross-section, known as endosiphoblades. The calcareous crests were able to form by concreting onto narrow sheets of infolded

conchiolin Conchiolins (sometimes referred to as conchins) are complex proteins which are secreted by a mollusc's outer epithelium (the mantle). These proteins are part of a matrix of organic macromolecules, mainly proteins and polysaccharides, that ass ...

, a condition unique to bisonocerids. Other distinctive traits of bisonocerids include long septal necks and a siphuncle which is swollen at the apex of the shell. Bisonocerids appear to be oncomyarian, meaning that they have a numerous small, undifferentiated muscle scars ringing around their

body chamber The body whorl is part of the morphology of the shell in those gastropod mollusks that possess a coiled shell. The term is also sometimes used in a similar way to describe the shell of a cephalopod mollusk. In gastropods In gastropods, the ...

. This is unlike endocerids, which have a few large muscle scars at the top of the body chamber, a dorsomyarian condition. When originally defined as a new order, Bisonocerida was suggested to be an independent descendant of Ellesmerocerida. It was later allied with the broad subclass

Multiceratoidea Multiceratoidea 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, ...

. However, a 2022

phylogenetic analysis In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

argued that Bisonocerida was closely related to Endocerida once more. Under this hypothesis, the two orders were

sister taxa In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...

within the subclass

Endoceratoidea Endocerida is an extinct nautiloid order, a group of cephalopods from the Lower Paleozoic with cone-like deposits in their siphuncle. Endocerida was a diverse group of cephalopods that lived from the Early Ordovician possibly to the Late Silurian ...

References

{{reflist Prehistoric nautiloids Prehistoric cephalopod orders Early Ordovician first appearances Wenlock extinctions