''Bactroceras'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of orthoceratoid

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 ...

that lived during the early Middle

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 ...

, from about 472—464 mya, existing for approximately 8 million years.Nautiloidea – Ellesmerocerida by W.M Furnish and Brian F. Glensister; Treatise on Invertebrate Paleontology Part K R.C. Moore (ed) Univ.Kans. press

Taxonomy

''Bactroceras'' was named by Holm (1898). Its type is ''Bactroceras avus''. It was assigned to the order Orthocerida and family Baltoceratidae by Furnish and Glenister (1964) and retained there by Evans (2005).D. H. Evans. 2005. The Lower and Middle Ordovician cephalopod faunas of England and Wales. Monograph of the Palaeontographical Society 623:1-81 King & Evans (2019) instead placed it in its own family (Bactroceratidae), as part of the newly named order Rioceratida. Some paleontologists have regarded ''Bactroceras'' as an early bactritid because of its spherical apex and ventral siphuncle. However, a more recent study has argued that the shell of ''Bactroceras'' has important differences from those of true bactritids. For instance, the first shell chamber of ''Bactroceras'' resembles that of other

orthocerids, such as ''Archigeisonoceras'' and ''Hedstroemoceras'': it is about 10 mm in diameter and is short, forming a spherical cap. True bactritids more strongly resemble late Silurian and Devonian orthocerids, whose first chamber is only about 5 mm across. Moreover, there is a large stratigraphic gap of nearly 50 million years between ''Bactroceras'' and the next orthocones with a spherical apex and ventral siphuncle.

Morphology

''Bactroceras'' had a thin, cone shaped shell. Its siphuncle, the tube connecting its chambers to the animal's body, was ventral, which means that it ran next to the shell wall on the underside of the shell. The first chamber of the shell was spherical. The soft anatomy of the animal is unknown, but it lacked mineral deposits in its shell, so it may have floated with the point of its shell upward, but somewhat inclined. ''Bactroceras'' differs from the later cephalopod '' Bactrites'' in that its shell was more circular in cross section and has deeper V-shaped ventral lobes.

Fossil distribution

Fossils were found in Middle Ordovician strata dating from the Whiterockian/ Llanvirnian age. Locations were varied and stretched from New South Wales, Australia to El Puente,

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to Antelope Valley,

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Taxonomy

Morphology

Fossil distribution

References

Further reading