''Bactroceras'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial n ...

orthoceratoid Orthoceratoidea is a major subclass of nautiloid cephalopods. Members of this subclass usually have orthoconic (straight) to slightly cyrtoconic (curved) shells, and central to subcentral siphuncles which may bear internal deposits. Orthoceratoi ...

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, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. ...

, 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 The ''Treatise on Invertebrate Paleontology'' (or ''TIP'') published by the Geological Society of America and the University of Kansas Press, is a definitive multi-authored work of some 50 volumes, written by more than 300 paleontologists, and co ...

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 Orthocerida is an order of extinct Orthoceratoid cephalopods also known as the Michelinocerida that lived from the Early Ordovician () possibly to the Late Triassic (). A fossil found in the Caucasus suggests they may even have survived until t ...

and family

Baltoceratidae Baltoceratidae is an extinct family of orthoconic cephalopods belonging to the subclass Nautiloidea endemic to what would be Asia, Australia, Europe, North America, and South America during the Ordovician living from about 480–460 mya, exis ...

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 Riocerida, originally named as Rioceratida, is an extinct order of Ordovician nautiloid cephalopods. They were the earliest-diverging members of the major group Orthoceratoidea, with which they share dorsomyarian muscle scars (a few large muscl ...

. Some paleontologists have regarded ''Bactroceras'' as an early bactritid because of its spherical apex and

ventral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

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 ...

. 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

orthocone An orthocone is an unusually long straight shell of a nautiloid cephalopod.; During the 18th and 19th centuries, all shells of this type were named '' Orthoceras'', creating a wastebasket taxon, but it is now known that many groups of nautiloids ...

s with a spherical apex and ventral siphuncle.

Morphology

''Bactroceras'' had a thin, cone shaped shell. Its

, 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 ) , nickname = , image_map = New South Wales in Australia.svg , map_caption = Location of New South Wales in AustraliaCoordinates: , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , e ...

to El Puente,

Bolivia , image_flag = Bandera de Bolivia (Estado).svg , flag_alt = Horizontal tricolor (red, yellow, and green from top to bottom) with the coat of arms of Bolivia in the center , flag_alt2 = 7 × 7 square p ...

Antelope Valley The Antelope Valley is located in northern Los Angeles County, California, and the southeast portion of Kern County, California, and constitutes the western tip of the Mojave Desert. It is situated between the Tehachapi, Sierra Pelona, and t ...

Nevada Nevada ( ; ) is a state in the Western region of the United States. It is bordered by Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. Nevada is the 7th-most extensive, ...

Taxonomy

Morphology

Fossil distribution

References

Further reading