{{Short description|Extinct genus of nautiloids}} {{Automatic taxobox | fossil_range = {{fossil range|middle Ordovician}} | taxon = Bactroceras | authority = Holm, 1898 | parent_authority = King & Evans, 2019 | subdivision = | synonyms = * ''Eobactrites'' <small>Schindewolf, 1932</small> }}

'''''Bactroceras''''' is a genus of orthoceratoid cephalopods that lived during the early Middle Ordovician, from about 472—464 mya, existing for approximately 8 million years.<ref name = Furnish>Nautiloidea – Ellesmerocerida by W.M Furnish and Brian F. Glensister; Treatise on Invertebrate Paleontology Part K R.C. Moore (ed) Univ.Kans. press</ref>

==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)<ref name = Furnish/> and retained there by Evans (2005).<ref>D. H. Evans. 2005. The Lower and Middle Ordovician cephalopod faunas of England and Wales. Monograph of the Palaeontographical Society 623:1-81</ref> King & Evans (2019) instead placed it in its own family ('''Bactroceratidae'''), as part of the newly named order Rioceratida.<ref name=":2">{{Cite journal |last1=King |first1=Andy H. |last2=Evans |first2=David H. |date=2019 |title=High-level classification of the nautiloid cephalopods: a proposal for the revision of the Treatise Part K |journal=Swiss Journal of Palaeontology |language=en |volume=138 |issue=1 |pages=65–85 |doi=10.1007/s13358-019-00186-4 |s2cid=133647555 |issn=1664-2384|doi-access=free |bibcode=2019SwJP..138...65K }}</ref>

Some paleontologists have regarded ''Bactroceras'' as an early bactritid because of its spherical apex and ventral siphuncle.<ref name = "Holland 2003"> {{cite journal | author = Holland, Charles H. | author-link = Charles Hepworth Holland | year = 2003 | title = Some observations on bactritid cephalopods | journal = Bulletin of Geosciences | volume = 78 | issue = 4 | pages = 369–372 | publisher = Czech Geological Survey | issn = 1214-1119 }}</ref>

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 Ordovician orthocerids, such as ''Archigeisonoceras'' and ''Hedstroemoceras'': it is about 10&nbsp;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&nbsp;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.<ref name = "Kroger & Mapes 2007"> {{cite journal | author = Kröger, Björn | author2 = Mapes, Royal H. | name-list-style = amp | year = 2007 | title = On the origin of bactritoids (Cephalopoda) | journal = Paläontologische Zeitschrift | volume = 81 | issue = 3 | pages = 316–327 | doi = 10.1007/BF02990181 | bibcode = 2007PalZ...81..316K | s2cid = 129091196 }}</ref>

==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.<ref name="Holland 2003"/> ''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, Bolivia to Antelope Valley, Nevada.

== References == {{Portal|Paleontology}} {{Reflist}}

== Further reading == * ''Fossils'' (Smithsonian Handbooks) by David Ward * [https://paleobiodb.org/classic/checkTaxonInfo?taxon_no=12332 PaleoBiology Database: ''Bactroceras'', basic info]

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Category:Prehistoric nautiloid genera Category:Middle Ordovician animals Category:Ordovician animals of Asia Category:Ordovician cephalopods of Europe Category:Ordovician cephalopods of North America Category:Ordovician animals of South America Category:Ordovician Bolivia Category:Fossils of Bolivia Category:Cephalopods of Asia Category:Prehistoric molluscs of Oceania Category:Cephalopods of South America Category:Fossil taxa described in 1898