''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 The ''Treatise on Invertebrate Paleontology,'' published from 1953–2007 by the Geological Society of America and the University of Kansas, then 2009–present by the University of Kansas Paleontological Institute, is a definitive multi-authore ...

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, from Ancient Greek ὀρθός (''orthós''), meaning "straight", and κέρας (''kéras''), meaning "horn", also known as the Michelinocerida, is an order of extinct Orthoceratoidea, orthoceratoid cephalopods that lived from the ...

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, exist ...

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 The Bactritida are a small order of more or less straight-shelled (orthoconic) cephalopods that first appeared during the Emsian stage of the Devonian period (407 million years ago) with questionable origins in the Pragian stage before 409 millio ...

because of its spherical

apex The apex is the highest point of something. The word may also refer to: Arts and media Fictional entities * Apex (comics) A-Bomb Abomination Absorbing Man Abraxas Abyss Abyss is the name of two characters appearing in Ameri ...

and

ventral Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. 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 biological tissue, tissue passing longitudinally through the mollusc shell, shell of a cephalopod mollusc. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the li ...

. 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

orthocerid Orthocerida, from Ancient Greek ὀρθός (''orthós''), meaning "straight", and κέρας (''kéras''), meaning "horn", also known as the Michelinocerida, is an order of extinct orthoceratoid cephalopods that lived from the Early Ordovician ...

s, 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 the long, cone-shaped Exoskeleton, shell belonging to several species of ancient nautiloid cephalopod—the prehistoric ancestors of today's marine cephalopod Mollusca, mollusks, including the cuttlefishes, nautiluses, octopuses an ...

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 New South Wales (commonly abbreviated as NSW) is a state on the east coast of :Australia. It borders Queensland to the north, Victoria to the south, and South Australia to the west. Its coast borders the Coral and Tasman Seas to the east. T ...

to El Puente,

Bolivia Bolivia, officially the Plurinational State of Bolivia, is a landlocked country located in central South America. The country features diverse geography, including vast Amazonian plains, tropical lowlands, mountains, the Gran Chaco Province, w ...

Antelope Valley The Antelope Valley is a valley primarily located in northern Los Angeles County, California, United States and the southeast portion of Kern County, California, Kern County, and constitutes the western tip of the Mojave Desert. It is situated ...

Nevada Nevada ( ; ) is a landlocked state in the Western United States. It borders Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. Nevada is the seventh-most extensive, th ...

Taxonomy

Morphology

Fossil distribution

References

Further reading