Dawsonoceratidae is an extinct family of orthoconic nautiloid cephalopods that lived in what would be North America

North America

North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

 and Europe

Europe

Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

 from the Late Ordovician

Ordovician

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

 through the Middle Devonian

Devonian

The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

 from about 480–390 mya, existing for approximately .

Taxonomic Position

Dawsonoceratidae was named by Flower (1962) and included in the Michelinoceratida. It was assigned to the Orthocerida by Walter Sweet in Teichert et al 1964 as part of the Orthocerataceae. The type genus is Dawsonoceras, named by Hyatt in 1883.

Morphology

Dawsonoceratidae are michelinocerids (Orthocerida) with the internal pattern of the Michelinoceratidae  except that the siphuncle segments, which are generally tubular, are constricted at the septal foremina (the openings through with the siphuncular tissues pass). This feature is also found in Silurian Kionoceras, but less developed. Septal necks are short and generally recumbent. The type genus, Dawsonoceras and its close relative Dawsonocerina are further characterized by having orthoconic shells with conspicuous annulations, covered by transverse, scalloped or festooned lirae which in some are also longitudinal. Annulations consist of narrow encircling ribs that may be somewhat oblique, separated by generally wider areas in between

Genera

The Dawsonoceratidae comprise six known genera, Dawsonoceras, the type, Anaspyroceras, Calocyrtocerina, Dawsonocerina, Metaspyroceras, and Palaeodawsonocerina.
Anaspyroceras, named by Shimizu and Obata (1935) and Metaspyroceras, named by Foeste (1932), were previously included in the orthoceratid subfamily Leuroceratinae, which is no longer considered a valid taxon, and reassigned to the Dawsonoceratidae. Anaspyroceras and Metaspyroceras differ from Flowers definition of the Dawsonoceratidae in that the septal necks are othochoanitic rather than recumbent and siphuncle segments are without noted constrictions. Calocyrtocerina, named by Chen (1981) was originally included in the Paraphragmitidae

Paraphragmitidae

Paraphragmitidae is an extinct family of actively mobile aquatic carnivorous cephalapods belonging to the subclass Orthoceratoidea endemic to what would be Asia and Europe during the Silurian living from 436—418.7 mya, existing for approximately ....

 of Flower and Kummel 1950. Palaeodawsonocerina started off as Spyroceras senckenbergi Teichert (1932) and redefined by Kroger and Isakar (2006) as the type species of Paleodawsonocerina (P. senckenbergi)

References

• Fossils (Smithsonian Handbooks) by David Ward