Dawsonoceratidae
Introduction '''Dawsonoceratidae''' is an extinct family of orthoconic nautiloid cephalopods that lived in what would be North America and Europe from the Late Ordovician through the Middle Devonian 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 of Flower and Kümmel 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