Nathorstoceras, Kröger, 2013

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 38

Genus Nathorstoceras gen. nov.

urn:lsid:zoobank.org:act:3415D639-EFD4-404C-90EA-DA8AAAA930E5

Type species

Nathorstoceras kallholnense gen. et sp. nov., Kallholn, Dalarna, Sweden, Boda Limestone, Boda Core Member, late Katian, Ordovician by montotypy.

Diagnosis

Straight or slightly curved longicones with annulated or smooth shell, ornamented with transverse lirae or conspicuous growth lines, which form shallow sinus on prosiphuncular side; septal spacing relatively narrow with ca. five chambers per distance equal to corresponding conch diameter; siphuncle subcentral or slightly eccentric on convex side of conch curvature; septal foramen narrow; siphuncular segments abruptly expanded at septal foramen, septal necks achoanitic; epi- and hyposeptal deposits present; endosiphuncular deposits not known at present.

Etymology

In honour of Alfred Gabriel Nathorst (1850–1921), Swedish palaeobotanist, geologist, and polar researcher, for his pioneering work on the Palaezoic of the Siljan structure ( Nathorst 1885).

Comparison

This genus is unique among Ordovician orthocerids. The rapidly expanding siphuncular segments are similar as compared with late Palaeozoic pseudorthocerids, such as Adnatoceras Flower, 1939 . However, characteristic pseudorthocerid endosiphuncular deposits are not known from specimens of Nathorstoceras gen. nov. With its annulate conch N. kallholnense sp. nov. resembles Mid Silurian paraphragmitids, which differ in having less expanded siphuncles and suborthochoanitic septal necks. In the Estonian Porkuni Formation smooth orthocones occur, with achoanitic septal necks and broadly expanded siphuncular segments ( Eriditidae gen. et. sp. indet. in Kröger 2007). These specimens are poorly preserved or very fragmentary, and it is difficult to compare them with the relatively large fragments of the specimens from the Boda Limestone, but their similarity in the septal neck/siphuncular segment shape suggest that they may represent species of Nathorstoceras gen. nov.

Monomuchites bacotens e Balashov, 1975 from the Molodov Horizon of Podolia is known from a single fragmentary specimen only. However, the polished cross section figured in Balashov (1975: pl. 1, fig. 7) shows a marked similarity with the Boda specimens of Nathorstoceras gen. nov. M. bacotense , differs from the Boda specimen mainly in having a circular conch cross section. Herein, it is interpreted as another species of Nathorstoceras gen. nov.

Species included

Nathorstoceras kallholnense sp. nov., Nathorstoceras adnatum sp. nov., Nathorstoceras bacotense ( Balashov, 1975) comb. nov.

Stratigraphic and geographic range

Boda Limestone, Dalarna, Sweden, late Katian, Late Ordovician.