Tyrioceras Strand, 1934

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 67

Genus Tyrioceras Strand, 1934

Type species

Tyrioceras kjaerulfi Strand, 1934 , Furuholmen, Asker District, Norway, Grimsøy Formation, late Katian, Late Ordovician; by original designation.

Emended diagnosis

Large straight, compressed to depressed conch cross section; ornamented with broad bands or narrower raised transverse lines, which form broad and well marked, but not very deep ventral sinus with ventrolateral and dorso-lateral salients, siphuncle eccentric; septal necks orthochoanitic, relatively long, siphuncular segments tubular.

Remarks

The external characters of this genus where described in detail by Strand (1934) and Frye (1982). Originally the diagnosis of the genus was based only on the type species. In order to include T. warburgae Frye, 1982 , which is similar to the type in many aspects, but differs in having a depressed, more rapidly expanding conch, the diagnosis is emended.

The placement of Tyrioceras within the Lituitina , was formerly based on the characteristically curved ornamentation alone, and therefore was somewhat questionable. The details of the internal characters, now known from T. warburgae , support these earlier assumptions. The shape and extent of the cameral deposits in T. warburgae are very similar to those of other lituitins. A syn-vivo dissolution of the connecting ring, which is typical of lituitins, is indicated by epichoanitic deposits ( Figs 29 , 30 ). Additionally, voluminous epi-, hyposeptal and mural deposits, and relatively long septal necks are very characteristic for Lituitina . The assignment within the family of Sinoceratidae is after Dzik (1984).

Figures

Fig. 29. Polished median sections of Tyrioceras Strand, 1934, Boda Limestone. A. Tyrioceras cf. kjaerulfi Strand, 1934, PMU 26897, Kallholn, detail of septal neck with massive endocameral and epichoanitic deposits and long orthochoanitic septal necks. B. Tyrioceras warburgae Frye, 1982, PMU 26900, Kallholn, detail of septal neck with epichoanitic deposits. C. Same specimen as in A, total fragment. Scale bars: A-B = 5 mm; C = 10 mm.

Fig. 30. Reconstruction of median section of Tyrioceras cf. kjaerulfi Strand, 1934, PMU 26897, Boda Limestone, Kallholn. Black = outer shell and septa; grey = cameral deposits.