Parvihebetoceras wahli Kröger, 2007

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 96-97

Parvihebetoceras wahli Kröger, 2007

Fig. 44

Parvihebetoceras wahli Kröger, 2007: 599 , figs 4c, 7c, 9.

Diagnosis

Same as for genus, by monotypy.

Type locality and horizon

Porkuni, Estonia, Porkuni Formation, Hirnantian, Ordovician.

Material

One specimen (PMU 26949), Kallholn, Dalarna, Sweden, Boda Limestone, Boda Core Member, late Katian, Ordovician.

Description

The specimen is a fragment of a body chamber with one apical chamber of the phragmocone ( Fig. 44 ). The conch is slightly curved, the cross section circular, 5.4–8.3 mm in diameter and 16 mm long (angle of expansion 10°). The shell surface is partly preserved and smooth. Apically the conch is truncated with a dome-shaped, deeply convex septum of truncation. The adapical surface of the septum of truncation is rugged. The convexity of the septum is 3.5 mm. The tip of the septal dome is slightly subcentral, off the center of the conch in direction toward the convex side of the conch curvature. A second septum is preserved, ca. 4 mm adorally of the septum of truncation, which is less convex and with a subcentral septal perforation ( Fig. 44C ). The shape of the septal neck is poorly preserved, but probably was achoanitic or very short suborthochoanitic; it appears longer on the prosiphuncular side of the conch. Parts of the connecting ring are preserved, which indicate a slightly expanded siphuncular segment. Because of the poor preservation of the internal details, the presence or absence of cameral deposits is difficult to evaluate, but thin hyposeptal deposits appear to be present at the adoral septum ( Fig. 44C ) and the rugged surface of the septum of truncation may be interpreted as the surface of episeptal deposits.

Remarks

The deeply convex, dome-shaped septum of truncation, the shape of the septal necks and the position and shape of the siphuncle are diagnostic characters of Parvihebetoceras . The conch size of the Boda specimen is identical to that of Parvihebetoceras wahli Kröger, 2007 from the Porkuni Formation of northern Estonia. P. wahli reaches its adult size with a diameter of ca. 7 mm and has a septum of truncation at diameters of ca. 5 mm. Because of the general similarity of the Boda specimen with the Estonian specimens described in Kröger (2007), it is regarded as conspecific with P. wahli . However, the Estonian specimens of P. wahli appear slightly more brevidomic and have slightly smaller adult sizes.

The specimen described herein shows features that can be interpreted as hyposeptal deposits on the adoral septum. However, because of the poor preservation of the internal characters, it is impossible to evaluate if these are primary or secondary deposits. Therefore, they are not included within the diagnosis of the species.

The rough, poorly preserved surface of the septum of truncation is similar to that in the middle Silurian Sphooceras Flower, 1962 , but in Sphooceras the shape of the septal necks and siphuncle differ and heavy cameral deposits are developed, which are absent in Parvihebetoceras .

Figures

Fig. 44. Parvihebetoceras wahli Kröger, 2007, PMU 26949, Kallholn, Boda Limestone. A. Lateral view. B. Adapical view. C. Median section with one septum preserved. Scale bar = 5 mm for all figures.