Charactoceras sp. B, Strand, 1934

Charactoceras sp. B

Fig. 22D View Fig

Material examined

ESTONIA • 1 spec.; Vormsi Island, Hosholm shore ; Adila Formation , Pirgu Regional Stage; GIT 878- 222 View Materials .

Description

The specimen is a fragment of a complete body chamber and an additional 14 chambers of the phragmocone. The body chamber is ca 90° long with an unconstricted aperture with a wide U-shaped hyponomic sinus. The total fragment has a conch diameter of 113 mm, and the diameter of the preceding whorl is 83 mm. At its base, the width of the body chamber is 48 mm and the height 39 mm (CHI = 0.81), at the aperture it measures 60 mm in width and 45 mm in height (CHI = 0.75). The septal distance at the base of the body chamber is 6 mm at the venter. There is no sign of mature septal crowding. The sutures run straight and transverse over flanks and venter. At a position where the whorl height is 33 mm, the septal foramen of the siphuncle is ca 2.5 mm in diameter and is located ca 6 mm from the ventral conch margin (RSP = 0.20).

Remarks

The fragment does not permit species level determination because the more juvenile growth stages are not preserved. These are critical for species determination because many species of Charactoceras are differentiated by the extent of the annulation of the early growth stages. The WER of 1.85 of this specimen is lower than that of comparable growth stages of the type of C. estonicum (WER>3.5, compare Fig. 22A, D View Fig ) and of other known species of Charactoceras . The species of Charactoceras with lowest known expansion rate is C. rotundum (WER: 1.9–2.4) from late Katian Strata of northern Greenland ( Troedsson 1926). However, this latter species differs in having a siphuncle that is located more distantly from the ventral conch margin.