Isorthoceras sp. E

Fig. 19E

Material examined

ESTONIA • 1 spec.; Vormsi Island, Saxby shore, Kõrgessaare Formation, Vormsi Regional Stage; GIT 426-1132 .

Description

The specimen is a 110 mm long, 27–32 mm high fragment of a phragmocone (angle of expansion ca 4°) with 16 chambers preserved (RCL 0.21–0.31), and with a circular cross section. The outer shell is not preserved. The siphuncle is eccentric (RSP = 0.31) and, where preserved, with a straight, tubular connecting ring. The septal necks are short (ca 0.8 mm long) orthochoanitic-suborthochoanitic and shallow endosiphuncular annuli occur.

Remarks

This specimen is unique among the larger Isorthoceras fragments in having a combination of relatively low angle of expansion (comparable to I. luhai at similar conch heights) and a relatively eccentric siphuncle position (comparable to I. saaremense at similar conch heights). Additionally, the siphuncle is more tubular than compared to the other species of Isorthoceras known from Estonia, and shallow endosiphuncular deposits occur. More material is needed with external characters preserved to allow for a species level determination.