Charactoceras estonicum Strand, 1934

Charactoceras estonicum Strand, 1934

Fig. 22A View Fig

Charactoceras estonicum Strand, 1934: 28–29, pl. 2 fig. 5, pl. 7 fig. 1.

Charactoceras estonicum – Balashov 1953b: 266, pl. 11 figs 1–2; 1962c: pl. 7 fig. 1. ― Stumbur 1962: 136. ― Dzik 1984: 44, 154, text-figs 12.30, 59.5.

Diagnosis

Charactoceras with smooth, rapidly expanding conch (WER ca 3–4), and adult diameter of ca 140 mm; whorl cross section with rounded venter and rounded umbilical margins, and with deeply impressed dorsal furrow, moderately compressed with WWI ca 1.1–1.3 with greatest width in the dorsal half; sutures with broad and shallow ventral lobes and ventro-lateral saddles; siphuncle close to ventral conch margin. (Adopted from Strand 1934.)

Material examined

ESTONIA • 2 specs; Vormsi Island, Hosholm shore ; Adila Formation , Pirgu Regional Stage; TUG 939- 68 , TUG 1745-274 • 5 specs; Vormsi Island, Hosholm shore (tower locality); Adila Formation , Pirgu Regional Stage; GIT 878-216 to GIT 878-220 • 2 specs; Vohilaid Island, Vohilaid shore (E); Adila Formation , Pirgu Regional Stage; GIT 878-198 , GIT 878-229 .

Type locality and horizon

Piirsalu, Estonia; Moe Formation, Pirgu Regional Stage.

Description

The species has been described in detail by Strand (1934). Additional measurements, taken from the Estonian specimens, contribute to the knowledge on its variability in WWI and WER. The measurements show that WER and WWI are nearly constant throughout different growth stages, with a mean WWI = 2.16 (1 st quartile 1.14–3 rd quartile 1.26, n = 10,) and a mean WER = 3.44 (1 st quartile 3.27–3 rd quartile 3.45, n = 5). Also, the shape of the whorl cross section does not change significantly at different growth stages with rounded venter and flanks and greatest width located in the dorsal half of the whorl.

The most complete specimen in the collection (apart from the holotype TUG 939-68, which has been described in detail by Strand 1934 and Balashov 1953b), is GIT 878-229. This specimen is a fragment of a phragmocone consisting of two whorls, and has a maximum diameter of 72 mm. The preceding whorl has a diameter of 40 mm (WER = 3.24). At a conch diameter of 44 mm, the next whorl has a diameter of 24 mm (WER = 3.36). The WWI is 1.13 at a conch width of 36 mm, 1.20 at a conch width of 24 mm, and 1.1 at a conch width of 11 mm. The septa are 8 mm apart at the venter where the whorl height is 32 mm. The septal foramen is located 3 mm from the ventral conch margin and has a diameter of 3 mm where the conch height is 28 mm (RSH = 0.11, RSP = 0.12).

Remarks

The diagnosis is adopted from Strand (1934: 28). Here, the WER and WWI are used to describe the relative conch dimensions and are included in the diagnosis; their values are based on the type specimen.

Comparison

Charactoceras estonicum is apart from the Laurentian palaeocontinent known only from Baltica (see Foerste 1926, 1928b, 1935 a, 1935b; Troedsson 1926; Strand 1934; Wilson 1961; Nelson 1963; Frye 1982). Species of Charactoceras are differentiated by whorl cross section shape, ornamentation, and expansion rate. All four species of Charactoceras described by Frye (1982) from the Boda Limestone, late Katian, Sweden, differ in being annulated.

Charactoceras estonicum is one of the species of Charactoceras with a smooth, non-annulated shell. Among those smooth species, C. laddi Foerste, 1935a , C. manitobaense Nelson, 1963 differ in having a flattened venter, and C. eximium ( Sweet & Miller, 1957) and C. triangulum Frye, 1982 differ in having a trapezoidal whorl cross section. An elliptically depressed whorl cross, but with smaller WWI than in C. estonicum , have: C. baeri (WWI = 1.4–1.5, Foerste 1924), C. hercules ( Billings, 1857) (WWI = 1.8 at whorl height 53 mm, Nelson 1963: 102), C. normale Wilson, 1961 (WWI = 3.6 at whorl height of 18 mm, Wilson 1961), and C. warranae Nelson, 1963 (WWI = 2 at whorl height of 45 mm). The two species which are most similar to C. estonicum with respect to whorl cross section and ornamentation are C. rotundum Troedsson, 1926 and C. schucherti Foerste, 1928b ; they differ in having a smaller expansion rate ( C. rotundum : WER = 2.3 at conch diameter 107 mm, Troedsson 1926), and ( C. schucherti : WER = 2.5 at conch diameter of 47 mm, Foerste 1928b), respectively.