Curtoceras teres ( Eichwald, 1840 )

Curtoceras teres ( Eichwald, 1840)

Figs 32–35 View Fig View Fig View Fig View Fig ; Table 23 View Table 23

Nautilus teres Eichwald, 1840: 105 .

Curtoceras estonicum Balashov, 1953: 207 , pl. 6 figs 1–2.

Lituites teres – Eichwald 1860: 1299. — Dewitz 1879: 177, pl. 4 fig. 4. — Schröder 1882: 57–58, pl. 2 fig. 2.

Discoceras teres – Schröder 1891: 161. — Schindewolf 1942: 351, text-fig. 5e; 1944: 218, text-fig.7.

Schroederoceras teres – Hyatt 1894: 467. — Schindewolf 1932: 171, text-fig. 3.

Discoceras (Schröderoceras) teres – Schindewolf 1939: 60, text-fig. 15, pl. 4 fig. 8.

Curtoceras teres – Balashov 1953: 246, pl. 5 fig. 2, pl. 7 fig. 4. — Dzik 1984: 42, 44.

Lituites Odini View in CoL – Verneuil in Murchison et al. 1845: 360, pl. 25 fig. 8. — Lossen 1860: 23.

Curtoceras estonicum – Stumbur 1962: 136. — Balashov 1962: pl. 9 fig. 7; 1974: pl. 9, fig. 7.

? „ Lituites“ teres – Holm 1885: 10, pl. 5 figs 5–8.

non Clymenia Odini Eichwald, 1840: 107 View in CoL .

non Clymenia Odini View in CoL – Eichwald 1860: 1304, pl. 51 fig. 27.

Diagnosis

Species of the genus Curtoceras with an adult conch diameter of 40–50 mm. The adult conch consists of up to 4 volutions; in the last whorl, coiling rate and conch width decrease (WER ah ~ 2.00 and CWI ~ 0.30), umbilical width increases (UWI ~ 0.50). Whorl profile rounded or weakly laterally and ventrally flattened, and very weakly embracing or not embracing (IZR varies between 0.00–0.06). Whorl profile changes from depressed in inner volutions (WWI ~ 1.65), in outer whorl typically equidimensional but may be weakly depressed or weakly compressed in some specimens (WWI as low as 0.82). Body chamber about 180 degrees in length, uncoiled after ~ 120 degrees; terminal aperture with ventral sinus and dorso-lateral and dorsal extension and slight flaring. Shell surface with distinct, regularly spaced lirae with acute and more or less symmetric crests on inner whorls, flattening out towards the outer whorl, where the lirae are slightly imbricated; ventral sinus of ornament present. Siphuncle about 0.16 of corresponding apertural height throughout ontogeny; siphuncle position ventral in initial chamber, central after first 180 degrees, shifted towards the dorsal shell wall in later growth stages (RSP varies between 0.60 and 0.75 of apertural height).

Material examined

ESTONIA • 1 spec.; Tallinn; Ordovician; Kirchner Coll.; illustrated in Fig. 33A View Fig ; MB.C.9715 • 1 spec.; Aari quarry; Lasnamägi Regional Stage ( Väo Formation ); GIT 426 -499 • 1 spec.; Aa; Lasnamägi Regional Stage ; Stumbur Coll.; illustrated in Fig. 34C View Fig ; GIT 426-530 • 1 spec.; Osmussaar cliff ; Lasnamägi Regional Stage ; Männil Coll.; GIT 697-567-1 • 1 spec.; Osmussaar; Lasnamägi Regional Stage ; Schmidt Coll.; illustrated in Fig. 34A View Fig ; GIT 225-1092 • 1 spec.; Aari quarry; Lasnamägi Regional Stage ; Schmidt Coll.; illustrated in Fig. 34B View Fig ; GIT 225-1062 • 1 spec.; Osmussaar 1 ; Lasnamägi Regional Stage ; TUG 860-1309 • 1 spec.; same data as for preceding; GIT 697-130 • 1 spec.; Sõjamäe; Lasnamägi Regional Stage ( Väo Formation ); GIT 426-498 • 1 spec.; Kadaka road ( Tallinn ); Darriwilian Stage; illustrated in Fig. 34D–E View Fig ; FMNH HS. P31459 .

GERMANY – Mecklenburg-Vorpommern • 1 spec.; Basedow-Stöckersoll; Ordovician; 2006; Meyer Coll.; illustrated in Fig. 32 View Fig ; MB.C.32183 . – Brandenburg • 1 spec.; Strausberg; Lasnamägi Regional Stage; Neben and Krueger Coll.; MB.C.32188 • 1 spec.; Leegebruch near Velten ; Lasnamägi or Uhaku regional stages ; 1951; Gräber Coll.; MB.C. 11548a-d • 1 spec.; Oderberg; Ordovician; Neben and Krueger Coll.; MB.C. 32187 • 1 spec.; Oderberg; Aseri Regional Stage ; 1964; Neben and Krueger Coll.; illustrated in Fig. 35B View Fig ; MB.C. 32186 • 1 spec.; Potsdam; Lasnamägi Regional Stage ; illustrated in Fig. 33B View Fig ; MB.C. 11544. – Locality unknown • 1 spec.; probably Brandenburg ; Ordovician; Neben and Krueger Coll.; illustrated in Fig. 33C View Fig ; MB.C. 32184 • 1 spec.; probably Brandenburg ; Ordovician; Neben and Krueger Coll.; illustrated in Fig. 35A View Fig ; MB.C. 32185 .

POLAND – West Pomerania • 1 spec.; Bydgoszcz; Darriwilian Stage; previously figured by ( Schindewolf 1933: text-fig. 8), re-illustrated here in Fig. 33D View Fig ; MB.C.646 • 1 spec.; Rybokarty near Gryfice ; Darriwilian Stage; Wietersheim Coll.; MB.C. 11545 .

Description

Specimen MB.C.32183 ( Fig. 32 View Fig ) is the most complete of the available specimens. It has a total conch diameter of 48 mm and is composed of two and three-quarter volutions. The conch is spirally coiled up to a diameter of 43 mm, the last short segment of the body chamber is detached from the preceding whorl and has an almost straight direction. At 43 mm conch diameter, the conch is subevolute (UWI = 0.40) with a very high coiling rate (WER = 2.39). The umbilical foramen is very small, measuring approximately 1× 2 mm. The entire specimen is covered with shell material. The ornament consists of coarse, lamellar growth lines with rursiradiate direction; they extend with a low lateral projection and form a very deep and wide ventral sinus.

Specimen MB.C.11544 ( Fig. 33B View Fig ) is an incomplete, but rather well-preserved conch of a nearly adult individual with 44 mm diameter. It consists of 3.25 whorls with the initial chamber preserved. In the last 0.75 volutions, in which the conch diameter grows from 25 mm to 43.5 mm, the conch becomes more discoidal (CWI decreases from 0.39 to 0.30) and the whorl profile changes from weakly depressed to equidimensional (WWI changes slightly from 1.11 to 1.00). Flanks and venter remain broadly convex and the whorls are only very weakly impressed dorsally or not impressed. In the course of the entire ontogeny (from 5.7 mm to 43.6 mm conch diameter), the coiling rate decreases significantly (WER decreases from 5.18 to 2.00) and the umbilicus widens (UWI increases from 0.16 to 0.47). The last quarter of the outer volution belongs to the body chamber, which begins to detach from the preceding whorl. The shell is ornamented with distinct, regularly spaced lirae with symmetric or only slightly imbricated crests; the lirae are sharp especially in the inner whorls, while in the outer whorl, they appear to flatten out. The ornament element extend with a ventral sinus. The phragmocone chambers are moderately long (RCL = 0.25 at dm = 30 mm). The suture line is straight.

Specimen GIT 225-1092 ( Fig. 34A View Fig ) is a nearly complete conch with a diameter of 52 mm and consists of 3.5 whorls; the body chamber occupies one third of the last whorl and is detached from the preceding volution. During ontogeny, the conch width and expansion become smaller (CWI decreases from 0.50 to 0.30, WER decreases from 2.64 to 2.04), while the umbilical width ratio increases (UWI changes from 0.39 to 0.47). The whorl profile is weakly depressed to equidimensional (WWI decreases from 1.30 to 0.99); it shows broadly convex flanks and venter in the inner whorls and becomes weakly flattened laterally and ventrally late in ontogeny. At the maximum conch diameter of 52 mm, the siphuncle is narrow (RSH = 0.13) and has a position between the centre and dorsal shell wall (RSP = 0.64). The shell wall is locally preserved; it is ornamented with sharp, evenly spaced lirae. The adult suture line shows a shallow lateral and a shallow ventral lobe.

Specimen GIT 426-530 ( Fig. 34C View Fig ) is a natural transverse section at one side. It is a conch with 50 mm diameter and consists of 3.25 volutions; the last quarter volution belongs to the body chamber. In the last two volutions, the conch becomes more widely umbilicate (UWI increases from 0.20 to 0.44) and less expanded (WER decreases from 3.88 to 2.07). At the maximum diameter of 50 mm, the conch is discoidal (CWI = 0.32) and the whorl profile is nearly circular (WWI = 1.05) with convex flanks and a weakly flattened venter. The initial part of the phragmocone is preserved but the first chamber cannot be measured accurately; the umbilical window is 0.5 × 0.2 mm in diameter. The shell surface is ornamented with sharp, equally spaced lirae, which extend with a steep ventral sinus; some indistinct spiral lines are visible on the body chamber. The phragmocone chambers are rather short (RCL = 0.20 in the last half a whorl). The suture line is almost straight, but extends with a very shallow lateral lobe and a shallow external lobe.

The four specimens MB.C.32187, MB.C.32184 ( Fig. 33C View Fig ), MB.C.9715 ( Fig. 33A View Fig ) and MB.C.11544 ( Fig. 33B View Fig ) are nearly identical in their conch proportions and the shell ornament to specimen GIT 426- 530. All of the specimens are adult or nearly adult conchs with about three whorls (dm = 37–47 mm). All show that the body chamber starts to uncoil after about 2.75 whorls (MB.C.9715 and MB.C.11545). In specimen MB.C.9715 (along with GIT 697-567-1 and GIT 225-1062), the aperture is preserved; it has a ventral sinus and a dorsolateral extension. The whorl profile at the maximum conch diameter ranges between weakly compressed and weakly depressed (WWI = 0.82–1.07) with broadly rounded flanks and venter; the dorsal impression is small. The umbilical width ratio (UWI) is between 0.42 and 0.50 and the conch width (CWI) between 0.29 and 0.35. The shell is ornamented with sharp, evenly distributed lirae with acute and more or less symmetric crests. The lirae are particularly distinct on the body chamber, where they form a deep ventral sinus. At the very end of the body chamber in MB.C.9715, the lirae are slightly frilled ventrally.

Specimen MB.C.646 ( Fig. 33D View Fig ) is a sagittal section through a subadult conch with 32 mm diameter and with the first 2.5 whorls preserved. The umbilical window is small (0.50× 0.25 mm). Throughout ontogeny, the coiling rate of the conch decreases markedly (WER is reduced from 5.56 to 2.18). The siphuncular diameter (RSH) increases during ontogeny from 0.12 up to 0.18. The siphuncle has a ventral position in the initial chamber but during the first three quarters of the length of the first volution migrates towards the dorsal shell wall (RSP = 0.69 at dm = 4.9 mm); up to the largest conch diameter, the relative position of the siphuncle remains approximately the same (RSP = 0.67 at dm = 31 mm). The septal necks are orthochoanitic or loxochoanitic and short (<0.2 of the segment length). They are thicker than the free part of the septum. The connecting rings are essentially tubular in the first whorl but then become slightly concavo-convex or concave in shape, especially on the ventral side of the siphuncle. The brownish inner layer of the connecting ring wall is homogenous and about as thin as the septa. The outer, dark-grey layer of the ring is much thicker and the thickness is unevenly distributed. The initial chamber of the phragmocone is curved and cup-shaped, 1.82 mm in height and about 2.3 mm in width; the caecum is ~ 0.5 mm long. In the first whorl, the chambers shorten from RCL ~ 0.60 to RCL ~ 0.30. Further chambers change between 0.22 and 0.15. The section through the shell wall in the specimen indicates that the shell was ornamented with rather regularly spaced, slightly imbricated lirae; in the most apical part of the first whorl, the shell surface appears to be smooth. Inside of the conch, the septa are thickened at the point where they are attached to the inner surface of the shell wall.

Two additional specimens, GIT 225-1062 ( Fig. 34B View Fig ) and GIT 426-499, were studied from sagittal sections. The conch morphology is generally the same as in specimen MB.C.646. A difference is that, in GIT 225-1062 ( Fig. 34B View Fig ), the siphuncle is thinner (RSH as low as 0.12) and its diameter does not increase during ontogeny; however, the section through the specimen is not perfectly in the median plane and the values might thus be artificially lower. Both specimens are adult individuals. More complete of the two is the specimen GIT 225-1062, which shows the initial chamber (0.85× 1.80 mm). The last 185 degrees of the conch is occupied by the body chamber, which detaches from the preceding volution and shows the aperture. At the dorsal surface of the uncoiled part of the body chamber, the shell ornament of sharp, regularly spaced and imbricated lirae is preserved.

Two specimens were sectioned transversely. The larger of the two is specimen MB.C.32185 ( Fig. 35A View Fig ), which is a conch 45 mm in diameter with three whorls preserved. During ontogeny (between 8 mm and 45 mm conch diameter), the conch changes from thinly pachyconic to extremely discoidal (CWI decreases from 0.61 to 0.29) and from subinvolute to subevolute (UWI increases from 0.18 to 0.44); the whorl expansion rate is lowered (WER decreases from 3.57 to 2.00). The whorl profile shows broadly rounded flanks and venter, it changes from weakly depressed to weakly compressed (WWI decreases from 1.24 to 0.96). The siphuncle varies in diameter between 0.17 and 0.14 of the apertural height; it has a subcentral position in the earliest whorl (RSP = 0.55) and then moves further towards the dorsal shell wall and maintains the relative distance of RSP = 0.73. The other transversely sectioned specimen MB.C.32186 ( Fig. 35B View Fig ) is nearly identical to specimen MB.C. 32185 in conch dimensions and ratios; the only difference is the more depressed whorl profile (WWI = 1.13 at dm = 38 mm).

Specimen FMNH HS.P31459 ( Fig. 34D View Fig ) is a nearly complete conch (only the innermost whorls are covered by sediment) of an adult individual with the total diameter of about 47 mm. One lateral side of the specimen is hidden in the surrounding rock and the first half of the outer volution is slightly distorted; this hindered some measurements. The body chamber occupies the last half of a whorl. The aperture is partly preserved and shows the presence of a rounded and deep hyponomic sinus and very slight lateral flaring. At the base of the body chamber, structures resembling muscle attachment impressions are preserved ( Fig. 34E View Fig ). At the diameter of 44 mm, the conch is subevolute or evolute (UWI ~ 0.45) with a high coiling rate (WER ~ 2.20). The suture lines are straight and relatively closely spaced (RCL ~ 0.25). The ornament consists of rursiradiate lirae that form a deep and wide ventral sinus; the lirae are sharp especially in the inner whorl and flatten out towards later whorls.

Remarks

The holotype of “ Nautilus Teres ” was not available for our study; it was described by Eichwald (1840) but not illustrated in that publication. It came from the Orthoceratite Limestone (C1 horizon, late Darriwilian, Middle Ordovician) of the Estonian island of Osmussaar (formerly Odensholm in Swedish and Odinsholm in German).

Balashov (1953: 246, pl. 5 fig. 2) reproduced and assigned, as “ holotype ”, a specimen of “ Lituites Odini ” figured by Verneuil (1845: 360, pl. 25 fig. 8). Eichwald (1860) had assigned this specimen to “ Nautilus teres ” but he did not state whether it was part of the type collection or not. Furthermore, the present repository of the specimen or the type collection of Eichwald could not be traced. The herein studied material agrees closely with the latter specimen, but designation of a neotype from the presently studied collection is avoided because it cannot be confirmed that the type collection of Eichwald (1840) was lost.

Here we assign 19 specimens to Curtoceras teres , which makes it the most abundant species in the studied material and allows some insight into the intraspecific variation. This concerns mainly the shape of the whorl profile. Generally, the WWI decreases as the conch grows ( Fig. 11 View Fig ). In the early growth stages, the whorl profile is moderately depressed (WWI = 1.66 in specimen MB.C.32186), but in the adult stage it is between weakly depressed (WWI = 1.13 in specimen MB.C.32186), equidimensional (MB.C.11544, MB.C.32187, MB.C.32185) and weakly compressed (WWI = 0.82 in specimen MB.C.9715); the whorl profile is usually broadly rounded or sometimes weakly flattened ventrally and laterally. The differences in adult whorl profile are not accompanied by differences in any other characters or parameters. The lowest variation is recognised in WER and CWI; both decrease during ontogeny ( Fig. 11 View Fig ), particularly in early growth stages (up to dm ~ 5 mm), after that the rate of decrease is getting smaller.

Besides the material described above,the Estonian geoscience data portal eMaapõu(https://geoloogia.info) lists several juvenile specimens of Curtoceras (TUG 1745-331 to TUG 1745-337, University of Tartu, Natural History Museum) from the Aseri Regional Stage, which appear to fall within the variation observed in C. teres . These specimens support the extension of the stratigraphic range of the species to the Aseri Regional Stage (Darriwilian Stage).

Curtoceras teres is similar to the stratigraphically coeval C. estonicum (Island of Osmussaar and Tallinn Region of Estonia). According to the original description of Balashov (1953), the latter differs by having a square-shaped whorl profile in later growth stages and more sinuous suture lines; also, the siphuncle is closer to the dorsal side in C. estonicum than in C. teres . In the herein studied collection, there are four specimens (GIT 225-1092, GIT 697-130, GIT 426-498, TUG 860-1309) which have weakly rectangular whorl profile producing a ventral lobe and dorso-lateral saddle in the suture lines. However, there is no difference in conch geometry between these specimens, the holotype of C. estonicum described by Balashov (1953) and the representatives of C. teres with rounded whorl profile studied herein. For this reason and considering the variability of whorl profile shape in late growth stages of the latter, Curtoceras estonicum is synonymised with C. teres here.

In contrast to Curtoceras teres , the early Sandbian C. kerstovense from the St Petersburg Region ( Russia) attains much larger adult size (up to 80 mm) with only the very end of the body chamber uncoiled from the preceding volution. Curtoceras abditus from the Keila Regional Stage (Late Ordovician) of Estonia differs from C. teres mainly in the reticulate shell ornament with strong transverse ornament elements, and in having a generally more robust conch in the early growth stage.

Among the species of Trocholites , Curtoceras teres is most similar to T. depressus , especially in umbilical width (UWI) and conch width (CWI). However, T. depressus has a much lower and much less rapidly decreasing coiling rate (particularly in early and mid-ontogeny; WER = 2.7–1.6 vs 9.9–1.9), the whorl profile tends to be more depressed (WWI = 1.67–1.2 vs 1.66– 0.82 in ontogeny) and the siphuncle is much wider (RSH ~ 0.30 vs 0.16) with a marginodorsal position; the ornament in T. depressus has ribs on inner whorls instead of just sharp lirae.

Geographic and stratigraphic occurrence

Sweden (Island of Öland), Estonia, St Petersburg and Kaliningrad regions of Russia, northern Germany and northern Poland; Aseri–Uhaku regional stages, Darriwilian, Middle Ordovician.