Trocholites depressus ( Eichwald, 1840 )

Trocholites depressus ( Eichwald, 1840)

Figs 1D, I View Fig , 5–7 View Fig View Fig View Fig ; Table 3 View Table 3

Nautilus depressus Eichwald, 1840: 106 .

Clymenia incongrua Eichwald, 1840: 108 .

Clymenia Odini Eichwald, 1840: 107 View in CoL .

Clymenia depressa – Eichwald 1860: 1305, pl. 50 fig. 5.

Clymenia incongrua – Eichwald 1860: 1306, pl. 50 fig. 7.

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

Palaeonautilus depressus – Remelé 1880: 246; 1890: 37.

Palaeonautilus incongruus – Remelé 1880: 247.

? Trocholites depressus – Rüdiger 1889: 48. — Sweet 1958: 93, pl. 7 figs 3–7, text-fig. 12. — Chen 1987: 173, pl. 7 figs 7–9, text-fig. 35.

Trocholites depressus – Schröder 1891: 13 (151), pl. 1(24) figs 4, 7. — Balashov 1953: 242, pl. 7 fig. 2. — Kröger & Keupp 2004: text-fig. 1.

? Trocholites incongruus – Schröder 1891: 154, pl. 2(25) figs 3–4.

Trocholites incongruus – Schröder 1891: 153, pl. 2(25) fig. 2. — Balashov 1953: 240, pl. 7 fig. 1; 1962: pl. 9 fig. 6; 1974: pl. 9 fig. 6.

Trocholites cf. orbis – Schröder 1891: 13 (151), pl. 1(24) fig. 6.

Trocholites Odini View in CoL – Foord 1891: 50. — Schröder 1891: 13 (151). — Balashov 1953: 243, pl. 7 fig. 3.

Trocholites orbis – Neben & Krueger 1971: pl. 32 figs 10–11. — Dzik 1984: pl. 6 fig. 3.

? Trocholites cf. incongruus – Neben & Krueger 1971: pl. 32 figs 8–9.

? Trocholites soraviensis – Neben & Krueger 1973: pl. 81 figs 10–11.

? Trocholites orbis – Dzik 1984: pl. 6 fig. 2.

? Trocholites cf. depressus – Babin & Gutiérrez-Marco 1992: 524, 534, text-fig. 4g.

non Trocholites incongruus – Angelin 1880: 11, pl. 9 figs 15–18.

non Trocholites cf. incongruus – Rüdiger 1889: 48.

Diagnosis

Species of the genus Trocholites with an adult conch diameter of 30–40 mm. Conch in the last whorl thinly discoidal (CWI = 0.35–0.40) and evolute (UWI ~ 0.50) with low coiling rate (WER ~ 1.65). Whorl profile weakly to moderately depressed (WWI = 1.25–1.60) with rounded flanks and slightly flattened venter in the adult stage. Whorl width slightly increasing, whorl height slightly decreasing in last quarter volution. Phragmocone chambers moderately long (RCL = 0.30–0.40). Suture line nearly straight. Ornament on inner whorls with lirae and narrow or distinct ribs, and on outer whorl with raised lirae, sometimes with annular ridges visible on the internal mould.

Type material

Neotype (designated here)

ESTONIA • Odinsholm (= Osmussaar ); Lasnamägi Regional Stage , late Darriwilian; Eichwald Coll.; illustrated by Eichwald (1860: pl. 50 fig. 5), Schröder (1891: pl. 1(24) fig. 4) and Balashov (1953: pl. 7 fig. 2), re-illustrated here in Fig. 5A View Fig ; PMSPU 1-1128 .

Additional material

ESTONIA • 1 spec.; Osmussaar; Darriwilian Stage; 1876; Dames Coll.; illustrated by Schröder (1891: pl. 1 fig. 7), re-illustrated here in Fig. 6F View Fig ; MB.C. 9719 • 1 spec.; Osmussaar 1 ; Lasnamägi Regional Stage ; illustrated in Fig. 5B View Fig ; TUG 1351-14 • 1 spec.; Osmussaar ; Lasnamägi Regional Stage ; Schmidt Coll.; illustrated in Fig. 6G View Fig ; GIT 225-1074 • 1 spec.; Osmussaar 1 ; Uhaku Regional Stage ; illustrated in Fig. 6A View Fig ; GIT 697-129 • 2 specs; Osmussaar 2 ; Uhaku Regional Stage ; Männil Coll.; illustrated in Fig. 6C View Fig ; GIT 697-410 • 1 spec.; Osmussaar 2 ; Uhaku Regional Stage ; Männil Coll.; GIT 697-404-1 • 1 spec.; Osmussaar ; probably Darriwilian Stage; TAM G432:787 • 1 spec.; Osmussaar cliff ; probably Darriwilian Stage; 1985; Männil Coll.; illustrated in Fig. 6B View Fig ; GIT 697-535 • 1 spec.; Väike-Pakri Island; Lasnamägi Regional Stage ; Orviku Coll.; illustrated in Fig. 5C View Fig ; GIT 145-1 • 1 spec.; Tallinn; Lasnamägi Regional Stage ; TUG 2-721 • 1 spec.; Odinsholm (= Osmussaar ); Lasnamägi Regional Stage , late Darriwilian; Eichwald Coll.; illustrated by Eichwald (1860: pl. 51 fig. 27), Schröder (1891: pl. 1(24) fig. 6) and Balashov (1953: pl. 7 fig. 3), re-illustrated here in Fig. 5D View Fig ; PMSPU 1-1127 • 1 spec.; Odinsholm (= Osmussaar ); Lasnamägi Regional Stage , late Darriwilian; Eichwald Coll.; illustrated by Eichwald (1860: pl. 50 fig. 7a), Schröder (1891: pl. 2(25) fig. 2) and Balashov (1953: pl. 7 fig. 1a, c); PMSPU 1-1129 • 1 spec.; Odinsholm (= Osmussaar ); Lasnamägi Regional Stage , late Darriwilian; Eichwald Coll.; illustrated by Balashov (1953: pl. 7 fig. 1b); PMSPU 1-2622 .

GERMANY – Brandenburg • 1 spec.; Niederfinow; Ordovician; Neben and Krueger Coll .; illustrated in Fig. 6D View Fig ; MB.C. 32160 . – Mecklenburg-Vorpommern • 1 spec.; Lebbin ; Lasnamägi Regional Stage ; Neben and Krueger Coll.; illustrated by Neben & Krueger (1971: pl. 32 figs 10–11), re-illustrated here in Fig. 6E View Fig ; MB.C. 32161 .

Description

Neotype PMSPU 1-1128 ( Fig. 5A View Fig ) is a nearly complete conch with 3.5 volutions preserved. The diameter of the conch is 36 mm; the body chamber is 240 degrees long and the aperture has a ventral sinus, ventrolateral extension and dorsolateral sinus. In the last 90 degrees, the conch is discoidal ( CWI ~ 0.40) and widely umbilicate ( UWI increases slightly from 0.46 to 0.48); the whorl profile is evenly convex from the venter to the flanks and increasingly depressed ( WWI increases from 1.36 to 1.48). Ornament is only locally preserved; it consists of fine lirae, which are accompanied by narrow but distinct ribs on inner whorls. The phragmocone chambers are moderately long ( RCL = 0.31); the suture line is straight.

Specimen MB.C.32160 ( Fig. 6D View Fig ) is an almost complete adult specimen with a conch diameter of 29 mm. The length of the body chamber is half a volution; its terminal apertural end has a ventral extension and thus produces a pronounced ventral sinus. Within the last half volution, the conch becomes progressively discoidal (CWI decreases from 0.44 to 0.38) and remains evolute (UWI rises weakly from 0.50 to 0.52) with a low coiling rate (WER = 1.64). The largest whorl is moderately depressed (WWI = 1.58) and weakly embracing (IZR = 0.09). The shell is only preserved in inner whorls, where there are distinct, smooth ribs and lirae; the internal mould of the adult body chamber has annular grooves that extend with a ventral sinus and a single more prominent impression near the beginning of the body chamber. The phragmocone chambers are moderately long (RCL = 0.30); the suture line is more or less straight.

Specimen MB.C.32161 ( Fig. 6E View Fig ) is an almost complete, well-preserved conch of an adult individual with 29 mm conch diameter. The length of the body chamber is half a volution; the terminal aperture extends into a ventral sinus. In the last half volution, the conch is increasingly discoidal (CWI decreases from 0.41 to 0.33), increasingly evolute (UWI increases slightly from 0.44 to 0.48) and moderately expanding (WER = 1.77). The whorl profile is weakly depressed (WWI decreases from 1.31 to 1.21) and weakly impressed dorsally (IZR = 0.10) with a broadly rounded venter and rounded flanks. The shell wall of the inner whorls is ornamented with unevenly spaced thin ribs and lirae; in the outer whorl, the ribs fade out and are replaced by raised lirae. There are annular impressions present on the internal mould of the terminal body chamber. The ornament elements extend straight across the flank and turn to form a deep ventral sinus. In the outer whorl, the phragmocone chambers are short or moderately long (RCL varies between 0.17 and 0.23). The suture line is nearly straight.

Specimen PMSPU 1-1127 ( Fig. 5D View Fig ) is a conch of 31 mm diameter, consisting of nearly five whorls; the last ~ 35 degrees of the conch belong to the body chamber. The initial volution of the phragmocone is preserved, the initial chamber is present, the umbilical window is ~ 0.25 mm across. In the last 90 degrees of the last whorl, the conch is widely umbilicate (UWI ~ 0.46). Ornament is poorly preserved but transverse elements are indicated locally on the surface. Phragmocone chambers are moderately long (RCL up to ~ 0.40); suture line is almost straight.

Specimen GIT 697-129 ( Fig. 6A View Fig ) has a conch diameter of 34 mm; the specimen possesses 5.25 whorls and is nearly complete. Only the aperture of the body chamber is missing. At a diameter of 30 mm, the conch is thinly discoidal (CWI = 0.39) and evolute (UWI = 0.49). The whorl profile is weakly depressed in the last whorl (WWI decreases from 1.43 to 1.35) with broadly rounded flanks and venter. The ornament consists of evenly spaced lirae, which are irregularly raised and form groups of more widely spaced, rib-like elements (0.5–1.0 mm apart in inner whorls). The phragmocone chambers are rather long (RCL = 0.40 at wh = 7 mm); the suture line is almost straight.

Specimen GIT 145-1 ( Fig. 5C View Fig ) is a median longitudinal section of an essentially complete conch with a maximum diameter of 40 mm, consisting of six volutions. During ontogeny, the coiling rate generally decreases from WER = 2.70 to WER = 1.70. The siphuncle remains marginodorsal (RSP = 1.00) and is relatively wide (RSH between 0.26 and 0.32). The septal necks are short and loxochoanitic, sometimes the brims are thickened. The connecting rings are thin with a deposit-like structure on the side facing the chamber, which extends also on the adapical side of septum. The phragmocone chambers are rather long especially in the early volutions (RCL 0.70–0.30); the suture line is straight. Ornament is preserved in small area on penultimate whorl and consists from lirae and low ribs; two annular ridges are present on the internal mould at about the beginning of the body chamber.

Specimen PMSPU 1-1129 represents a juvenile growth stage of a conch in which four whorls are preserved; traces of at least one more volution are visible. The maximum diameter of the conch is 22 mm; at this size the conch has UWI of 0.41 and RCL of ~ 0.25. The suture line is straight. Fine transverse ornament is visible on the shell surface.

Specimen PMSPU 1-2622 is a median transverse section of a conch in the juvenile growth stage, consisting of four volutions and 22 mm in diameter. In the last 1.5 volutions, the conch changes from thickly to thinly discoidal (CWI = 0.52–0.45) and is evolute (UWI = 0.48–0.50); coiling rate is moderate to low (WER = 1.82–1.70). Whorl profile of the outer volution is rounded from ventral and lateral sides but inner whorl profiles appear to be slightly ventrally flattened; the whorls are moderately depressed (WWI = 1.64–1.67) and moderately to weakly impressed (IZR = 0.19–0.14). The siphuncle is marginodorsal (RSP = 1.00) and rather wide (RSH ~ 0.30).

Specimen MB.C.9719 ( Fig. 6F View Fig ) is a subadult phragmocone with 26 mm diameter; the body chamber is missing. At this diameter, the conch is thinly discoidal (CWI = 0.41) and evolute (UWI = 0.45). In the last three quarters volution, the whorl profile has broadly rounded flanks and venter and changes from moderately to weakly depressed (WWI decreases from 1.67 to 1.38). The shell wall is ornamented with fine lirae with symmetric crests, but low ribs are locally visible on one side of the specimen. The phragmocone chambers increase in length in the last three quarters of the terminal volution (RCL changes from 0.24 to 0.37); the suture line is almost straight.

The other specimens are in a subadult or adult growth stage and range from 31 to 43 mm in conch diameter. In the last whorl, these conchs become more discoidal (CWI decreases from 0.45 to 0.36) and more widely umbilicate (UWI increases from 0.44 to 0.50) ( Fig. 7A–B View Fig ). The whorl profile in the last whorl is rounded, moderately to weakly impressed and becomes less depressed (WWI decreases from 1.61 to 1.20) during ontogeny ( Fig. 7C View Fig ). In some specimens, the WWI increases in the last half of the body chamber (e.g., GIT 697-535). The siphuncle has a marginodorsal position (RSP = 1.00) and the RSH decreases from 0.35 to 0.26 (n = 5). The complete body chamber is preserved in GIT 697-410 ( Fig. 6C View Fig ), where it is about half a volution long and the aperture extends into a ventral sinus. In the specimens TUG 2-271 and TUG 1351-14 ( Fig. 5B View Fig ), the latest phragmocone chambers are significantly shortened (down to RCL = 0.10).

Remarks

Three late Darriwilian species of Trocholites have been described from the Estonian islet of Osmussaar (formerly Odinsholm or Odensholm): T. depressus , T. incongruus and T. odini . These species were established by Eichwald (1840), who gave short descriptions but no illustrations of any type material. One specimen of each species was illustrated later by him ( Eichwald 1860), but he did not explicitly state that these belonged to the respective type collections. The originals of Eichwald (1860) were studied and re-illustrated by Schröder (1891) and Balashov (1953); the latter author regarded them as holotypes (see discussion below). Trocholites incongruus and T. odini were not properly revised and are unequivocally known only from the probable type material. New material was assigned only to Trocholites depressus by Kröger & Keupp (2004). Other assignments to T. depressus are more or less questionable (e.g., Sweet 1958; Chen 1987).

High-resolution photographs of the originals of Eichwald (1860) are accessible for our study. These, along with the historical descriptions and illustrations, suggest that these specimens differ only slightly in conch geometry and shell ornament. Furthermore, the original specimens of T. incongruus appear to only represent an earlier ontogenetic stage of representatives of the other two species.

Among the material currently examined from northern Germany and Estonia, there are over a dozen specimens that appear to closely resemble the probable types of the three species discussed above. These specimens are all discoidal, slowly expanding with a wide umbilicus and weakly depressed, weakly impressed and laterally broadly rounded whorl profile. The siphuncle is marginodorsal (RSP = 1.00) and rather wide; the shell ornament consists of lirae raised at irregular intervals. The variation concerns the ww / wh ratio (WWI = 1.58 in specimen MB.C.32160 but 1.21 in specimen MB.C. 32161 in the adult growth stage; Fig. 7C View Fig ) and the dorsal impression (IZR = 0.09 in specimen MB.C.32160 and 0.16 in specimen GIT 697-404-1). At least some of these differences may be due to the variation in adult conch size (between 30 mm and 40 mm) resulting in the comparison of non-corresponding growth stages. Ribs are developed on the shell surface in most specimens but these ribs are of variable thickness (GIT 697- 129 and MB.C.32160). Therefore, the results of our investigation conclude that the differences between Trocholites depressus , T. incongruus and T. odini are likely attributable to intraspecific variation and based on ontogenetic differences. The three taxa are consequently synonymized herein; the species name depressus has priority.

Eichwald (1840) did not figure any specimens and later ( Eichwald 1860) he did not indicate whether the illustrated specimen was part of the type collection. Therefore, the specimen of Eichwald (1860), later re-illustrated by Balashov (1953: pl. 7 fig. 2), cannot be considered the holotype of “ Nautilus depressus ”. Nevertheless, we refer to it here as a neotype. This is justified by the following reasons: Eichwald (1860) clearly regarded the specimen as a representative of his earlier described species Nautilus depressus Eichwald, 1840 ; Balashov (1953) physically examined and re-photographed the specimen; photographs of the specimen were available for the present study, confirming that it corresponds to the original as well as to the currently understood range of the species. The neotype is deposited in the Natural History Collections of St Petersburg State University under collection number PMSP-1-1128.

It is important to note that there is some inconsistency in the scientific literature regarding the use of the species name odini . The confusion occurred after Verneuil (1845: 360, pl. 25 fig. 8) included a specimen with a nearly circular whorl profile and an uncoiled terminal body chamber in the species and used for this the combination Lituites odini . This contradicts the original description by Eichwald (1840). Consequently, Eichwald (1860) and others (Remelé 1890; Foord 1891) correctly assigned that specimen to Curtoceras teres . However, many authors confused the two species and used the names odini and teres as synonyms; both Eichwald and Verneuil were sometimes stated as authors of the name odini . The fact that the probable holotype of T. odini was first figured by Eichwald as late as in 1860 possibly contributed to this ambiguity. Nevertheless, the material examined by us confirms that they are clearly two separate species but also represent two different genera, Trocholites and Curtoceras (see under C. teres below).

Specimen GIT 145-1 ( Fig. 5C View Fig ) is extraordinary as it has a healed perforation of the phragmocone, which was previously discussed in detail by Kröger & Keupp (2004). This specimen is shortly re-described and re-figured here for completeness.

Representatives of Trocholites depressus are similar to specimen MB.C.32182 of T. sp. 4 and the type specimens NRM Mo 152354 and GIT 362-721-2 of T. tureki sp. nov. in conch width and whorl profile shape. However, specimen MB.C.32182 has a narrower siphuncle shifted from the dorsal shell wall and its inner volutions are not flattened ventrally; both specimens NRM Mo 152354 and GIT 362-721-2 have a more regularly developed ornament with low ribs / annuli present also on the surface of the outer whorl.

The subadult holotype of Trocholites chaloupkai from the early Katian part of the Zahořany Formation of the Prague Basin is also similar to the representatives of T. depressus , particularly in conch geometry and siphuncular position and diameter ( Aubrechtová et al. 2023). However, the Bohemian specimen differs by a more significant ventral flattening of the conch, presence of deep ventral lobe of suture line and a unique, web-like shell ornament.

Compared to the species of Curtoceras , T. depressus is most similar to C. teres ; the similarities concern especially the umbilical width (UWI) and conch width (CWI). The specimens of C. teres differ from the specimens of T. depressus , however, in the much higher and much more rapidly decreasing coiling rate (particularly in early and mid-ontogeny; WER = 9.9–1.9 vs 2.7–1.6), the whorl profile is less depressed, equidimensional or even slightly compressed (WWI = 1.66–0.82 vs 1.67– 1.2 in ontogeny) and the siphuncle is much narrower (RSH ~ 0.16 vs 0.30) with some distance from the dorsal shell wall; also, the shell ornament in C. teres lacks strong ribs on inner whorls.

Geographic and stratigraphic occurrence

Northern Estonia, northern Germany, north-west Poland, Sweden,? Norway and?Tibet; Darriwilian, Middle Ordovician.