Trocholites hospes ( Remelé, 1880 )

Trocholites hospes ( Remelé, 1880)

Figs 1G–H View Fig , 8–12 View Fig View Fig View Fig View Fig View Fig ; Table 4 View Table 4

Palaeonautilus hospes Remelé, 1880: 249 , pl. 2 fig. 3.? Palaeonautilus hospes Remelé, 1880: 249 , pl. 2 fig. 4.

Palaeonautilus hospes – Remelé 1881: 2, 13, text-fig.1. — Foerste 1930: 286, pl. 43 fig. 2, pl. 44 figs 4–5. — Balashov 1962: pl. 9 fig. 5.

Palaeonautilus (Trocholites) hospes – Remelé 1890: 40, pl. 2 fig. 3.

? Palaeonautilus (Trocholites) hospes – Remelé 1890: pl. 2 fig. 4, pl. 5 fig. 8.

? Trocholites hospes – Schröder 1891: 17 (155), pl. 1(24) figs 8–9.

? Palaeonautilus hospes – Schindewolf 1939: 60, text-fig. 14, pl. 4 fig. 7.

Trocholites hospes – Balashov 1953: 244, pl. 5 fig. 3. — Sweet 1958: 96, pl. 8 figs 3, 5. — Neben & Krueger 1971: pl. 32 figs 12–14.

Diagnosis

Species of the genus Trocholites with an adult conch diameter of 43–60 mm. Conch in the last whorl thickly discoidal (CWI = 0.52–0.60) and subevolute (UWI = 0.30–0.40) with moderately high coiling rate (WER = 1.75–1.90). Whorl profile weakly to moderately depressed (WWI = 1.40–1.80), rounded, venter sometimes slightly flattened, usually moderately embracing (IZR = 0.20–0.30). Whorl width and whorl height stagnant or slightly increasing in last quarter volution. Terminal aperture with ventral and lateral flares. Shell ornament with fine lirae extending with rather deep, rounded ventral sinus. Phragmocone chambers moderately long (RCL = 0.20–0.24). Suture line straight.

Material examined

BALTIC REGION • 1 spec.; locality unknown; Aseri or Lasnamägi or Uhaku regional stages ; Deubel Coll.; illustrated in Fig. 10A View Fig ; MB.C. 7638 .

GERMANY – Brandenburg • 1 spec.; Herzsprung (Uckermark); upper Lasnamägi Regional Stage; illustrated by Foerste (1930: pl. 44 fig. 4), re-illustrated here in Fig. 8A View Fig ; MB.C. 11557 • 1 spec.; Oderberg; Lasnamägi or Uhaku Regional Stage ; Neben Coll.; illustrated by Neben & Krueger (1971: pl. 32 fig. 14), re-illustrated here in Fig. 8C View Fig ; MB.C. 32162 • 1 spec.; Karlstein (Neumark); probably Darriwilian; Neben and Krueger Coll.; illustrated in Fig. 9A View Fig ; MB.C. 32163 • 1 spec.; Oderberg; upper Lasnamägi Regional Stage; 1936: Neben Coll.; illustrated by Neben & Krueger (1971: pl. 32 figs 12–13), re-illustrated here in Fig. 9B View Fig ; MB.C. 11556 • 1 spec.; Eberswalde; Lasnamägi or Uhaku Regional Stage ; Remelé Coll.; illustrated by Neben & Krueger (1971: pl. 32 figs 1–2), re-illustrated here in Fig. 8D View Fig ; MB.C. 11555.2 • 1 spec.; Oderberg; upper Lasnamägi Regional Stage; Neben and Krueger Coll.; illustrated in Fig. 9C View Fig ; MB.C. 32164 .

POLAND – West Pomerania • 1 spec.; Cieszeniewo; upper Lasnamägi Regional Stage; 1890; Dorow Coll.; illustrated by Foerste (1930: pl. 43 fig. 2, pl. 44 fig. 5), re-illustrated here in Fig. 8B View Fig ; MB.C. 11558 • 1 spec.; Ustronie Morskie (West Pomerania); Lasnamägi or Uhaku Regional Stage ; Müldner Coll.; illustrated in Fig. 9D View Fig ; MB.C. 32165 • 1 spec.; Ustronie Morskie (West Pomerania); Lasnamägi or Uhaku Regional Stage ; Neben and Krueger Coll.; MB.C. 32166 .

Description

Specimen MB.C.11557 ( Fig. 8A View Fig ) is a conch in the terminal stage of growth and has a diameter of about 56 mm. The conch is thickly discoidal (CWI = 0.56) and subevolute (UWI = 0.38). The last whorl is moderately depressed; the WWI decreases from 1.64 to 1.58 during the last quarter volution of the conch. The length of the terminal body chamber is 270 degrees; its venter is slightly flattened. Its aperture possesses ventral and lateral flares producing a deep ventral sinus and prominent ventrolateral lappets. The shell ornament is preserved only in the penultimate whorl and in a very small area at the ventral margin of the aperture; it consists of irregularly spaced, coarse lirae. The internal mould has very low annuli on the venter and in the ventrolateral area. The phragmocone chambers are rather long (RCL = 0.40). The suture line is almost straight and extends with a very broad and shallow external lobe.

Specimen MB.C.11558 ( Fig. 8B View Fig ) is an almost complete conch of a fully grown individual with a conch diameter of 51 mm. The conch is thickly discoidal (CWI = 0.58) and subevolute (UWI = 0.37). The whorl profile is moderately depressed; the WWI increases from 1.73 to 1.83 in the last 90 degrees of the volutions. The body chamber has a length of about 220 degrees and possesses ventral and ventrolateral flares; a ventral sinus is developed at the aperture. Small remains of the shell ornament are preserved only in the ventrolateral area near the aperture and in the inner volutions; they consist of fine and irregularly developed lirae. The phragmocone chambers are moderately long (RCL = 0.23). The suture line is almost straight with the exception of a very shallow and broad external lobe. At the beginning of the last whorl, the moderately wide siphuncle has a subdorsal position (RSH = 0.24; RSP = 0.84).

Specimen MB.C.32162 ( Fig. 8C View Fig ) is an almost complete adult conch with a conch diameter of 51 mm and has nearly five whorls preserved. The conch is thickly discoidal (CWI = 0.57) and subevolute (UWI = 0.42); the coiling rate is moderate (WER = 1.77). The whorl profile is moderately depressed (WWI = 1.75) and moderately embracing (IZR = 0.24). The specimen is almost completely exfoliated; small remains of the shell ornament are preserved only in the inner whorls, where fine and regularly spaced lirae are present. The phragmocone chambers are moderately long (RCL = 0.22). The suture line is almost straight, but extends with a very shallow and broad external lobe. At about half of the last whorl, the rather thin siphuncle has a subdorsal position (RSH = 0.18; RSP = 0.80). At the base of the body chamber, traces of muscle scar imprints are preserved.

Specimen MB.C.32163 ( Fig. 9A View Fig ) is an almost complete, well-preserved adult conch with a diameter of 50 mm. The conch is thickly discoidal (CWI = 0.58) and subevolute (UWI = 0.33) with a moderately high coiling rate (WER = 1.82). The whorl profile is weakly depressed (WWI = 1.50) and strongly impressed dorsally (IZR = 0.33) in the terminal half volution of the conch. The terminal body chamber is slightly longer than half of a volution; its aperture possesses ventral and lateral flares; a ventral sinus is indicated. Remain of the shell are preserved only in small areas; the ornament consists of rather regular, fine lirae.

Specimen MB.C.11556 ( Fig. 9B View Fig ) is a well-preserved, almost complete conch, which already represents the final stage of growth with a diameter of 43 mm. In this stage of growth, the conch is thickly discoidal (CWI = 0.56), subevolute (UWI = 0.37) and moderately expanding (WER = 1.83); the whorl profile in the last half volution is rounded, moderately depressed (WWI = 1.59) and moderately impressed dorsally (IZR = 0.27). The length of the terminal body chamber is 250 degrees; its aperture possesses ventral and lateral flares. The shell ornament is preserved only near the aperture and consists of irregularly thick lirae, which turn back already from the umbilical seam and form a rather deep, blunt V-shaped ventral sinus. The phragmocone chambers are moderately long (RCL = 0.24). The suture line extends almost straight across the flank and form a very broad and shallow external lobe. Faint traces of muscle scar imprints are preserved in the rear part of the body chamber.

Specimen MB.C.32164 ( Fig. 9C View Fig ) is a well-preserved, subadult individual with 47 mm conch diameter; the conch is thinly discoidal (CWI = 0.52) and subevolute (UWI = 0.35) and the profile of the last whorl is moderately depressed (WWI = 1.53). The whorl profile is crescent-shaped with broadly rounded venter and flanks; an umbilical margin is not discernible. The shell ornament is well-preserved in large areas; it consists of irregularly spaced, fine and slightly thickened lirae. They are directed backwardly right from the umbilical seam and form a deep, blunt V-shaped ventral sinus.

Specimen MB.C.32165 ( Fig. 9D View Fig ) is, with a conch diameter of 64 mm, the largest specimen of Trocholites in our collection. The last ~ 0.50 a whorl corresponds to the body chamber. The specimen has a well-preserved shell ornament that consists of fine and almost equally spaced lirae. On the body chamber, the lirae are fine on the flanks, but on the venter they become more prominent and form a rounded, nearly rectangular sinus.

Specimen MB.C.7638 (Fig, 10A) is an incomplete conch in the early growth stage with 26.5 mm diameter and was sectioned. During ontogeny (between 4.0 mm and 26.5 mm conch diameter), the conch changes from thickly pachyconic to thinly pachyconic (CWI decreases from 0.82 to 0.71) and from involute to subinvolute (UWI increases from 0.15 to 0.29); the coiling rate decreases markedly (WER drops from 2.60 to 1.67). The whorl profile changes only slightly and remains moderately depressed (WWI = 1.98–1.84), but changes from moderately to strongly impressed (IZR increases from 0.18 to 0.42) ( Fig. 10B–D View Fig ). The shell ornament is not well-discernible; it appears to consist of thin ribs or raised lirae in the internal whorl and only lirae in the outer whorl. The phragmocone chambers are rather long (RCL = 0.39 in the external volution). The siphuncle remains close or in contact with the dorsal shell wall and changes from RSH 0.15 to 0.20 in ontogeny.

Remarks

Remelé (1880) described and figured two specimens of “ Palaeonautilus hospes ” from late Darriwilian erratics near Heegermühle (Brandenburg, Germany); subsequently these specimens were re-described and re-figured by him ( Remelé 1881, 1890). Only one of these two specimens (MB.C.11555.2) is available for study; it was also illustrated by Neben & Krueger (1971: pl. 32 figs 1–2) and is re-illustrated here in Fig. 8D View Fig . However, this syntype was assigned to Trocholites remelei by Neben & Krueger (1971). Its internal mould shows some annular ridges in the outermost preserved whorl, but the shell surface is ornamented only with fine lirae, not clusters of raised lirae as in the lectotype of T. remelei (see below).

Alternatively, it is possible that the specimen MB.C.11555.2 ( Fig. 8D View Fig ) belongs to T. contractus , but the corresponding growth stage (dm = 26 mm) of that species has a wider conch (CWI ~ 0.65 in the neotype of T. contractus , but only ~ 0.55 in specimen MB.C.11555.2) with a more depressed whorl profile (WWI ~ 2.12 vs ~ 2.00). The unprofessional preparation of specimen MB.C.11555.2 by previous researchers makes it difficult to decide whether this syntype belongs to T. hospes or another species of Trocholites such as T. contractus . For this reason, this syntype is not considered here.

The other syntype presented by Remelé (1880, 1881, 1890) was not available for study, but according to original descriptions, as well as figures, it can be reliably assigned to T. hospes .

Some specimens of Trocholites hospes have a more depressed whorl profile (WWI = 1.83 at 52 mm diameter in MB.C.11558; WWI = 1.75 at 51 mm diameter in MB.C.32162) than others (WWI = 1.50 at dm at 50 mm diameter in MB.C.32163) in the terminal growth stage ( Fig. 12 View Fig ). These differences probably result from size variations of fully adult conchs, which range in their diameters between 43 mm and 60 mm.

Trocholites hospes is easily recognised among the species of Trocholites by its rather large and stout conch, the deep and moderately wide umbilicus, the fine lirae on the shell surface and the terminal body chamber with apertural flares. Most similar to T. hospes is T. splendor sp. nov., which differs by its more slender conch (CWI = 0.50 at dm = 49 mm), a less strongly depressed whorl profile (WWI = 1.34 at dm = 49 mm) ( Fig. 12 View Fig ) and the presence of thin ribs or raised lirae on the shell surface of the inner whorls.

The lectotype of the Darriwilian Trocholites remelei from Sweden also resembles specimens of T. hospes , but the shell in T. remelei possesses annuli or groups of raised lirae, which are visible not only on the internal mould but also on the shell surface. Furthermore, T. remelei , which is known only from a single specimen, has more strongly expanding, less depressed and narrower whorl than corresponding growth stages of T. hospes ( Fig. 12 View Fig ).

Another species that could be confused with juvenile growth stages of Trocholites hospes is T. contractus ( Fig. 12 View Fig ); specimens of both species have similarly stout conchs (CWI ~ 0.60) with a deep umbilicus, distinctly depressed whorl profiles (WWI ~ 2.00), similar whorl overlap (IZR ~ 0.30) and lirae on the shell surface. However, T. contractus reaches a much smaller terminal conch diameter (about 33 mm) and its body chamber markedly decreases in height, width and imprint zone rate toward the aperture.

Among the non-Baltic trocholitid species, the specimens of Trocholites hospes resemble the holotype of T. hubeiensis (Kuniutan (Guniutan) Formation, Darriwilian, Hubei Province of China). It is difficult to make an accurate measurement of the holotype from the only figure and description available, but it appears that the holotype has a similar conch geometry, including a high expansion rate, a narrow umbilicus, a moderately depressed and continuously rounded whorl profile and a subdorsal, comparatively thin siphuncle. On the other hand, the phragmocone chambers appear to be shorter in the holotype of T. hubeiensis . Nothing is known of the adult conch size. However, the similarity is rather high and it may thus be possible that T. hubeiensis is a junior synonym of T. hospes . This would be another species of coiled cephalopods that occurs in both Baltica and South China; the other species include the tarphyceratid Discoceras rarospira ( Eichwald, 1860) from Middle–Late Ordovician strata of Baltoscandia and the Kuniutan (Guniutan) Formation (Middle Ordovician) of Yunnan ( Zhang & Chen 2002), and the lituitid Cyclolituites lynnensis Kjerulf, 1865 that is known from the late Darriwilian / early Sandbian boundary strata of Norway ( Kjerulf 1865; Sweet 1958) and the Sandbian of the Hunan Province of China ( Fang et al. 2017, 2021). Chen (1987) also reported an occurrence of T. depressus in Ordovician strata of northern Tibet.

Similar in conch geometry to the specimens of Trocholites hospes is also the holotype of T. ruedemanni ( Fig. 12 View Fig ) from early Sandbian (?) strata of New York ( USA), except that the adult conch size in the latter is considerably smaller (29 mm vs 43–60 mm), the terminal body chamber is slightly uncoiled and the shell ornament on the umbilical wall in the last two whorls has nodes and fine costae ( Flower 1943).

Geographic and stratigraphic occurrence

Norway, Sweden (Island of Öland), northern Germany, northern Poland, St Petersburg Region of Russia; Darriwilian (Middle Ordovician).