Trocholites macrostoma Schröder, 1882

Trocholites macrostoma Schröder, 1882

Figs 1A View Fig , 13–14 View Fig View Fig ; Table 5 View Table 5

Trocholites macrostoma Schröder, 1882: 99 .

Trocholites macrostoma – Schröder 1891: 149, pl. 1(24) fig. 1. — Neben & Krueger 1971: pl. 32 figs 15–16.

Diagnosis

Species of the genus Trocholites with an adult conch diameter of about 42 mm. Conch in the last whorl thinly discoidal (CWI ~ 0.43) and evolute (UWI ~ 0.53) with very low coiling rate (WER ~ 1.52). Whorl profile moderately depressed (WWI ~ 1.73) with rounded venter and flanks, moderately embracing (IZR = 0.18–0.28). Aperture with ventral flares. Shell ornament with irregularly thick, rursiradiate lirae, accumulations of lirae and growth lines, with shallow but relatively sharp ventral sinus; surface of internal mould with low and wide annular ridges.

Type material

Neotype (designated here)

GERMANY – Brandenburg • Oderberg; Darriwilian; Neben and Krueger Coll.; figured by Neben & Krueger (1971: pl. 32 figs 15–16), re-illustrated here in Fig. 13 View Fig ; MB.C. 32167 .

Additional material

GERMANY – Brandenburg • 1 spec.; Niederfinow ; Lasnamägi Regional Stage ; Neben and Krueger Coll.; illustrated in Fig. 14A View Fig ; MB.C. 32168 .

Description

Neotype MB.C.32167 ( Fig. 13 View Fig ) is a well-preserved, nearly complete adult specimen with a conch diameter of 40 mm. The terminal body chamber is slightly longer than half a volution and possesses a ventral flare at the aperture. In the last half volution, the conch becomes more discoidal ( CWI decreases from 0.48 to 0.43) and more evolute ( UWI increases from 0.46 to 0.53); the terminal whorl is moderately depressed ( WWI decreases from 1.81 to 1.72) and moderately embracing ( IZR = 0.28). The coiling rate of the conch is very low ( WER = 1.48). Shell ornament is well-preserved in the inner whorls and part of the last whorl; it consists of distinct and irregularly thick lirae or clusters of lirae; the internal mould has very low and widely spaced annular ridges. The elements of the ornament are rursiradiate in their direction and form a shallow, subangular ventral sinus.

Specimen MB.C.32168 ( Fig. 14A View Fig ) is a conch with 35 mm diameter; the specimen was sectioned and displays five whorls. Conch width and coiling rate generally decrease during ontogeny between 3.4 mm and 35 mm conch diameter (CWI changes from 0.96 to 0.43 and the WER from 2.66 to 1.56), while the umbilicus becomes wider (UWI increases significantly from 0.18 to 0.53). The whorl profile shows rounded flanks and a slightly flattened venter only in some inner whorls; it is always weakly impressed and strongly or moderately depressed (WWI decreases from 2.65 to 1.74). Whorl height and whorl width increase during growth but in the last half volution, the width appears to decrease slightly (ww changes from 15.4 to 15.1 mm). The siphuncle has a subdorsal position and a relative diameter (RSH) of about 0.20, however, both distance and diameter cannot be accurately measured due to the insufficient preservation of the specimen.

Remarks

A single specimen of Trocholites macrostoma was described by Schröder (1882) and subsequently illustrated by him ( Schröder 1891: pl. 1(24) fig. 1; see also Fig. 1A View Fig herein). This holotype originated from the Echinosphaerites Limestone (Darriwilian Stage, Middle Ordovician) of an unspecified locality in “Ostpreussen” (present day Kaliningrad Region of Russia). Schröder (1891) stated that the holotype was kept in the Mineralogical Museum of the University of Königsberg (now Kaliningrad). However, this collection was most probably destroyed in World War II (pers. com. D. Weyer, Berlin, 2022). Therefore, we propose here specimen MB.C.32167 as the neotype for T. macrostoma . This specimen is nearly identical in size and dimensions to the holotype. The only difference is that the last whorl in the holotype is slightly less impressed (IZR ~ 0.23 vs ~ 0.28) and that the last whorl of the holotype appears to have had an even lower coiling rate than that calculated for neotype MB.C.32167. However, the precise value cannot be determined reliably from the illustration of the holotype. The two specimens come probably from the same stratigraphic horizon (Darriwilian, Middle Ordovician).

The neotype of Trocholites macrostoma closely resembles the specimens of T. vodickai sp. nov., but it is separated from these by the slowly expanding, moderately impressed and moderately depressed last whorl and by the presence of distinct, rursiradiate lirae on the shell surface.

The other specimen (MB.C.32168) assigned here to T. macrostoma differs from the neotype only by the small decrease in absolute whorl width in the latest ontogeny. Specimen MB.C.32168 additionally differs from T. freyjae sp. nov., T. baldri sp. nov., T. glacialis sp. nov., T. sp. 1, T. sp. 2 and T. sp. 3 in the ontogenetic trajectories of several characters between conch diameters of 5 to 25 mm. In this growth interval, the CWI decreases and UWI increases significantly in MB.C.32168; the WWI is higher than in the other species ( Fig. 11 View Fig ). In addition, the whorl width (but not whorl height) of the terminal body chamber appears to decrease close to the aperture.

In Trocholites depressus , the last whorl tends to be more discoidal (CWI as low as 0.33 vs over 0.40 in T. macrostoma ) and more rapidly expanding (WER ~ 1.65 vs 1.52) with a less depressed (WWI = 1.25– 1.60 vs ~1.73) whorl profile; the shell ornament has ribs (vs irregularly thick lirae in T. macrostoma ). The specimens of Trocholites macrostoma differ from the holotype of T. luna sp. nov. in that the last whorl of the conch is less expanded (WER ~ 1.52 vs 1.60), the whorl profile is more depressed (WWI ~ 1.73 vs 1.55; the venter of body chamber lacks annuli, only lirae are present.