Trocholites kadakaensis, Aubrechtová & Korn, 2025

Aubrechtová, Martina & Korn, Dieter, 2025, The coiled Middle Ordovician cephalopod genera Trocholites and Curtoceras (Tarphyceratida) from Baltoscandia and north-central Europe, European Journal of Taxonomy 982, pp. 1-78 : 41-43

Trocholites kadakaensis sp. nov.

urn:lsid:zoobank.org:act:DF7112C8-6722-44CB-B68C-E09E540C98E6

Fig. 20 ; Table 11

Diagnosis

Species of the genus Trocholites with an adult conch diameter of more than 42 mm. Conch in the last whorl thickly discoidal (CWI ~ 0.48) and evolute (UWI ~ 0.46). Whorl profile moderately depressed (WWI ~ 1.80) with rounded flanks and venter, moderately to weakly embracing (IZR = 0.35–0.15). Whorl width and whorl height slightly decreasing in last half volution; the coiling rate decreases to WER ~ 1.70. Phragmocone chambers at 30 mm diameter moderately long (RCL ~ 0.30). Suture line nearly straight.

Etymology

From Kadaka near Tallinn ( Estonia), the locality of the holotype.

Type material

Holotype

ESTONIA • Kadaka (Tallinn); Väo Formation, late Darriwilian Stage; 1920; Deubel Coll.; illustrated in Fig. 20 ; MB.C. 11563 .

Paratype

ESTONIA • 1 spec.; Tallinn ; late Darriwilian Stage; Prantl and Zázvorka Coll.; NM-S6032 .

Description

Holotype MB.C.11563 ( Fig. 20 ) is an incompletely preserved conch, which is almost in the terminal growth stage, measuring 42 mm in diameter. It consists of 3.25 volutions of which the last quarter volution belongs to the body chamber. In the last half volution, both the whorl width and whorl height decrease (from 20.1 mm to 19.8 mm and from 11.5 mm to 10.9 mm, respectively). The conch is discoidal (CWI = 0.48) and subevolute to evolute (UWI = 0.46). The whorl profile is moderately depressed (WWI = 1.82) with broadly rounded flanks and venter; it changes from strongly to weakly embracing (IZR decreases from 0.36 to 0.15). The coiling rate (WER) decreases from a value of 1.76 to 1.67 (WER wh changes from about 2.60 to 1.85 in the last half a whorl). Only small remains of the shell are preserved; its ornament consists of indistinct, low lirae. On the venter of the terminal body chamber, there are extremely fine and low longitudinal (spiral) lines. The phragmocone chambers are moderately long (RCL = 0.30 at dm = 30 mm). The suture line is almost straight and forms a very shallow and wide external lobe.

Paratype NM-S6032 is a partially preserved conch in the late ontogenetic stage with a maximum measurable diameter of 39 mm. Two volutions are preserved of which the last ~150 degrees of the outer volution belong to the body chamber. In the last half volution, the whorl height decreases from 10.8 to 9.9 mm. The conch is subevolute to evolute with UWI = 0.47 at the largest conch diameter. The whorl profile has broadly rounded flanks and a broadly rounded venter. The coiling rate based on whorl height (WER wh) decreases from 2.55 to 1.80 in the last half whorl. The suture line is almost straight with a very shallow and wide external lobe. At a diameter of 29 mm, remains of the siphuncle are preserved; the siphuncle is located near the dorsal shell wall but it is not in contact with it.

Remarks

Holotype MB.C.11563 belongs to the representatives of Trocholites with a relatively large conch. Aside from the size, it has moderately long phragmocone chambers (RCL ~ 0.30) and the body chamber is contracted in both width and height. Very similar to this specimen is holotype MB.C.9718 of T. zaryensis sp. nov., but this is smaller in the adult size (36 mm) and differs in the shape of the terminal body chamber. In T. zaryensis , the height of the last whorl shows an increase in contrast to a decrease in T. kadakaensis sp. nov.; it also shows a slight flattening of the venter.

Similar in conch geometry is also the holotype of T. baldri sp. nov. In its last whorl, however, the decrease of the body chamber height is greater than in the holotype of T. kadakaensis sp. nov.; the former is also smaller in adult size (~ 30 mm).

The decrease of the body chamber width and height is present also in the neotype of Trocholites contractus . In that specimen, however, the proportions of the body chamber change much more rapidly and the adult conch diameter is smaller (~ 33 mm vs 41 mm). In addition, umbilicus in the last whorl of T. contractus is wider (UWI ~ 0.53 vs 0.46), the expansion rate is lower (WER ~ 1.60 vs 1.70) and the whorl profile is more depressed (WWI ~ 1.95 vs 1.80); phragmocone chambers are longer (RCL ~ 0.23 vs 0.30).

The two specimens of T. vodickai sp. nov. differ because their last whorl is more discoidal (CWI ~ 0.40 vs 0.48) and the whorl profile is much less depressed (WWI ~ 1.25–1.50 vs 1.80) without any distinct decrease in width and height towards the terminal aperture.

The last whorl in the holotype of Trocholites splendor sp. nov. differs in having a narrower umbilicus (UWI ~ 0.35 vs 0.46), less depressed (WWI ~ 1.35 vs 1.80) whorl profile with increasing instead of decreasing whorl width and whorl height of the terminal body chamber.

The holotype of Trocholites ruedemanni is similar to the holotype of T. kadakaensis sp. nov. in expansion rate and imprint zone rate of the adult growth stage but differs from it by the slight uncoiling of the mature body chamber, shell ornament (nodes and costae) and a narrower umbilicus (UWI = 0.32 vs 0.43 at dm ~ 30 mm).

In the apertural part of the body chamber of the specimen MB.C.11563, there are several circular openings with a diameter of ~ 3 mm; these are identified as borings that belong to the ichnospecies Trypanites sozialis Eisenack, 1934 (see Knaust et al. 2023; Vinn et al. 2023).

Figures

Fig. 20. Trocholites kadakaensis sp. nov.; holotype MB.C.11563 (Deubel Coll., 1920) from Kadaka (Tallinn; Estonia); dorsal, lateral and ventral views. Scale bar units = 1 mm.