Trocholites Conrad, 1838

Genus Trocholites Conrad, 1838 View in CoL

Type species

Trocholites ammonius Conrad, 1838 View in CoL ; by original designation.

Diagnosis (after Frey 1995; Kröger & Aubrechtová 2018; emended)

Genus of the family Trocholitidae View in CoL with adult conch diameters of generally less than 100 mm, five or more volutions. Conch discoidal or pachyconic (CWI = 0.30–0.80) with moderately wide to very wide umbilicus (UWI = 0.30–0.60) and low or moderate whorl expansion (WER = 1.50–2.00). Body chamber 0.50–0.75 of a volution in length; not uncoiling from the preceding whorl. Whorl profile weakly to strongly depressed (WWI = 1.20–2.20), broadly reniform, weakly to strongly impressed dorsally (IZR up to 0.45), broadly rounded across venter and flanks. Aperture commonly flared with ventral sinus and ventrolateral extensions. Siphuncular diameter 0.16–0.30 of the whorl height, marginodorsal or subdorsal in position in all growth stages; septal necks short (<0.20 of chamber height), orthochoanitic and / or loxochoanitic in shape. Ornament composed of growth lines and lirae, less commonly of narrow annuli or ribs; ornament elements extended into ventral sinus. Suture line straight or with shallow lobes.

Species included

North America ( Conrad 1838; Miller & Dyer 1878; Hyatt 1894; Ruedemann 1926; Foerste 1930; Flower 1943): Trocholites ammonius Conrad, 1838 View in CoL ; T. canadensis Hyatt, 1894 ; T. circularis Miller & Dyer, 1878 ; T. dyeri Hyatt, 1894 ; T. faberi Foerste, 1930 ; T. gracilis Flower, 1943 ; T. major Ruedemann, 1926 ; T. minusculus Miller & Dyer, 1878 ; T. planorbiformis Conrad, 1838 ; T. ruedemanni Flower, 1943 .

Great Britain ( Hyatt 1894): Trocholites blakei Hyatt, 1894 .

North Europe and St Petersburg Region of Russia ( Eichwald 1840; Kjerulf 1865; Remelé 1880; Schröder 1882, 1891; Foerste 1932; Kröger & Aubrechtová 2018; this paper): Trocholites baldri sp. nov.; Palaeonautilus broeggeri Foerste, 1932 ; T. contractus Schröder, 1891 ; Nautilus depressus Eichwald, 1840 ; T. freyjae sp. nov.; T. gennadii Kröger & Aubrechtová, 2018 ; T. glacialis sp. nov.; P. hospes Remelé, 1880 ; T. kadakaensis sp. nov.; T. luna sp. nov.; T. macrostoma Schröder, 1882 ; Lituites nakholmensis Kjerulf, 1865 ; T. remélei Schröder, 1891 ; T. splendor sp. nov.; T. tureki sp. nov.; T. triangulus sp. nov.; T. vodickai sp. nov.; T. vortex sp. nov.; T. zaryensis sp. nov.

South and Central Europe ( Babin & Gutiérrez-Marco 1992; Aubrechtová et al. 2023): Trocholites fugax Babin & Gutiérrez Marco, 1992 View in CoL ; T. chaloupkai Aubrechtová et al., 2023 View in CoL .

South Korea ( Kobayashi 1934): Trocholites ammonioides Kobayashi, 1934 . China ( Frech 1911; Chen & Liu 1974, 1976; Lai 1981; Lai & Wang 1981; Gao et al. 1982; Chen & Zou 1984; Guo 1998): Palaeonautilus hubeiensis Chen & Liu, 1974 ; Trocholites huanxianese Chen in Gao, Lai & Wen, 1982; T. jiangxiense Lai, 1981 ; T. lativentrosus Lai & Wang, 1981 ; T. mirabilis Lai & Wang, 1981 ; T. wuhaiensis Chen in Chen & Zou, 1984; T. xiazhenensis Chen & Liu, 1976 ; T. yushanensis Lai, 1981 ; T. yunnanensis Reed, 1917 ; T. zhejiangense Lai, 1981 .

Australia ( Stait et al. 1985): Trocholites costatum Stait et al.1985 .

Remarks

Representatives of the genus Trocholites are usually rather small with only 20 to 60 mm conch diameter in the terminal growth stage. They possess evolute or subevolute conchs and have a distinctly depressed whorl profile; the whorl profile possesses a small dorsal imprint zone. The terminal body chamber does not deviate from the preceding whorl, but often has an aperture with ventral and lateral flares.

Trocholites is similar to Curtoceras and both genera may be confused under certain circumstances. The conch in Curtoceras is more discoidal (CWI = 0.30–0.40) and the whorl profile is weakly depressed or nearly equidimensional (WWI = 1.00–1.50). The terminal body chamber uncoils from the preceding whorl at least in some species of Curtoceras . However, there are species with transitional morphology that are difficult to assign to one genus or the other, especially if the adult growth stage is not known.

Discoceras is distinguished from Trocholites by the much greater adult conch size (usually exceeding 100 mm) and coarser shell ornament with elements that are characteristically frilled and imbricate; the siphuncle in the species of Discoceras is usually not marginal in position. In adult growth stages of Discoceras , the conch is more discoidal (CWI <0.35) and the whorls are usually only weakly embracing. The whorl profile in Discoceras is very variable between species and during ontogeny. It is usually weakly depressed in earlier stages of growth (WWI = 1.00–1.50), but becomes equidimensional or compressed during ontogeny (WWI = 0.50–1.00). In some species of Discoceras , the whorl profile is oval or circular, while in others it is subrectangular, subquadratic, trapezoidal or heart-shaped. In many species of Discoceras , the last volution uncoils from the preceding whorl to varying degrees.

Schröder (1882, 1891) discussed or established several species of Trocholites , most of which could be reliably identified in the collection currently studied. Only three species remain problematic: T. orbis , T. macromphalus and T. soraviensis .

Trocholites orbis , according to its original description, is very similar to T. depressus and differs from this species only in being somewhat more discoidal and lacking strong ribs on the shell surface. Unfortunately, T. orbis is only known from the original description and illustration of the holotype ( Schröder 1891: pl. 1(24) fig. 3; see also Fig. 1C View Fig herein). This specimen originated from an Ordovician erratic boulder at “Nasser Garten bei Königsberg” (present day Portowoje, south of Kaliningrad, Russia) and belonged to the collection of the “Provinzial-Museum zu Königsberg i. Pr.” (Regional Museum of Königsberg in Prussia), which was most probably destroyed during World War II (pers. com. D. Weyer, Berlin, 2022). The species has never been revised and the only two other specimens assigned to it by Neben & Krueger (1971) and Dzik (1984) probably belong to T. depressus . From the collection at hand, we could not unequivocally assign any specimens to the species T. orbis . The available information on the (probably lost) type material do not permit further comparison of T. orbis with T. depressus or any other species of the genus.

Trocholites macromphalus is known only from the holotype ( Schröder 1891: pl. 1(24) fig. 5; see also Fig. 1E View Fig herein). It originated from Darriwilian strata of the Estonian island of Osmussaar and was said to be deposited at the “Museum der Akademie der Wissenschaften zu St. Petersburg ” (Museum of the Academy of Sciences of St Petersburg). The holotype was never re-examined and its current location and status could not be confirmed. In the currently examined collection, some specimens of Curtoceras teres are most similar based on the original description and illustration. However, since Schröder (1891) noted that the holotype had a dorsal siphuncle and ribs on internal whorls, the species is transferred to Curtoceras but not synonymised with C. teres .

The holotype of Trocholites soraviensis originated from an Ordovician erratic boulder at Żory ( Poland) and was included in the “Sammlung der Kgl. geologischen Landesanstalt und Bergakademie zu Berlin“ (Collection of the Royal Geological Survey and Mining Academy of Berlin). After its description by Schröder (1891), it has never been restudied and remains the only unequivocal representative of the species. The holotype ( Schröder 1891: pl. 2 fig. 1; see also Fig. 1F View Fig herein) is most similar to specimens herein assigned to Trocholites freyjae sp. nov., T. baldri sp. nov. and T. glacialis sp. nov. However, reliable comparison is not possible based on the available information. The present location of the holotype is unknown and it is possible that it was destroyed during World War II. This cannot be confirmed, though, and thus, neotype designation and comparison to representatives of other species of Trocholites is avoided.

Some species of Trocholites recorded from various regions of China, especially T. huanxianese , T. yunnanensis and T. xiazhenensis , may in fact belong to Curtoceras or Discoceras , but the currently available data do not allow a reliable taxonomic assignment.

The holotype of the Ordovician species Trocholites zhuozishanensis described by Guo (1998) from Inner Mongolia (northern China) has ventrally shifted siphuncle and thus should not be assigned to Trocholites or any other genus of the family Trocholitidae .

Trocholites lativentrosus from the Sandbian Kanling Formation of the Tarim Basin of north-west China appears to resemble some species studied here ( Trocholites freyjae sp. nov., T. baldri sp. nov. and T. glacialis sp. nov.) in conch geometry, especially whorl profile shape and impression rate, but information available on the type material is not sufficient to make reliable comparisons and draw conclusions regarding possible synonymy.

Myagkova (1955) reported the occurrence of the type species Trocholites ammonius from Middle Ordovician strata of the Middle Urals of Russia. However, the illustration and description suggest that the specimen should rather be assigned to a species of Curtoceras or possibly Discoceras ; the type species is thus only unequivocally known from North America.

The late Darriwilian species Trocholites scoticus from Great Britain is transferred to Curtoceras because the two type specimens of Blake (1882) reportedly have only weakly depressed whorl profile and the body chamber in the holotype is adaperturally uncoiled.

Geographic and stratigraphic occurrence

North America, Europe, St Petersburg Region of Russia, China, South Korea, Australia, Middle–Late Ordovician; Great Britain, Llandovery Series (early Silurian).