Type genus

Proterocameroceras Ruedemann, Reference Ruedemann1905; by original designation.

Diagnosis

Adopted from King (Reference King1990, p. 73). “Slender, small to medium-sized, straight or slightly cyrtoconic endogastric conchs, with narrow camerae. Siphuncle ventral; septal necks nearly achoanitic to holochoanitic, connecting rings thick. Endosiphuncular structures usually simple, becoming relatively more complex in later forms.”

Genus Anthoceras Teichert and Glenister, Reference Teichert and Glenister1954

Type species

Anthoceras decorum Teichert and Glenister, Reference Teichert and Glenister1954; from the Early Ordovician (Emanuel Formation) of Western Australia; by original designation.

Diagnosis

Adopted from Kröger (Reference Kröger2012, p. 118). “Prominently annulated, orthoconic longicones with circular or slightly depressed conch cross section; ornamented with fine transverse striae; sutures straight or slightly sinuous and transverse; siphuncle large, marginal; septal necks subholochoanitic to holochoanitic, connecting rings moderately thick; endocones asymmetric longitudinally, more heavily developed on side directed toward conch center.”

Anthoceras ? sp. A

Figure 3.1–3.3, 3.6

Figure 3.

Endoceratid cephalopods from the Šárka Formation (late Darriwilian Stage, Middle Ordovician). (1–3) Anthoceras? sp. A, NM-L 65339, Těškov-channels (northern field): (1) ventral view; (2) lateral view (rectangle indicates position of detail in (3); (3) detailed view of the shell surface. (4, 5) Anthoceras? sp. B, CGS CW12, Osek near Rokycany: (4) general view of the surface of the shell counterpart (i.e., negative surface), rectangle indicates position of detail in (5); (5) detail of shell ornament with multiple healed shell injuries (arrows). (6) Anthoceras? sp. A, NM-L 65340, Šárka–field, wide and rounded annuli on the surface of the counterpart.

Occurrence

Darriwilian (Middle Ordovician) of Central Bohemia.

Description

Specimen NM-L 65339 is a 41-mm-long fragment representing the adapical portion of the body chamber; the adapical end of the specimen is strongly corroded. The transverse section is circular (w/h = 1), 27.3 mm in diameter. At 3°, the apical angle of shell expansion is small. The surface of the shell is annulated (seven annuli are present). The annuli are rounded at the top and 7 mm apart at the middle of the lateral side of the shell. The annuli are nearly directly transverse dorsally, laterally inclined adapically towards the venter (Fig. 3.2), and form a low, broad, and symmetric sinus ventrally (Fig. 3.1). The surface is covered by fine lirae (Fig. 3.3), which are parallel to the course of the annuli; the thickness, height and distance of the lirae varies slightly, but about 34 lirae are present in a length of 5 mm.

A second specimen, NM-L 65340, is a fragment of an external mold of a shell, 38 mm long and 40 mm wide (estimated shell diameter ~60 mm). The mold shows three directly transverse annuli that are 9 mm apart and rounded at the top. The distance between annuli is 7 mm (= 0.12 of the estimated diameter of the shell). The annulation fades toward the (adapertural?) end of the specimen. Transverse and/or longitudinal striae or lirae forming a shell ornament are not discernible.

Materials

NM-L 65339, from Těškov-channels (Fig. 1, locality 2) (northern field); Šárka Formation, upper Darriwilian Stage, Middle Ordovician; F. Novotný Collection NM-L 65340, from Šárka–field (Fig. 1, locality 5); Šárka Formation, upper Darriwilian Stage, Middle Ordovician; F. Hanuš Collection.

Remarks

Although the two specimens lack evidence indicating the internal morphology of the phragmocone, NM-L 65339 bears a marked resemblance in its external form to Anthoceras Teichert and Glenister, Reference Teichert and Glenister1954. It is similar to Anthoceras vaginatum (Schlotheim, Reference Schlotheim1820) in possessing annulations that are slightly inclined to the normal of the conch axis, forming a lobe on one side of the conch. The relative frequency of the annulations is also within the range of A. vaginatum (Kröger, Reference Kröger2012, fig. 7), but the density of the striae is substantially greater in NM-L 65339. The stratigraphical range indicated for A. vaginatum by Kröger (Reference Kröger2012, p. 126) is early to middle Darriwilian. The specimens described here may be slightly younger (late Darriwilian).

A specimen similar to Anthoceras? sp. A was figured by Evans et al. (Reference Evans, Ghobadi Pour and Popov2013, fig. 5A–E) from the early Darriwilian of Iran and assigned by him to Protocyclendoceras sp. A. All species of Protocyclendoceras Balashov, Reference Balashov1968, were synonymized under A. vaginatum by Kröger (Reference Kröger2012). The only difference between the external forms of the Iranian and Bohemian specimens is the circular (versus slightly depressed, w/h = 1.16) cross-section seen in NM-L 65339.

Preservation of specimen NM-L 65340 does not permit a precise taxonomic assignment. However, the similarity of the annulation to that of NM-L 65339 suggests that it may belong with Anthoceras? sp. A.

Anthoceras ? sp. B

Figure 3.4, 3.5

Occurrence

Darriwilian (Middle Ordovician) of Central Bohemia.

Description

Specimen CGS CW12 represents a fragment of the counterpart of the juvenile portion of a shell. The portion is 60 mm long and exhibits the shell ornament. The maximum measurable diameter is 11.4 mm (maximum reconstructed diameter is 12.5 mm) while the cross section is circular or subcircular; the shell expands moderately (estimated angle of expansion of 8°). The external mold shows straight, directly transverse annuli (Fig. 3.4, 3.5) that are rounded at the top; they are evenly spaced, 4 mm apart adaperturally (distance of the annuli is roughly the same as their width). In the adapical part of the specimen, the annulation is less prominent, but two comparatively strong annuli are visible at the extreme adapical end of the specimen. The surface of the counterpart is covered in narrow, sharp lirae that are spaced in a highly irregular manner (6–14 lirae per cycle of annulation, 20 lirae per 5 mm adaperturally). Several growth anomalies and healed injuries are indicated by the course of the lirae (Fig. 3.5).

Materials

CGS CW12, from the Rokycany region; from Osek (Fig. 1, locality 1) near Rokycany, Šárka Formation, upper Darriwilian Stage, Middle Ordovician.

Remarks

The shell surface ornament and angle of expansion indicate that the specimen probably represents the endoceratid genus Anthoceras. As with Anthoceras? sp. A, no internal structures are preserved, and the specimen is assigned to the genus with a degree of uncertainty. It differs from Anthoceras? sp. A, in that the annulations lie in a plane approximately normal to the conch axis while the frequency of the striae is lower.

There is some discrepancy about the locality from which the specimen originated. The adapical part of the specimen was collected by Š. Rak before 2014, reportedly at the locality Těškov, and donated to the Czech Geological Survey in Prague as part of the collection of V. Kozák. The adapertural part of the specimen, which was collected by O. Zicha in 2023 at the locality Osek near Rokycany, also was donated to the Czech Geological Survey. The two localities are ~7 km apart. Siliceous nodules of the Šárka Formation are present and are collected from fields in both areas. However, because the locality information for the adapertural half of the specimen is known with certainty, we consider Osek near Rokycany to be the more likely location.

Regardless of the correct identity of the annulated specimens as Anthoceras, their occurrence in the Prague Basin appears to be extremely rare. For example, although the collections of the National Museum, Prague, includes ~100 endocerids from the Šárka Formation, none possesses annulate conchs.

Subclass Orthoceratoidea Teichert, Reference Teichert, Teichert and Yochelson1967

Order Pseudorthoceratida Flower and Caster, Reference Flower and Caster1935

Family Kionoceratidae Hyatt in Zittel, Reference Hyatt, von Zittel and Eastmann1900

Type genus

Kionoceras Hyatt, Reference Hyatt1884; by original designation.

Diagnosis

Adopted from Sweet in Moore (Reference Sweet and Moore1964, p. K229). “Conchs with more or less well-developed longitudinal surficial ornament of lirae, ribs, ridges, or combinations of these, with or without subordinate transverse ornament of similar nature, or with faint transverse ornament of similar nature, or with faint transverse annulations.”

Genus Kionoceras Hyatt, Reference Hyatt1884

Type species

Orthoceras doricum Barrande, Reference Barrande1868; from the Ludlow Series (Silurian) of Bohemia; by original designation.

Diagnosis

Adopted from Sweet in Moore (Reference Sweet and Moore1964, p. K229). “Slender to rapidly expanding, apically curved, longitudinally fluted orthocones of circular to subcircular section. Surface ornament of prominent longitudinal ribs separated by concave interspaces and, in most species, less conspicuous longitudinal and transverse lirae or striae. Early-formed part of test may be faintly annulated. Siphuncle central to subcentral, empty in Silurian type-species and in most Ordovician species, but with annulosiphonate deposits in some questionably congeneric Silurian and later species; necks straight, or suborthochoanitic, segments cylindrical or weakly expanded within camerae.”

Kionoceras ? sp. indet.

Figure 4

Figure 4.

Orthoceratid Kionoceras? sp. indet. from the Králův Dvůr Formation (Katian Stage, Late Ordovician), Praha–Pankrác, NM L-67102: (1) general view; (2) detailed view of the shell ornament.

Occurrence

Katian (Late Ordovician) of Central Bohemia.

Description

The specimen is a strongly flattened fragment of an orthoconic juvenile shell, preserved in gray clay shale. The estimated shell length is 47 mm (the length cannot be precisely measured because the adapical half of the shell is obscured by another specimen); the maximum measurable shell diameter is 5.3 mm. The angle of expansion is low, but the value can only be estimated (~8°) because the shell is incomplete on the right side. The sutures are straight and evenly spaced (2.5–3.5 chambers correspond to the shell diameter). The shell ornament is preserved in the adapertural half of the specimen; it consists of relatively widely spaced annuli (2 mm apart), and slightly irregularly spaced (3–6 per mm), sharp longitudinal ridges that are of equal thickness (Fig. 4.2).

Materials

NM L-67102; from Praha–Pankrác (Fig. 1, locality 6) (street Na Strži, subway construction site); Králův Dvůr Formation, upper Katian Stage, Upper Ordovician.

Remarks

Except for sutures indicating the distance of septa, the internal structures are not preserved, and thus, only the external shell characters can be used for taxonomic determination. In this respect, representatives of three orthoceratoid genera (Kionoceras and Calorthoceras Chen in Chen et al., Reference Chen, Liu and Chen1981, and Gorbyoceras Shimizu and Obata, Reference Shimizu and Obata1935) bear some resemblance to the described specimen. Kionoceras is characterized by a slowly to rapidly expanding and adapically weakly curved shell. In the adapical part, the shell surface possesses longitudinal ridges alternating with narrower lirae and combined with faint, closely spaced transverse lirae. Weak annulations may also be apparent in the more adapical part of the conch (Barrande, Reference Barrande1868, pl. 269; Sweet, Reference Sweet and Moore1964, p. K229). By contrast, the shell in Calorthoceras is slowly expanding, while the surface ornament is composed of prominent but widely spaced annulations and longitudinal lirae, combined with closely spaced transverse lirae at all growth stages (Barrande, 1868; Chen in Chen et al., Reference Chen, Liu and Chen1981; Manda and Turek, Reference Manda and Turek2024). Apart from slight suppression of annulations on the body-chamber, Gorbyoceras is annulate throughout its length and where the conch sculpture is known (e.g., Flower, Reference Flower1946; Kröger, Reference Kröger2013, Reference Kröger2025) it consists of prominent longitudinal and transverse lirae, the former often exhibiting several orders of dominance.

The apparent lack of transverse lirae or striae in the specimen described above may have been because they were originally very fine and easily obscured or destroyed during diagenesis. Nevertheless, since the transverse lirae and striae in Gorbyoceras are generally coarser and more prominent, and therefore more likely to survive diagenesis, this, combined with an apparent adoral decrease in the prominence of the annulations, suggests that this specimen is likely to belong to Kionoceras rather than Gorbyoceras. The specimen also lacks the narrow but prominent, widely spaced annulations characteristic of Calorthoceras (Manda and Turek, Reference Manda and Turek2024). Given these factors, on balance, this specimen is questionably assigned to Kionoceras until better-preserved material providing an indication of the internal morphology is known.

The specimen described above is obscured adapically by another cephalopod specimen, or by a representative of some other fossil group; likely a cornulitid tubeworm similar to Conchicolites Nicholson, Reference Nicholson1872. This is, however, difficult to resolve due to the insufficient preservation.

Family Sactorthoceratidae Flower, Reference Flower1946

Type genus

Sactorthoceras Kobayashi, Reference Kobayashi1934; by original designation.

Diagnosis

Adapted from Kröger et al. (Reference Kröger, Beresi and Landing2007, p. 1273). “Smooth or annulated, orthoconic or slightly cyrtoconic conchs with straight sutures. Relatively close septal spacing of ellesmeroceridan aspect (five or more chambers over a distance comparable to conch cross section diameter). Siphuncle is central or located between conch wall and center, but never marginal. Siphuncle is narrow, tubular, or slightly expanded between the septa. Connecting rings are relatively thicker than in other Orthocerida. Septal necks are suborthochoanitic or orthochoanitic.” Endosiphuncular and cameral deposits currently unknown.

Remarks

Kröger et al. (Reference Kröger, Beresi and Landing2007) expanded the composition of the Sactorthoceratidae to include forms possessing cameral and endosiphuncular deposits and stated that the connecting rings of representatives of the family possessed connecting rings that were thicker than those of most orthocerids. Previously, the family had been regarded as comprising Sactorthoceras, Sigmorthoceras, and Centroonoceras (Flower, Reference Flower1946; Sweet, Reference Sweet and Moore1964), while Flower and Kummel (Reference Flower and Kummel1950) assigned all three genera to the Michelinoceratidae. While including a number of genera that are not currently known to possess cameral and/or endosiphuncular deposits, Kröger et al. (Reference Kröger, Beresi and Landing2007) also included Cartersoceras Flower, Reference Flower1964a, Murrayoceras Foerste, Reference Foerste1926, and Scipioceras Zhuravleva, Reference Zhuravleva1964. All three of the latter genera possess cameral and endosiphuncular deposits. Illustrations of the siphuncle of Sactorthoceras (Kobayashi, Reference Kobayashi1934; Yun, Reference Yun2011, pl. 10, fig. 6b; pl. 12, fig. 1; pl. 13, fig 3; pl. 14, fig. 1c) show connecting rings that are surrounded and partly obscured by sparite infilling the camerae, making it difficult to locate the boundaries of the connecting rings and assess their thickness. Sections through Sactorthoceras banestanensis Evans in Dastanpour et al., Reference Dastanpour, Evans and Bassett2006, from the upper part of the Katkoyeh Formation (Sandbian) of Kerman Province, Iran, are not obscured by sparite and appear to exhibit connecting rings no thicker than those of other orthocerids (Evans in Dastanpour et al., Reference Dastanpour, Evans and Bassett2006 fig. 3a, f, m, s). Cartersoceras and Murrayoceras were placed in the Baltoceratidae by Flower (Reference Flower1964a) and the dissidoceratid family Cyptendoceratidae by Zhuravleva (Reference Zhuravleva1994). Zhuravleva (Reference Zhuravleva1964) assigned Scipioceras to the Actinocerida. It is possible that assignment within the Pseudorthoceratida would be more appropriate for Scipioceras, but all three genera differ from Sactorthoceras in their general form and are better placed in families other than the Sactorthoceratidae, leaving the family with no known occurrences of endosiphuncular or cameral deposits.

Material belonging to Sactorthoceras banestanensis Evans in Dastanpour et al. (Reference Dastanpour, Evans and Bassett2006, fig. 1) demonstrates that the siphuncle undergoes a change from suborthochoanitic septal necks combined with inflated connecting rings to orthochoanitic septal necks combined with tubular connecting rings, while the relative diameter of the siphuncle also decreases substantially during growth of the conch. Although the species and material described by Kobayashi (Reference Kobayashi1934) and Yun (Reference Yun2011) from the Jigunsan Formation (early Darriwilian) of South Korea require further investigation of ontogenetic variations in their morphology, they appear to exhibit similar variations to those of S. banestanensis (Kobayashi, Reference Kobayashi1934, pl. 16, fig. pl. 7, fig. 10). Evans (in Dastanpour et al., Reference Dastanpour, Evans and Bassett2006) noted that such variation is characteristic of several families that were included in the Pseudorthocerataceae by Sweet (Reference Sweet and Moore1964), and in the Proteoceratidae in particular. Some species of the proteoceratid Isorthoceras Flower, Reference Flower1962, including the type species, Orthoceras sociale Hall, Reference Hall and Miller1877, bear a resemblance to Sactorthoceras, and it is possible that the two genera are closely related. However, cameral and endosiphuncular deposits, present in the type species and many other species assigned to Isorthoceras, remain unknown in Sactorthoceras. If present in Sactorthoceras, these structures must be restricted to the extreme apical end of the conch. Furthermore, while there are similarities between sagittal sections of the two genera, as well as indications that may be proportionately similar, at present there is insufficient data to make a full comparison. Determination of the relationship between the two genera must await a more comprehensive morphometric comparison and the discovery of specimens of Sactorthoceras that can be demonstrated to possess endosiphuncular deposits or that represent mature and complete conchs and are devoid of such features. For now, the two genera should be treated as distinct entities.

Genus Sactorthoceras Kobayashi, Reference Kobayashi1934

Type species

Sactorthoceras gonioseptum Kobayashi, Reference Kobayashi1934; from the Middle Ordovician (Jigunsan Formation) of South Korea; by original designation.

Diagnosis

Adopted from Sweet in Moore (Reference Sweet and Moore1964, p. K233). “Smooth or striated, closely camerated, straight or slightly curved longicones with slender subcentral empty suborthochoanitic siphuncle expanded slightly within camerae. No cameral deposits.”

Sactorthoceras pustulatum new species

Figure 5

Figure 5.

Pseudorthoceratids assigned to Sactorthoceras pustulatum n. sp. from the Šárka Formation (late Darriwilian Stage, Middle Ordovician): (1–7, 11) holotype NM-L 65338, Praha–Šárka (brickyard): (1) ventral view; (2) lateral view; (3) general view of the surface of the shell counterpart (i.e., negative surface), rectangle indicates position of detail in (11); (4, 5) micro-CT scans images of internal structure of the shell, indicating the shape and position of septa and the shape of the siphuncle in dorso-ventral (4) and lateral (5) section; (6, 7) shape of transverse section as shown by the internal mold (6) and seen in a micro-CT scan (7); (11) detailed view of the shell surface; (8–10) paratype NM-L 65341, Praha–Šárka: (8) general view; rectangle indicates positions of details in (9) and (10); (9) detail showing the exposed remains of the subventral siphuncle; (10) borings attributed to Trypanites? isp.

Holotype

NM L 65338, Praha–Šárka (brickyard; Fig. 1, locality 5), Šárka Formation, upper Darriwilian Stage, Middle Ordovician.

Paratype

NM L 65341, from Praha–Šárka (Fig. 1, locality 5), Šárka Formation, upper Darriwilian Stage, Middle Ordovician.

Diagnosis

Sactorthoceras with slightly cyrtoconic shell shape and depressed shell cross-section. Surface ornamented with transverse (~3 per mm) and longitudinal (~6 per mm) lirae; evenly spaced, regular rows of fine pustules developed at the crossing of longitudinal and transverse lirae.

Occurrence

Late Darriwilian (Middle Ordovician) of Central Bohemia.

Description

Holotype NM L 65338 (Fig. 5.1–5.7, 5.11) is an internal mold of a phragmocone, partly embedded in a siliceous nodule, and its counterpart (negative) with shell ornament, 70 mm long. The shell is weakly cyrtoconic (endogastric), septa are shallow and closely spaced (chamber length ~0.20 of shell height). The shell cross-section is slightly depressed (Fig. 5.6, 5.7); in the adapical part, the shell width is 27.3 mm, and the shell height is 24.5 mm (w/h = 1.11). At the adapertural end, the maximum width is ~32 mm, but a precise measurement of shell height cannot be obtained due to incomplete preservation of the dorsal side. The shell is gradually expanding (estimated angle of expansion is ~5°). In the adapical half of the shell, the siphuncle is subventrally located, relatively narrow (sh/h = 0.16) with a depressed cross-section (sw = 4.5 mm and sh = 3.8 mm; sw/sh = 1.18). The siphuncular segments (Fig. 5.4, 5.5) are almost tubular or weakly concave in the dorso-ventral section and slightly more concave in the lateral section. The septal necks are orthochoanitic, but their lengths cannot be measured due to lack of preservation of their terminal zones. The shell is ornamented (Fig. 5.3, 5.11) with distinct, transverse lirae (3 per mm) and densely spaced, fine longitudinal lirae (5–7 per mm). The latter are most pronounced over the crests of the transverse lirae, giving rise to evenly spaced, regular rows of fine pustules.

Paratype NM L 65341 (Fig. 5.8–5.10) is an internal mold of a phragmocone, partly embedded in a siliceous nodule, 83 mm long. The phragmocone is weakly endogastrically cyrtoconic with shallow and closely spaced septa (estimated chamber length 0.20 of shell height). The dorsal side of the shell is incomplete, but the cross-section appears to be weakly depressed. The estimated angle of expansion is 7°. The siphuncle is partially exposed in two weathered chambers in the adapertural half of the specimen (Fig. 5.9); the siphuncle is subventrally located and appears similar in proportions and shape to that of the previous specimen. The surface of the internal mold shows very fine longitudinal grooves, which are most prominent in the adapertural part of the respective chamber infilling. These are not visible on the convex surfaces of the septa.

Specimen NM L 46572 is a short fragment of the adapertural part of a phragmocone, obliquely truncated apically from the dorsal side. Its maximum measurable diameter is larger than the other two specimens (h = 35.9 mm; w = 37.6 mm); the cross-section is nearly circular (w/h = 1.05). The relative chamber length and siphuncular position and proportions are similar to the two above specimens. The shell is ornamented by relatively distinct transverse ornament elements and very fine longitudinal lines.

Etymology

From the Latin pustula, referring to the ornament of the holotype, which forms fine pustules.

Materials

NM L 65338 (holotype), Praha–Šárka (brickyard; Fig. 1, locality 5). NM L 65341 (paratype), Praha–Šárka (Fig. 1, locality 5). NM L 46572 from Osek near Rokycany (Fig. 1, locality 1). All specimens from the Šárka Formation, upper Darriwilian Stage, Middle Ordovician.

Remarks

The surface of the internal mold of paratype NM L 65341 shows drill-holes assigned to Trypanites? isp. (Fig. 5.10).

Evans in Dastanpour et al., Reference Dastanpour, Evans and Bassett2006, provided a detailed discussion regarding the scope and diagnostic characters of the problematic genus Sactorthoceras. The genus was not emended by the former author, but the discussion listed several characters important for the definition of the genus. These include the central or subcentral position of the relatively narrow siphuncle (~0.20–0.10 of shell diameter in ontogeny), the ontogenetic change from suborthochoanitic to orthochoanitic septal necks and from expanded to tubular connecting rings, the relatively short phragmocone chambers (~0.40–0.10 of shell diameter in ontogeny), absence of cameral and endosiphuncular deposits or their restriction to only the extreme apical parts of the shell.

The specimens from the Prague Basin studied herein appear to correspond to these characters and are thus assigned to Sactorthoceras. The forms most closely related to the Bohemian specimens appear to be the representatives of S. banestanensis Evans in Dastanpour et al., Reference Dastanpour, Evans and Bassett2006. Similarities include the angle of shell expansion and the position and diameter of the siphuncle. However, the Bohemian specimens differ from S. banestanensis and other species of Sactorthoceras in the combination of a weakly cyrtoconic shell shape, depressed shell cross-section, and shell ornament consisting of fine but distinct longitudinal lirae. A new species, Sactorthoceras pustulatum n. sp., is therefore established.

Another possible member of the Sactorthoceratidae (Evans et al., Reference Evans, Ghobadi Pour and Popov2013), Orthoceras tardigradum Barrande, J., Reference Barrande1870, from the Šárka Formation (Barrande, Reference Barrande1870, pl. 414; Barrande, Reference Barrande1874, p. 155) differs from the above studied specimens of Sactorthoceras in the circular shell cross-section and subcentral (versus subventral) position of the siphuncle.

Order Orthoceratida Kuhn, Reference Kuhn1940

Family Dawsonoceratidae Flower, Reference Flower1962

Type genus

Dawsonoceras Hyatt, Reference Hyatt1884; by original designation.

Diagnosis

Adopted from Kröger and Isakar (Reference Kröger and Isakar2006, p. 152). “Annulated orthocones with slender, central to subcentral, tubular or moderately expanded siphuncle. Septal necks very short and can vary between achoanitic, suborthochoanitic, or very short cyrtochoanitic in some developed species. Shell ornamented with a reticulate pattern of growth lines and fine longitudinal raised lines, or with transverse festoons. Apical shell large, without cicatrix, without initial constriction, apex blunt, annulated or smooth, transverse or longitudinal ornamentation may occur. Incipient annulosiphuncular deposits, and mural cameral deposits are known.”

Genus Dawsonoceras Hyatt, Reference Hyatt1884

Type species

Orthocera annulata Sowerby, Reference Sowerby1816; from the Silurian of England; by original designation.

Diagnosis

Annulated longiconic orthocones; annuli accompanied with numerous, tiny, frilled, undulated transverse lirae.

Siphuncle subcentral (shifted slightly ventrally) in early growth stages, central in late growth stages. Connecting rings almost tubular to slightly expanded, adnate to septa. Septal necks achoanitic or short cyrtochoanitic (modified from Horný, Reference Horný1956).

Remarks

The most recent diagnosis of the genus Dawsonoceras by Kröger and Isakar (Reference Kröger and Isakar2006) does not reflect the revision and emended diagnosis given by Horný (Reference Horný1956). We therefore include here the latter following Kröger (Reference Kröger2025).

Dawsonoceras ? sp. indet.

Figure 6.1, 6.3

Figure 6.

(1, 3) Orthoceratid Dawsonoceras? sp. indet. from the Králův Dvůr Formation (Katian Stage, Late Ordovician), Králův Dvůr or Kosov Hill area, NM L-46571: (1) general view; rectangle indicates position of detail in (3); (3) detailed view of the surface ornament seen on (1). (2) Holotype of Orthoceras evictum Barrande, 1868, NM L-11266, from the Králův Dvůr Formation (Katian Stage, Late Ordovician), Lejškov, which is re-interpreted as a conulariid in this work. Note the inscription on the specimen reads “Libomyschl”, which is stratigraphically coeval to the strata of the nearby locality “Leiskov” (Lejškov) stated by Barrande, 1868, for the specimen (P. Štorch, personal communication, 2024).

Description

The specimen is a moderately flattened external mold preserved in a brown-gray, micaceous clay shale. This fragment is 38 mm long and has a maximum preserved width of 14 mm. The shell is ornamented with distinct, crenulated transverse lamellae (13 per 5 mm adaperturally), which are slightly raised and overlapping. Arches of the individual transverse elements are 8–9 per 5 mm and are extended adaperturally in the form of sharp ridges that do not extend all the way to the next transverse element (i.e., the longitudinal elements of ornament are discontinuous). There are no distinct annuli on the shell surface.

Materials

NM L-46571; from Králův Dvůr (Fig. 1, locality 3) or Kosov Hill area (Fig. 1, locality 4); Králův Dvůr Formation, upper Katian Stage, Upper Ordovician.

Remarks

In his revision of the genus Dawsonoceras, Horný (Reference Horný1956) placed emphasis on the frilled transverse ornament on the shell surface. The ornament of the specimen described above thus strongly resembles that of the representatives of the genus Dawsonoceras. However, since no internal shell structures are preserved, the specimen is questionably assigned to Dawsonoceras.

Horný (Reference Horný1956) restricted the genus to Silurian forms only. Kröger and Isakar (Reference Kröger and Isakar2006) and Kröger (Reference Kröger2013) also reported the genus from the Late Ordovician of Baltoscandia and worldwide. However, the occurrence of Dawsonoceras in Ordovician strata has been questioned by Sweet in Moore (Reference Sweet and Moore1964) and most recently by Kröger (Reference Kröger2025). This is an additional reason for questionably assigning the above specimen to the genus Dawsonoceras.

If the specimen is truly a representative of Dawsonoceras, then the absence of distinct annuli and the discontinuous form of longitudinal ornament would be unique among species of the genus. However, the ornament appears to resemble a specimen illustrated by Blake (Reference Blake1882, pl. 4, fig. 6) from the Sholeshook Limestone Formation (Katian 3) of South Wales and assigned by him to Orthoceras annulatum Sowerby, Reference Sowerby1816 (= Orthocera annulata Sowerby, Reference Sowerby1816). The ornament in the Bohemian specimen differs only in the even stronger suppression of surface annuli. The original specimen of Blake (Reference Blake1882), as well as newly collected material from South Wales, is currently under study by one of the present authors (DE); one of these specimens shows a part of the siphuncle with slightly expanded and shorter segments compared to the Silurian representatives of the genus. If the Welsh material is found to be conspecific or closely related to the Bohemian specimen, this would indicate paleogeographic connections between the Prague Basin and Avalonia.

Reticulate shell ornament was also described in the stratigraphically coeval specimen NM L-11266, which was used by Barrande (Reference Barrande1868) as the holotype of Orthoceras evictum (as Kionoceras evictum [Barrande, Reference Barrande1868] in Marek, Reference Marek1999). The latter is a flattened fragment of a large individual consisting of remains of the shell wall (Fig. 6.2). The transverse elements of the ornament are much more densely spaced (16 per 5 mm) and only faintly crenulated, while the longitudinal ridges are low, widely spaced (1.3 per 5 mm), and continuous, with extremely fine longitudinal lines in between. These differences might be explained by the much later growth stage exhibited by specimen NM L-11266.

However, several features of the holotype indicate that its identity as a cephalopod is questionable and suggest, rather, that the specimen represents a fragment of one side of a large conulariid theca. These features include: (1) the dark color of the remaining test, indicating a high primary content of organic matter in the shell wall; (2) the wall is completely flat but not longitudinally cracked, as would typically be the case in a compacted conical cephalopod shell; (3) the ornament is perfectly transverse except at intersections with the indistinct longitudinal ornament elements, and not in any way sinuous as would typically be expected along the sides of a compacted cephalopod shell; (4) the presence of grooves similar to lamellae; and (5) the layered nature of the wall.

Genus Palaeodawsonocerina Kröger and Isakar, Reference Kröger and Isakar2006

Type species

Spyroceras senckenbergi Teichert, Reference Teichert1930; from the Kõrgessare Formation, Vormsi Regional Stage (Katian) of Saaremõisa (Lyckholm), Estonia.

Diagnosis

Adopted from Kröger (Reference Kröger2025, p. 31). “Straight or slightly cyrtoconic, circular or subcircular shell, with prominent, narrowly spaced annulations, growth lines, and numerous subordinate filiform longitudinally raised lines, producing a fine reticulate pattern; siphuncle central or slightly eccentric, slightly expanded, with achoanitic to very short orthochoanitic septal necks; apex blunt, slightly cyrtoconic, annulated.”

Remarks

Kröger and Isakar (Reference Kröger and Isakar2006) considered that some of the species assigned to Spyroceras Hyatt, Reference Hyatt1884, and Anaspyroceras Shimizu and Obata, Reference Shimizu and Obata1935, probably belonged in Palaeodawsonocerina because of the similarity of their conch ornament, but hesitated from assigning them to Palaeodawsonocerina because their internal characters were unknown and Anaspyroceras was described by Sweet (Reference Sweet and Moore1964) as possessing an orthochoanitic siphuncle with short septal necks and connecting rings that were probably tubular. Since Foerste’s description of the holotype of the type species of Anaspyroceras, Orthoceras anellus Conrad, Reference Conrad1843 (Foerste, Reference Foerste1928, p. 177–178, pl. 40, fig.4) only refers to the diameter and location of the siphuncle. Sweet’s (Reference Sweet and Moore1964) description of the structure of the siphuncle seems likely to have originated from the original description of Anaspyroceras williamsae Flower, Reference Flower1946, where the structure is known. However, at 1 per mm, the density of the longitudinal lirae is substantially less than that of the type species (7–9 per mm). Foerste (Reference Foerste1928) reported the presence of longitudinal lirae in the type species, but no reference to the presence of transverse striae or lirae, and his illustration of the holotype is of no help in this matter. Without restudy of this specimen and/or other specimens that can be confidently assigned to the type species, the full details of the conch ornament and structure of the siphuncle in Anaspyroceras can only be surmised.

Palaeodawsonocerina obscurum (Barrande, 1868) new combination

Figure 7

Figure 7.

Orthoceratid Palaeodawsonocerina obscurum n. comb. from the Králův Dvůr Formation (Katian Stage, Late Ordovician), Králův Dvůr (NM-L 10356, NM-L 10357) and Kosov (NM-L 67108, NM-L 67107): (1, 2) NM-L 10356, lectotype: (1) internal mold, (2) counterpart with shell ornament; (3) NM-L 67108, internal mold; (4) NM-L 67107, internal mold; (5) NM-L 10357, paralectotype, internal mold.

Reference Barrande1868   Orthoceras obscurum Barrande, pl. 277.

Reference Barrande1874   Orthoc. obscurum; Barrande, p. 279.

Reference Marek1999   Anaspyroceras obscurum; Marek, p. 415, text-fig. 1.

Reference Kröger and Isakar2006   Orthoceras obscurum; Kröger and Isakar, p. 154.

Lectotype

NM L-10356, from Králův Dvůr (Fig. 1, locality 3); uppermost Králův Dvůr Formation, upper Katian Stage, Upper Ordovician; J. Barrande collection, illustrated by Barrande (Reference Barrande1868, pl. 277, figs 11, 12) and re-illustrated here (Fig. 7.1, 7.2).

Paralectotype

NM L-10357, from locality Králův Dvůr (Fig. 1, locality 3); uppermost Králův Dvůr Formation, upper Katian Stage, Upper Ordovician; J. Barrande collection, illustrated by Barrande (Reference Barrande1868, pl. 277, fig. 13) and re-illustrated here (Fig. 7.5).

Diagnosis

Shell moderately large (adult shell diameter ~30 mm), orthoconic with low expansion rate (~3–6°). Sutures weakly sinuous, crossing annulations at a low angle. Distance between sutures the same as the annulations. Shell ornament of straight to weakly sinuous, directly transverse, or weakly oblique distinct annuli. Annuli distance greater adaperturally (~6 mm apart) and smaller adapically (~3 mm apart), rounded on top and asymmetric (with steeper side inclined adaperturally). Sharp reticulate ornament of transverse lirae (7–8 per mm, finer adapically) and longitudinal lirae (6–8 per mm).

Description

Lectotype NM L-10356 (Fig. 7.1, 7.2) is a strongly flattened part and counterpart of a fragment of an orthoconic shell preserved in brown-gray clay carbonate. This fragment is 35 mm long with a maximum measurable width of 29 mm (reconstructed diameter 18.5 mm). The shell ornament consists of weakly sinuous annulations (increasing from 2.3 to 2.8 mm distant in the adapertural direction). The annulations are slightly asymmetric in cross-section (more steeply inclined on their adapertural sides) with rounded tops. They are covered in a fine but distinctly reticulate ornament formed by transverse lirae with a density of 7–8 per mm and longitudinal lirae with a density of 6–8 per mm. Transverse lirae are more distinct adaperturally. At the adapical end of the external mold, the sutures of at least four septa are visible, each separated by distance similar to the distance between the annulations. The sutures appear to be faintly sinuous and cut across the annulations at a low angle indicating that they are inclined to the annulations. The presence of distinct sutures only at the adapical end of the specimen suggests that it represents part of a body-chamber with a short length of phragmocone attached.

Paralectotype NM L-10357 (Fig. 7.5) consists of a 96-mm-long fragment of an orthoconic shell preserved in brown-gray clay carbonate. This fragment is more flattened on one side and less flattened (convex) on the other. The maximum measurable diameter is 26.3 mm (reconstructed diameter 16.7 mm) and the minimum measurable diameter 25 mm (15.9 mm). Based on a length of the conch where the lateral extremities of the annulations on both sides of the conch appear to be rounded, the estimated angle of shell expansion is about 2.5°. Farther adapically, the expansion rate appears to be greater, but the lateral extremity of the conch may be missing in this zone. The surface annuli are straight and directly transverse, being more widely spaced adaperturally (5.7 mm apart), but oblique and more closely spaced adapically (2.6–2.7 mm apart). However, the more flattened side of the shell has adapical annuli that are almost directly transverse and straight. The annuli are clearly asymmetric in cross-section, being inclined adaperturally with rounded tops. In the most adapertural region, the annuli disappear, which may reflect the adult growth stage of the specimen. The reticulate ornament is preserved mainly on the less-flattened side of the specimen, where it consists of transverse lirae, while the longitudinal lirae are less well developed. The density of the lirae cannot be precisely measured due to poor preservation of the shell surface.

The additional material contains six flattened fragments of annulate orthoconic shells (Fig. 7.3, 7.4). The fragments from the Králův Dvůr locality and the largest fragment from the Kosov locality (NM L-67106) show fine reticular ornament identical to that described from the lectotype.

Materials

NM L-10356 (lectotype), from Králův Dvůr (Fig. 1, locality 3); uppermost Králův Dvůr Formation, upper Katian Stage, Upper Ordovician. NM L-10357 (paralectotype), from Králův Dvůr (Fig. 1, locality 3); uppermost Králův Dvůr Formation, upper Katian Stage, Upper Ordovician. Three specimens (NM L-67103–NM-L 67105) from Králův Dvůr (Fig. 1, locality 3), three specimens (NM L-67106–NM-L 67108) from Kosov (Fig. 1, locality 4); uppermost Králův Dvůr Formation, upper Katian Stage, Upper Ordovician; J. Barrande collection.

Remarks

Marek (Reference Marek1999) assigned Orthoceras obscurum to the dawsonoceratid genus Anaspyroceras. Later, Kröger and Isakar (Reference Kröger and Isakar2006) indicated that the surface ornament of Orthoceras obscurum, until now figured only by Barrande (1868), resembled that of their newly defined genus Palaeodawsonocerina from the Late Ordovician of Estonia (and Sweden, see Kröger, Reference Kröger2013), but did not assign the species to Palaeodawsonocerina because internal features were not known in Orthoceras obscurum. The precise nature of the internal morphology of the type species of Anaspyroceras (Orthoceras anellus) remains uncertain, and until additional specimens that can be confidently assigned to the species are included, the range of variation of the shell sculpture in the species cannot be delimited with any certainty. Thus, a precise diagnosis of Anaspyroceras based on the type species is yet to be achieved and the possibility that Palaeodawsonocerina is a junior synonym cannot be ruled out. Nevertheless, although the internal structure of Orthoceras obscurum is unknown, the relative distances between annulations in O. obscurum lie within the range of those of P. senckenbergi (Kröger, Reference Kröger2025, fig. 14c), while the conch ornament is almost identical, and the expansion rate is also similar to that of P. senckenbergi. Given the strong similarities between the external form of O. obscurum and P. senckenbergi, the former is assigned here to Palaeodawsonocerina. Palaeodawsonocerina obscurum n. comb. differs from P. senckenbergi in possessing sutures that cut across the annulations at a low angle, whereas those of P. senckenbergi are parallel to the annulations.

Possibly similar to the Bohemian specimens of P. obscurum n. comb. are forms currently under study by one of the present authors (DE) from the late Katian, Narrow Vein Mudstone Formation of the Corris area, North Wales, indicating possible paleogeographic connections between the Prague Basin and Avalonia.

Note that Kröger and Isakar (Reference Kröger and Isakar2006) defined Palaeodawsonocerina based on Spyroceras senckenbergi Teichert, Reference Teichert1930. Throughout their work, however, the former authors also use the combination Palaeodawsonoceras senckenbergi. Later, Kröger (Reference Kröger2013, Reference Kröger2025) used only the combination Palaeodawsonocerina senckenbergi. We thus consider Palaeodawsonoceras as an incorrect subsequent spelling of Palaeodawsonocerina.