The Diceratidae are the stem group of all uncoiled rudists (Skelton 2013a). The characteristic combination of right-valve attachment, sub-equivalve to inequivalve shells, external parivincular ligament and outward-coiled prosogyrate umbos is restricted to the genera Diceras Lamarck, 1805 and Valletia Munier-Chalmas, 1873 (Skelton and Smith 2000; Skelton 2013b; Schneider and Skupien 2015).

Valletia tombecki Munier-Chalmas, 1873 [= Valletia germani (Pictet & Campiche, 1868)], by monotypy.

Valletia differs from Diceras in the possession of a shortened external ligament; the attachment of the myophores to the hinge plate; and a derived inverse dentition sensu Douvillé (1887, 1896): a large anterior (2a) and a small posterior tooth (2b) in the left valve are separated by a deep central socket, which accommodates a corresponding, usually arched, large central tooth (1) in the right valve (Skelton 2013a; Schneider and Skupien 2015; Rineau et al. 2020).

Valletia with quickly expanding subequal valves, both with faint to distinct, bluntly rounded dorsoventral angulation, separating shell into approximately equal-sized anterior and posterior portions. Commissure sub-circular to oblique-D-shaped in outline, its posterior-ventral part rather well rounded. Right (attached) valve with single prominent arcuate tooth 1, curving around deep socket 2a’ below, which is separated from body cavity by narrow rim. Left (free) valve with massive, columnar anterior tooth 2a and much smaller to almost indistinct posterior tooth 2b, separated by deep, arcuate socket 1’, curving around the back and top of 2a.

Large left (free) valve; BSPG 2019 IV 3. Left (free) valve; BSPG 2016 XXI 1391. Juvenile left valve; BSPG 2019 IV 2.

Late Kimmeridgian limestones at Pierre Châtel, Département Ain, France.

Left valves from Saal very small to moderately sized (L = 8–32 mm). Shell moderately inflated, very quickly expanding in diameter; oblique-D-shaped in outline, with anterior margin forming the straight line of the D; posterior, dorsal and ventral margins well rounded. Juvenile shell oblique-short-ovate in outline. Umbo strongly prosogyrate, coiled, slightly projecting. Rounded angulation extending from umbo to anterior-ventral corner, separating shell into faintly sloping anterior and more steeply sloping posterior portions of approximately equal size. Ornament of commarginal costellae preserved on posterior portion of juvenile specimen. Somewhat reticulate ornament of irregular radial and commarginal costellae occurring on anterior slope of adult individual. Shell interior only exposed in one specimen but markedly corroded. Columnar, slightly upward bent anterior tooth (2a) and deep, arcuate central socket (1’) well visible. Posterior tooth (2b) absent, presumably corroded. Nymph also corroded. Adductor muscle attachment scars not discernible, but indicated by downward sloping hinge plate, both anteriorly and posteriorly. No right valves available from Saal.

The three specimens from Saal are left valves and, to our opinion, represent juvenile to adult individuals. These left valves clearly were not attached to a substrate, thus represent Diceratidae. The larger specimen (Fig. 2E) has a rather irregularly growing early shell portion, probably due to space restrictions, but has no attachment surface. The medium-sized specimen has a wide hinge plate tapering ventrally along the anterior and posterior shell margins to accommodate the adductor muscles on these shelves (Fig. 2C); there are no myophores below. Thus, the specimens represent Valletia, not Diceras. With regard to shell shape, the specimens closely resemble Valletia auris as figured by Favre and Richard (1927), particularly the specimens in their plate 1, figures 5 to 10. Favre (in Favre and Richard 1927) remarked that, among other characters, V. auris differed from Valletia antiqua Favre in Joukowsky and Favre 1913 in its non-carinate left valve. In fact, the specimens from Saal do show a marked angulation separating the anterior and posterior shell portions (as does the right valve in pl. 1, fig. 10 of Favre and Richard), but this angulation is blunt and rounded, very unlike the sharp carina in V. antiqua, where flank and posterior slope meet at angles below 90 degrees. As a result, the posterior-ventral part of the shell is sharply pointed in V. antiqua, while it is well rounded in V. auris (see also Schneider and Skupien 2015).

The medium-sized left valve, where the hinge is exposed, clearly shows the columnar, upward-bent anterior tooth (2a) typical of Valletia auris, as well as the deep arcuate socket (1’) for the prominent tooth of the right valve; this hinge configuration is distinct from the one in Diceras. The much smaller but usually distinct posterior tooth normally present in V. auris is absent. However, we are rather convinced that this is a result of the poor preservation, where much of the shell surface, in- and outside, also including the posterior adductor muscle scar, is corroded.

Like the specimens from Saal, the type material of V. auris comes from upper Kimmeridgian strata and thus is of largely the same age; this also makes it the stratigraphically oldest species in Valletia. As mentioned by Schneider and Skupien (2015), aged individuals of this species were described as variety crassa by Favre (in Favre and Richard 1927). Sano and Skelton (2011) also referred specimens from the Bau Limestone of Sarawak (Borneo Island, Malaysia) to Valletia auris, which had previously been assigned to Heterodiceras (Lau 1973).

Yanin (2021) assigned material from Berriasian strata of the Crimean Mountains (Yayla Mountains), Crimea, Ukraine, to Valletia auris. These very well-preserved specimens have a short-ovate commissure and seem to expand slightly less quickly than the specimens of the type series, as well as the material studied by us. However, given the general variability of these characters seen in the type series of Valletia auris, the Crimean records are retained in this species here.

A peculiarity of the specimens from Saal is the preservation of external ornament, which to date was not only unknown in Valletia auris, but in Valletia in general. As can be seen in various Diceratidae and Epidiceratidae, this ornament is restricted to the outermost, originally calcitic shell layer, which, somewhat surprisingly, is often not preserved, while the originally aragonitic shell portion below is. Interestingly, the juvenile shell from Saal only has ornament on its posterior slope, but it is unclear whether this is a pristine feature or rather a matter of preservation. This ornament consists of short, more or less commarginal costellae, not the radial/spiral costae commonly seen in Jurassic rudists (Fig. 2A). In contrast, the adult specimen only preserves ornament on its anterior shell portion, which is more likely to be an artefact, given that the remainder of the shell does have a weathered aspect. This is the first report of Valletia auris from the Kelheim area, and only the fourth one of this species in general.

Late Kimmeridgian strata of Pierre Châtel, Département Ain, France (Favre and Richard 1927). Bau Limestone of Sarawak, Borneo, Malaysia (Sano and Skelton 2011). Late Kimmeridgian Kelheim Member, Frankenalb Formation, of Kelheim-Saal, Bavaria, Germany (this study). Early Berriasian strata of the Crimean Mountains (Yayla Mountains), Crimea, Ukraine (Yanin 2021).

All Hippuritida that were attached by the left valve are contained in the Requieniidina and its sole superfamily, Requienioidea (Skelton 2013a).

The distinction between Epidiceratidae and Requieniidae Kutassy, 1934, the two families included in Requienioidea, is primarily based on the shift of the commissural plane relative to the coiling axis of the left valve (Gourrat et al. 2003; Scott et al. 2010; Fig. 3). In Epidiceratidae, the angle between the commissural plane and the coiling axis is relatively large (usually 45 degrees or more), resulting in a comparatively erect growth habit (Fig. 3A). In Requieniidae, this angle is distinctly smaller. As a result, the commissural plane intersects the spiral portion of the shell (Fig. 3B), while this is generally not the case in Epidiceratidae. This change in whorl geometry brings the usually flattened anterior face of the requieniid left valve into close contact with the substrate. The resulting shell forms were termed frictional or attached clingers by Gili et al. (1995), and thus also differ ecologically from epidiceratids.

In Epidiceras, Plesiodiceras, Heterodiceras and Saalidiceras gen. nov., the plesiomorphic, epidiceratid condition of this character is clearly met. In Hypelasma fritzlangi sp. nov., the commissural plane intersects the spiral shell in most specimens, but the growth habit remains relatively erect; this species thus might link Epidiceratidae and Requieniidae phylogenetically, but is assigned to the latter herein.

Diceras sinistra Deshayes, 1824, by original designation.

Small to very large Epidiceratidae (maximum commissural dimensions vary from 25 to more than 150 mm) characterised by sub-equivalve to moderately inequivalve shells with sub-circular, slightly high-ovate or D-shaped commissures. Outside of shell well-rounded to distinctly posteriorly carinate. Anterior and posterior adductor muscles inserting on distinct myophoral ledges, which extend along the anterior and posterior walls of the shell cavity, below the hinge plate.

The name Epidiceras was proposed as a new genus in a general account of hippuritid evolution by Douvillé (1935). Most subsequent workers (including the first author) overlooked that Douvillé (1935) had not selected a type species, and his new generic name was thus unavailable. The name Epidiceras was made available by Dechaseaux (1952), who selected Diceras sinistra Deshayes, 1824 as the type species.

Epidiceras has a long external ligament and generally well-pronounced, free myophoral ledges, which are extending along the anterior and posterior walls of the shell cavity below the hinge plate. In contrast, both adductor muscle scars are placed on downward extensions of the hinge plate in Plesiodiceras and Heterodiceras. As a result, the shell cavities of Plesiodiceras and Heterodiceras are much smaller and their internal moulds much slenderer than those of Epidiceras, which additionally bear the incisions left by the dissolved myophoral ledges. Furthermore, Epidiceras typically has sub-equivalve shells; the free right valve is often only moderately smaller than the attached left valve. In Plesiodiceras, the right valve is much smaller, almost flat, resembling a gastropod operculum. In Heterodiceras, the right valve is often larger than in Plesiodiceras, but still far from sub-equal to the left valve.

Gemmellaro had proposed a new subgenus, Pseudodiceras, in the programme of the first congress of the Società italiana per il progresso delle scienze in 1875 (Gemmellaro 1875: 28; original ‘programme’ not available to the authors). This subgenus was supposed to contain the carinate forms Diceras carinatum (here assigned to Epidiceras) and D. affine (here referred to Heterodiceras) from Sicily, which Gemmellaro thought to be distinguished by the auricular shape of their posterior cardinal tooth (3b) in the right valve. However, after consulting Bayle’s (1873) monograph, which displayed specimens with cardinal teeth of intermediate shapes, Gemmellaro refrained from formally proposing the new name. As a result, Pseudodiceras was established in synonymy (Gemmellaro 1875: 28, 29) and is thus a nomen nudum. The name is also not available from Cox (in Dechaseaux et al. 1969: N778), who designated Diceras carinatum Gemmellaro, 1875 as the type species, but did not use Pseudodiceras as a valid name.

Pchelintsev (1959) established four new genera, Eodiceras (type species: Diceras ursicinum Thurmann, 1853), Megadiceras (type species: Diceras beyrichi var. porrecta Boehm, 1883), Mesodiceras (type species: Mesodiceras enissalense Pchelintsev, 1959) and Paradiceras (type species: Chama speciosa Münster in Goldfuss, 1837), which were regarded as junior subjective synonyms of Epidiceras by Skelton (1999, 2013a), based on their attachment by the left valve and the presence of myophoral ledges; this view is followed here. Pchelintsev’s (1959) generic diagnoses are based on minor differences in outline shape of the commissure, shell size, shape of the hinge teeth, and the size and elevation of the posterior myophores, which we largely regard as significant at species level, but within the intrageneric variability of Epidiceras.

Large Epidiceras with up to 90 mm high, D-shaped commissure, the anterior margin forming the straight line of the D. Shell outside with a sharp carina extending from umbo to anterior-ventral corner, dividing the exterior of the shell into flattened anterior and posterior portions. Posterior myophore prominent and wide; anterior myophore less wide but distinct.

The type series of Diceras carinatum Gemmellaro, 1875 (late Tithonian; Palermo region, Sicily, Italy) consists of two originally figured specimens: a right valve (Gemmellaro 1875, pl. 5, fig. 9; accession number MGUP-20-407.1), designated lectotype herein, and a left valve (Gemmellaro 1875, pl. 5, fig. 10; MGUP-20-407.2), thereby becoming the only paralectotype. Note that Gemmellaro’s (1876) three-volume book, which is often cited as the original source of his species descriptions, is a collection of (parts of) 13 previously published journal articles by Gemmellaro and Gemmellaro and Di Blasi, which were reprinted with updated synonymies, modified or omitted introductions and in different order. Based on in-text citations and synonymies, the dates given (1868, 1869, 1871) are clearly erroneous, and the entire book was likely published in 1876, the date of the introduction. The original descriptions of the new rudist taxa from the ‘Calcare a Terebratula janitor’ are contained in the journal article by Gemmellaro (1875).

The type series of Diceras bavaricum Boehm, 1881 (late Kimmeridgian; Kelheim Member, Frankenalb Formation; Kelheim area, Bavaria, Germany) originally consisted of 20 specimens (Boehm 1881: 157). Four specimens were figured, and three of these are preserved: a right valve (Boehm 1881, pl. 26, fig. 4; BSPG 1872 XI 556), designated lectotype herein; a left valve (Boehm 1881, pl. 26, fig. 3; BSPG 1872 XI 557), designated paralectotype 1; and an internal mould of a specimen with closed valves (Boehm 1881, pl. 27, figs 3, 4; BSPG 1872 XI 558), designated paralectotype 2. The fourth figured specimen, an individual with closed valves (Boehm 1881, pl. 25, fig. 4, pl. 26, figs 1, 2), as well as the remaining 16 non-figured specimens are lost.

Numerous specimens are available from Saal, of which a representative suite was selected for measurements and/or figures for this study: a single slightly fragmentary juvenile right valve (BSPG 2016 XXI 1394); six double-valved specimens (BSPG 2016 XXI 69, BSPG 2016 XXI 1401, BSPG 2016 XXI 1402, BSPG 2020 CIII 30, BSPG 2020 CIII 31, BSPG 2020 CIII 32); thirteen left valves (BSPG 2016 XXI 13, BSPG 2016 XXI 1214–1218, BSPG 2016 XXI 1222, BSPG 2016 XXI 1223, BSPG 2016 XXI 1225, BSPG 2016 XXI 1403, BSPG 2016 XXI 1523, BSPG 2020 CIII 6, BSPG 2020 CIII 26); and twenty right valves (BSPG 2016 XXI 67, BSPG 2016 XXI 68, BSPG 2016 XXI 1200–1213; BSPG 2016 XXI 1219–1221, BSPG 2016 XXI 1522). In addition, fragmentary internal moulds of two right valves (BSPG 2019 IV 1, BSPG 2019 IV 11) and one left valve (BSPG 2019 IV 12) are available.

According to Gemmellaro (1875), the type material of Diceras carinatum was collected ‘in the district of Fovara near Villabate, near Carini, in the surroundings of Torretta and in the district of Santa Maria di Gesù near Palermo’ [translated from Italian]. The museum catalogue lists three locality names, Carini, Favara and Favarotta, for the two originals archived under accession numbers MGUP-20-407.1 and MGUP-20-407.2. Consequently, the type locality cannot be established. Tithonian shallow marine limestones in the Palermo region occur within two different structural units, and the mentioned localities relate to both. Tithonian strata near Carini and Torretta conform to the informal Calcari di Piano Battaglia within the platform succession of the Panormide Unit (Catalano 2011b; Catalano et al. 2013b). In contrast, Tithonian strata near Villabate and Santa Maria di Gesù expose the informal Ellipsactinia breccias member of the Crisanti Formation, forming part of the basinal Imerese Unit (Catalano 2011a; Catalano et al. 2013a).

Large Epidiceras, with commissure typically between 70 and 90 mm high when adult. Sub-equivalve to moderately inequivalve, with rather gradually and slowly expanding whorls. Commissure D-shaped in outline, with anterior margin forming the straight line of the D; dorsal, posterior and ventral shell margins evenly rounded. Both valves strongly carinate, with carina extending from umbo to anterior-ventral corner, dividing exterior of shell into flattened anterior and posterior portions, the anterior one often being slightly larger. Right valve generally rather regularly coiled, auriculate in outline and shape in early growth (Fig. 7A–C, 8A–D). Left valve often more openly coiled and stretched, the largest, slightly fragmentary specimen from Saal surpassing 200 mm in height (Fig. 6E). Ornament of radial costellae (spiral ornament, if one would apply gastropod terminology) preserved anterior to the carina in two large left valves, one of which is figured here (Fig. 8G).

Hinge plate of left valve high, with a single, strong, arcuate cardinal tooth 2 in sub-horizontal position, extending from near the anterior shell margin approximately halfway back. Cardinal tooth flanked by a shorter, moderately deep, sloping socket 3a’ ventrally within the arch, and an extensive, very deep socket 3b’ dorsally and posteriorly. Hinge plate of right valve equally high, dominated by very large, arcuate posterior cardinal tooth 3b paralleling the dorsal shell margin; initially forming a raised arch, but becoming increasingly tongue-shaped and pointed, and bending upward in adulthood. Small peg-like anterior cardinal tooth 3a situated ventrally, above a pronounced buttress, separated from 3b by a deep, arcuate socket 2’. Nymphs not very prominent but distinct and long. Anterior myophore in both valves sharp and distinct, but forming a relatively low elevation below the hinge plate. Posterior myophore prominent and wide in both valves. Both myophores extending ventrally far below the hinge plate, providing space for large adductor muscles.

Before conducting the description of his new species, Gemmellaro (1875: 29) had received two right valves of ‘Diceras’ from the Kelheim area, sent from Munich by Karl Alfred von Zittel. These specimens are still part of the collections of the museum in Palermo. Based on comparison with his own material from Sicily, Gemmellaro (1875) considered the Sicilian and Bavarian specimens as conspecific and assigned them to his newly established species, Diceras carinatum. He further mentioned that Diceras carinatum, Diceras speciosum and Diceras muensteri were found in the same deposits at Kelheim.

Boehm (1881) did not agree with Gemmellaro’s (1875) attribution of the Bavarian specimens and instead assigned them to Diceras bavaricum. He credited Zittel with the authorship of D. bavaricum, who had previously used this name on collection labels (Boehm 1881: 82). However, the name was not published and thus not available before Boehm’s (1881) monograph, and Diceras carinatum clearly takes priority, if considered conspecific. Boehm (1881) distinguished D. bavaricum and D. carinatum based on differences in the dentition of the right valve (which were not further explained), but also stated that the hinges of the 20 specimens of D. bavaricum at his disposal were highly variable. He further claimed that D. carinatum was almost equivalve, while D. bavaricum was strongly inequivalve. Both statements, however, are misleading. In the right valve figured by Gemmellaro, designated as the lectotype herein, the upper part of the large posterior cardinal tooth is broken – which accounts for the main ‘difference’ in dentition compared to Diceras bavaricum of Boehm (1881). Secondly, Gemmellaro’s (1875) figured type material consists of one left valve and one right valve, which admittedly are of approximately the same size, but are not part of the same individual; their hinges do not match (Fig. 4). Consequently, it is impossible to assess to which degree the specimens from Sicily are equivalve. Furthermore, both specimens with closed valves figured by Boehm (1881, pl. 25, fig. 4, pl. 26, figs 1, 2 and pl. 27, figs 3, 4) are only slightly to moderately inequivalve. Moreover, the size and degree of uncoiling of the attached left valve of all epidiceratids varies greatly, depending on the substrate and immediate surroundings of an individual. Upon comparison of the type material of both Diceras carinatum and D. bavaricum, we revert to Gemmellaro’s (1876) opinion and consider D. bavaricum a junior subjective synonym of D. carinatum. The three specimens from Plagne, Département Ain, France, assigned to Epidiceras bavaricum by Skelton (1999: pl. 1, figs 4–6), also belong to Epidiceras carinatum (Gemmellaro, 1875).

Epidiceras acutum (Boehm, 1883) from Štramberk in Czech Republic is the only species of Epidiceras that has a similarly carinate shell, but the carina is sharper; as a result, the anterior and posterior parts of each valve meet at a lower angle than in E. carinatum. Moreover, the anterior shell margin seems less straight, and the commissure thus less clearly D-shaped than in the latter. Given that Boehm (1883) only figured a single specimen with closed valves (which unfortunately has been lost), both the morphologic variability and internal characters of E. acutum are unknown, and it remains unclear whether it should be treated as a junior subjective synonym of E. carinatum or not.

Several large left valves from Saal show remnants of ornament consisting of radial costellae, which are clearly restricted to the anterior slope of the shell, up to near the crest of the carina. Behind the carina, the outer calcitic shell layer is smooth, with fine growth lines only (Fig. 8G).

Tithonian strata of Palermo region, Sicily, Italy (Gemmellaro 1875). Late Kimmeridgian strata of Valfin lès Saint Claude, Departement Jura, and the area of Oyonnax, Departement Ain, France (Skelton 1999; personal observation S.S.). Late Kimmeridgian Kelheim Member, Frankenalb Formation; Kelheim region, Bavaria, Germany (Boehm 1881; herein).

Saalidiceras syllai gen. et sp. nov., by original designation herein.

Moderately large (Holotype: L = 63 mm; H = 85 mm), distinctly inequivalve epidiceratid with regularly coiled valves. Commissure crescent in outline. Two strong, broadly rounded, highly raised keels at the endpoints of the crescent enclose a broad, evenly rounded anterior trough. Coiling axes of both valves directed steeply downward-inward and slightly backward. Left valve with approximately 1.75 whorls, resembling a corkscrew in shape. Attachment surface indistinct, very small. Right valve with half a whorl only, forming a rapidly widening, incurved anterior trough. Shell interior, including hinge, unknown.

From the type locality, Saal an der Donau, and the genus name ‘Diceras’, indicating its placement in the basal Hippuritida.

The hinge of Saalidiceras gen. nov. is unknown, because the holotype and the three paratypes of Saalidiceras syllai gen. et sp. nov. are preserved with contiguous valves, and the additional specimens are strongly corroded fragments. Nevertheless, we think the assignment to the Epidiceratidae is well justified. The two valves are distinctly unequal in size, with the left, attached valve being significantly larger than the right one. Both valves are coiled, the left one attaining nearly two whorls, while the right one attains not more than two thirds of a whorl. Furthermore, the ligament is relatively long and external, sitting on elongate nymphs.

As for genus.

In honour of the late Jürgen Sylla, amateur fossil collector from Sauerlach, Upper Bavaria, Germany, who collected the holotype and one of the paratypes.

Holotype, specimen with contiguous valves; BSPG 2016 XXI 1366. Paratype 1, specimen with contiguous valves; BSPG 2016 XXI 1368. Paratype 2, specimen with contiguous valves; BSPG 2022 X 5. Paratype 3, specimen with contiguous valves; BSPG 2022 X 6. Two fragments of left valves (not figured); BSPG 2016 XXI 1367, BSPG 2022 X 7. Poorly preserved right valve (not figured); BSPG 2016 XXI 369.

Frankenalb Formation, Kelheim Member; late Kimmeridgian; Saal an der Donau, Lower Bavaria, Germany.

Shells moderately large for epidiceratids; total shell length of 63 mm and shell height of 85 mm in holotype; distinctly but moderately inequivalve. Commissure crescent in outline, 63 mm high in holotype, with two massive, very prominent, broadly rounded keels joining the endpoints of the crescent, enclosing a broad anterior trough with evenly rounded slopes. Coiling axes steeply inclined; directed downward-inward and slightly backward. Left valves of holotype and paratype 3 with approximately 1.75 whorls, resembling a corkscrew in shape. Attachment surface indistinct, very small in holotype; not preserved in paratypes. Right valve only with half a whorl or slightly more, forming a rapidly widening, incurved anterior trough. Shell interior, including hinge, unknown.

With its corkscrew-shaped left valve and the curved, trough-shaped right valve, Saalidiceras syllai is so entirely distinct from any other Jurassic rudists that it cannot be confused. The four double-valved specimens illustrated in Figs 9, 10 display a certain amount of variability with regard to the dimensions of the commissure relative to shell size, and also with regard to the elongation of the left valve. In paratype 3, the left valve is distinctly more openly coiled than in the other specimens; at the same time, its commissure is shorter and less high (Fig. 10D).

While the bizarre morphology of the species is well preserved in the type series, its hinge and musculature remain unknown, and all specimens are affected by diagenesis, where parts of the original shell are decayed to chalky matter that is barely distinguishable from the surrounding matrix, making preparation a difficult task. The calcitic outer shell layer, which we assume was present, is no longer preserved; as a result, it is unknown whether the shell surface was ornamented or not.

Not having observed the specimens in situ, we can only speculate about their life style and potential functional aspects of their peculiar morphology. The indistinct or very small attachment surface seems unsuited to keep such large shells upright. Possibly, part of the left valve was submerged in soft sediment, anchoring the animal in the same way a manmade screw would do. Alternatively, adults may have been edgewise recliners sensuSeilacher (1984), resting either on their anterior side, in a similar fashion as suggested for Opisoma by Seilacher (1990) and Aberhan and Hillebrandt (1999), or on their posterior keels, as advocated for the same taxon by Posenato et al. (2013).

Only known from the type locality.

Diceras valfinense Boehm, 1881, by original designation.

For distinction from Epidiceras and Heterodiceras, see remarks under Epidiceras above.

Shell up to 60 mm high, with broadly sub-circular, ventrally faintly to distinctly trilobate commissure. Left valve short-conical in shape, expanding quickly during ontogeny. Right valve operculum-shaped. Both valves coiled, with 1.5 to 2 whorls clearly traceable. Right valve hinge with small knob-like tooth 3a and prominent, obliquely triangular tooth 3b. Left valve hinge with distinct, sharply arcuate tooth 2. Right valve adductor muscle scars situated on downward extensions of hinge plate. Left valve adductor muscle scars situated on elevated, inwardly inclined buttresses.

Syntypes of Diceras valfinenseBoehm (1881) (Kimmeridgian limestones of Valfin lès Saint Claude, Departement Jura, France): double-valved specimen (not figured; BSPG 1871 VII 1004); left valve (pl. 25, fig. 2a; BSPG 1871 VII 752), designated lectotype herein; bouquet of left valve and double-valved specimen (not figured; BSPG 1871 VII 1005); right valve (pl. 25, fig. 2b, BSPG 1871 VII 1002), left valve (not figured; BSPG 1871 VII 1003).

Originals and reference material of Diceras muensteri (Goldfuss, 1837) sensuBoehm (1881) (late Kimmeridgian; Kelheim Member, Frankenalb Formation; Kelheim area, Bavaria, Germany): left valve (pl. 25, fig. 3a; BSPG 1872 XI 571); left valve (not figured; BSPG 1872 XI 573); left valve (not figured; BSPG 1872 XI 574) (?, specifically indeterminate); right valve (pl. 25, fig. 3b; BSPG 1872 XI 572); internal mould of double-valved specimen (not figured; BSPG 1872 XI 575) (?, specifically indeterminate).

Numerous specimens from Saal, of which a representative suite was selected for this study. Twenty-six double-valved specimens (BSPG 2016 XXI 15a, b [valves isolated], 73, 1399, 1400, 1413, 1414 [two individuals], 1519; BSPG 2019 IV 4; BSPG 2020 CIII 2, 4, 5, 11, 15, 18, 19, 21, 25, 33 [overgrown by left valve], 34–40). Questionably, one internal mould of a double-valved specimen (BSPG 2016 XXI 1393). Nine left valves (BSPG 2016 XXI 1395, 1409–1411; BSPG 2020 CIII 1 [on branching coral], 12, 13, 16, 41). Fifty-one right valves (BSPG 2016 XXI 74, 1371–1385, 1387–1390, 1392, 1396 [overgrown by Hypelasma fritzlangi], 1412, 1415, 1521, 1524–1543; BSPG 2019 IV 13; BSPG 2020 CIII 3, 7–10).

Late Kimmeridgian strata of Valfin lès Saint Claude, Departement Jura, France.

Small epidiceratid with up to 60 mm high shells. Outline of commissure broadly sub-circular, ventrally faintly to distinctly trilobate; the largest, central lobe tracing the position of the body cavity; the smallest, anterior lobe and the slightly larger posterior one tracing the outer margins of the adductor muscle scars. Left valve short-conical in shape, with high whorl expansion rate. Right valve small, shaped like an operculum, with umbo not extending above commissure. Both valves coiled, with 1.5 to 2 whorls clearly traceable. Right valve hinge with small knob-like tooth 3a and prominent, broad, oblique, dorsally bending, obliquely triangular tooth 3b, extending to or projecting slightly above commissure; rather sharply bounded lunate recess on its anterior side marking the extent of rather deep socket 2’. Left valve hinge with prominent, sharp-crested, elongate-arcuate tooth 2, flanked by small, pit-like socket 3a’ ventrally, and large, deep, arcuate socket 3b’ posterior-dorsally. Right valve adductor muscle scars situated on downward extensions of hinge plate; the anterior one inserting just below and anterior to tooth 3a; the posterior one significantly larger, lunate in outline, extending downward from behind tooth 3b. Left valve adductor muscle scars situated on elevated, inwardly inclined buttresses, more or less at the level of the hinge plate.

Plesiodiceras valfinense and Plesiodiceras? muensteri (Goldfuss, 1837), which is discussed below, are in the same size category and have similar overall proportions, including a reduced, operculum-like right valve. However, compared to P.? muensteri, P. valfinense gains much more quickly in diameter or, in gastropod terms, has a greater whorl expansion rate, resulting in a short-conical left valve. The umbonal area is rarely well-enough preserved to establish the exact number of whorls developed during ontogeny, but 1.5 to 2 whorls are distinct in the specimens of P. valfinense studied by us. In contrast, the lectotype of P.? muensteri shows less than 1.5 whorls.

In specimens growing without space restrictions, the commissure is broadly equidistant in P. valfinense, with its dorsal part generally well rounded, with a slight incurvature near the umbo in most specimens. The remainder of the commissure is arranged in three faintly indicated lobes – a central, largest, ventral one tracing the body cavity, and anterior and posterior ones, which trace the adductor muscle scars – the latter usually the larger of the two. In P.? muensteri, the commissure is higher than long and markedly reniform; its shape is not determined by soft body anatomy.

When Boehm (1881) compared Diceras muensteri and his new Diceras valfinense, he only discussed his own material from Kelheim, and specimens from Cerin [‘Cirin’ in Boehm 1881] and Valfin lès Saint Claude in France; he made no direct reference to Goldfuss’ (1837) syntypes of Chama muensteri. Critically also, Goldfuss’ material comprises no single valves, which makes comparison of internal characters impossible. Boehm (1881) found that the distinguishing character of the specimens from Valfin was the presence of a ledge bounding the left-valve posterior adductor muscle scar anteriorly. However, he also noted that the precise shape of the hinge teeth, as well as the arrangement of buttresses and muscle scars in the specimens from Valfin varied greatly. Seven of Boehm’s (1881) specimens treated under the names Diceras valfinense and D. muensteri, including the four shells figured by him, are preserved in the collections at Munich. We regard these as conspecific, and the variability discussed by Boehm (1881) as intraspecific; the specimens are all assigned to Plesiodiceras valfinense here. Notably, an even greater range of morphologies in topotypic P. valfinense was illustrated by Loriol (1888).

Diceras sanctaeverenae Thurmann, 1853 from the Oxfordian of the ‘Ste Vérène limestone’ north of Solothurn, Switzerland, is based on an unpublished manuscript name coined by Gressly. The species was compared with Plesiodiceras? muensteri by Thurmann (1853) and regarded as distinct, but was later synonymised with P.? muensteri by Greppin (1870: 99). Boehm (1881) had no clear opinion on its validity. Given that the species is only documented by Thurmann’s sketchy drawings (Thurmann 1853; Thurmann and Étallon 1863), its status is impossible to assess without revisiting type or topotypic material, and we regard it as a nomen dubium.

The drawings of Diceras monsbeliardense Contejean, 1860 (originally spelled monsbeliardensis; incorrect gender agreement), from the Kimmeridgian of Montbéliard, Departement Doubs, France, depict a double-valved specimen, likely a Plesiodiceras, with ornament of radial striae on both valves. With regard to its general morphology, i.e. a low valve expansion rate and reniform rather than trilobate commissure, this specimen rather resembles Plesiodiceras? muensteri than P. valfinense. However, Contejean’s (1860) figures are generally not photorealistic and should be interpreted with care, particularly also because no description of the species was provided. Diceras monsbeliardense is thus only tentatively assigned to Plesiodiceras? muensteri below.

Pchelintsev (1959) described seven new species of Plesiodiceras from Oxfordian (‘Rauracian’) strata of the Crimean Mountains (Yayla Mountains), Crimea, Ukraine. Pchelintsev’s (1959) species are mainly distinguished by subtle variations of their growth form, which we think are intraspecific. Six of his new species, Plesiodiceras subvalfinense, P. uzuntaschi, P. orthogonale, P. yailense, P. kuruuzense and P. koreizense are based on relatively well-preserved type material, consisting exclusively of articulated specimens, thus no internal characters are known. However, they all show the relatively slowly expanding left valves typical of Plesiodiceras? muensteri, together with a more or less distinctly reniform commissure. Some specimens preserve fine radial striae on both valves. A specimen of Plesiodiceras uzuntaschi was refigured by Yanin (1989: pl. 6, fig. 1). These six species are treated as junior subjective synonyms of P.? muensteri below. The holotype of the monotypic Plesiodiceras acutiforme Pchelintsev, 1959 is relatively poorly preserved and difficult to interpret; it is considered a nomen dubium at present. The single early Kimmeridgian specimen of Plesiodiceras aff. monsbeliardense (Contejean) figured by Pchelintsev (1959) needs revisiting to establish its identity.

The four species of Plesiodiceras from the Pamir described by Pchelintsev (1960) are relatively poorly preserved and illustrated. Most specimens seem to represent only left valves, and no internal characters are known. The holotype by monotypy of Plesiodiceras orientale Pchelintsev, 1960 (pl. 2, fig. 1a, b) is double-valved and has subequal valves, thus certainly is not a Plesiodiceras. Based on the occurrence of Plesiodiceras, Pchelintsev (1960) proposed an Oxfordian (‘Lusitanian’ or ‘Rauracian’) age for this fauna. However, the gastropod Trochacteon pamiricum Pchelintsev, 1960 (pl. 2, figs 3–5), described from the same stratum, would rather indicate a Late Cretaceous age for the assemblage (Kollmann 2014), suggesting that the ‘Plesiodiceras’ may be requieniids. Given their poor preservation, paired with an inconclusive age assignment, Plesiodiceras angustum Pchelintsev, 1960, Plesiodiceras orientale Pchelintsev, 1960 and Plesiodiceras capuliforme Pchelintsev, 1960 are here declared nomina dubia.

Plesiodiceras sulejoviense Karczewski, 1969 from the upper Oxfordian of Sulejów, Łódź Voivodeship, Poland, is distinguished by its arrangement of muscle scars and hinge elements, which were described but not figured. Only a double-valved specimen is shown (Karczewski 1969: pl. 25, fig. 14a–c). With regard to its general morphology, including the quickly expanding left valve and the shape of the sub-circular commissure, the figured specimen resembles Plesiodiceras valfinense. However, its right valve is more openly coiled than we have observed in the latter. Numerous small specimens of Plesiodiceras figured by Karczewski (1969) under the names Plesiodiceras yailense Pchelintsev, 1960, P. muensteri Goldfuss, 1837 and P. orthogonale Pchelintsev, 1960 include material resembling P.? muensteri (e.g. pl. 25, figs 5, 10–12), but also specimens close to P. valfinense (e.g. pl. 25, figs 8, 9), and should be restudied to make definite assignments.

The Plesiodiceras shells recently described from Saudi Arabia by Schneider and Al-Mojel (2022) are of approximately the same age as the material from Saal. However, due to their limited preservation, an assignment to one of the species treated herein is impossible.

Late Kimmeridgian; Valfin lès Saint Claude, Departement Jura, and Oyonnax and Cerin, Departement Ain, France (Boehm 1881 [as Diceras valfinense and D. muensteri]; Loriol 1888; Dechaseaux 1941 [as Heterodiceras (Plesiodiceras) muensteri]; Skelton 1999 [as Plesiodiceras muensteri]; personal observation S.S.). Late Kimmeridgian; Kelheim Member, Frankenalb Formation; Kelheim region, Bavaria, Germany (Boehm 1881 [as Diceras muensteri]; herein).

Shell approximately 60 mm high, with reniform commissure. Left valve coiled-cylindrical in shape, expanding rather slowly during ontogeny, with distinct anterior sulcus. Right valve operculum-shaped. Both valves coiled, with less than 1.5 whorls clearly traceable. Interior shell characters unknown.

Syntypes of Chama muensteri Goldfuss, 1837 (late Kimmeridgian; Kelheim Member, Frankenalb Formation; Kelheim area, Bavaria, Germany): Double-valved specimen (pl. 138, fig. 7a, b; BSPGAS VII 2075), designated lectotype herein. Two double-valved syntypes with preserved shells (not figured; BSPGAS VII 2076, BSPGAS VII 2077). The fourth syntype is an internal mould of a double-valved specimen (not figured; BSPGAS VII 2078).

“Occurs in the Portland stone near Regensburg.” (Goldfuss 1837: 205; translated from German). Late Kimmeridgian; Kelheim Member, Frankenalb Formation; near Regensburg, Bavaria, Germany.

Only one double-valved specimen of Chama muensteri was figured by Goldfuss (1837: pl. 138, fig. 7a, b), and no other specimens were explicitly mentioned in his description of the species. However, the same collection tray contains another three double-valved specimens, two of them with reasonably well preserved shells and the third one an internal mould. We thus treat all four specimens as syntypes, and designate the figured specimen, BSPGAS VII 2075, as the lectotype.

In the lectotype, the umbo of the left valve including the attachment surface is broken. The right valve is distinctly coiled but operculate, and the commissure is distinctly higher than long, with a marked anterior sinus. The anterior sulcus in the left valve is relatively deep, separating strongly bulging upper and lower parts of unequal size. Of the two additional shell-preserved specimens, BSPGAS VII 2076 has an incompletely preserved left valve and a slightly ambiguous commissure, but most likely represents P.? muensteri. In contrast, BSPGAS VII 2077 clearly shows the characters of Plesiodiceras valfinense, most notably the quickly expanding, short-conical left valve. The internal mould, BSPGAS VII 2078, is too poorly preserved to be specifically assigned.

As discussed above, we consider Plesiodiceras? muensteri as different from Plesiodiceras valfinense, based on its reniform commissure and anterior sulcus, and the more slowly expanding left valve. Given that its interior shell characters remain unknown, its assignment to Plesiodiceras is tentative. Hypelasma fritzlangi sp. nov. described below also lacks the anterior sulcus and distinctly reniform commissure.

Plesiodiceras? muensteri (Goldfuss, 1837) does not occur at Saal. Its exact type locality ‘near Regensburg’ is unknown, but cannot have been far from Saal. Given that it is also the earliest potential Plesiodiceras described and the lectotype had never been photographed, it seems appropriate to discuss this species in the present context.

Late Kimmeridgian; Kelheim Member, Frankenalb Formation; Regensburg region, Bavaria, Germany (Goldfuss 1837). Tentatively: Kimmeridgian; Montbéliard, Departement Doubs, France (Contejean 1860). Oxfordian; Crimean Mountains (Yayla Mountains), Crimea, Ukraine (Pchelintsev 1959; Yanin 1989).

Requieniidae are diagnosed as strongly inequivalve, with a trochospirally to helicospirally coiled left (attached) valve, mainly differing from Epidiceratidae in the smaller angle between the coiling axis and the commissural plane; the latter, as a result, intersects the earlier whorls in adult requieniids (Gourrat et al. 2003; Scott et al. 2010; Skelton 2013; Fig. 3B). The much smaller right valve is poorly inflated or entirely flat. Scott et al. (2010) reported a thicker outer calcitic shell layer as an additional diagnostic character. However, in Jurassic representatives, the outer shell layer seems to be equally thin to that in Epidiceratidae, and Plesiodiceras also has a similarly reduced, operculum-like right valve as most Requieniidae, making the coiling geometry the only reliably diagnostic character.

Matheroniinae are diagnosed as requieniids with a low-spirogyrate left valve and a slightly convex right valve, as opposed to a higher-spired left valve and a flat to faintly convex right valve in Requieniinae (Scott et al. 2010). The left valve ‘myophores are expanded plates on the valve wall’ and the right valve ‘posterior myophore plate extends from the cardinal platform’ (Scott et al. 2010: 451–452).

Hypelasma colloti Paquier, 1898, by monotypy.

Hypelasma was considered monospecific and diagnosed as very inequivalve by Paquier (1898), with a non-spirogyrate right valve and a much larger left one with a prominent umbo. Dentition was described as rather robust, with the posterior socket 3b’ occupying a major part of the hinge plate in the left valve. In the right valve, the posterior adductor muscle inserts on a downward extension of the hinge plate, while it inserts on a weak myophore in the left valve.

This diagnosis was emended by Gourrat et al. (2003), who included a second species, Hypelasma salevensis (Favre in Joukowsky and Favre 1913), in the genus, to comprise small to medium-sized requieniids with slowly expanding left valves with a prominent, projecting umbo. An initially rather acute but progressively more rounded carina in the left valve separates a smaller flattened anterior flank, from a well-rounded posterior one. The right valve is posteriorly carinate. In the left valve, the posterior adductor muscle inserts on a small but distinct posterior myophoral ledge below the hinge plate; in the right valve it inserts on the downward extended hinge plate. The anterior adductor muscle inserts on the shell wall in both valves.

Here, we include a third, new species in Hypelasma, which shares all of these characters apart from the pronounced carinae in either valve.

Hypelasma with bluntly angled rather than distinctly carinate left valve and non-carinate right valve, and with sub-circular to sub-ovate commissure. Right valve cardinal tooth 3a small in juveniles, possibly reduced in adults; corresponding left valve cardinal socket 3a’ absent.

In honour of Fritz Lang, amateur fossil collector from Hirschaid, Upper Franconia, Germany, who collected part of the type series.

Holotype, left valve; BSPG 2016 XXI 1408. Paratype 1, left valve; BSPG 2016 XXI 1405. Paratype 2, specimen with contiguous valves; BSPG 2019 IV 5. Paratype 3, left valve; BSPG 2016 XXI 1397. Paratype 4, juvenile specimen with contiguous valves; BSPG 2019 IV 9. Paratype 5, juvenile left valve; BSPG 2019 IV 6. Paratype 6, juvenile right valve; BSPG 2019 IV 7. Paratype 7, juvenile right valve; BSPG 2019 IV 8. Paratype 8, double-valved specimen; BSPG 2016 XXI 1396. Two more left valves (not figured); BSPG 2016 XXI 1404, BSPG 2020 CIII 14. Three more right valves (not figured); BSPG 2016 XXI 1406, BSPG 2016 XXI 1407, BSPG 2019 IV 10.

Frankenalb Formation, Kelheim Member; late Kimmeridgian; Saal an der Donau, Lower Bavaria, Germany.

Shell up to 50 mm high. Commissure sub-circular in outline in juveniles; higher than long, sub-ovate, with a blunt ventral angulation in adults. Left valve expanding rather slowly during ontogeny, with a coiled-cylindrical rather than conical adult portion, its anterior, poorly inflated flank portion separated from the more distinctly rounded posterior one by a broad, rounded bulge. Right valve operculum-shaped, with umbo not extending above commissure. Both valves coiled, with generally less than 1.5 whorls clearly traceable. Ornament of dense, very fine radial striae preserved in some specimens (Fig. 15C, D). Right valve hinge with small, short-tongue-shaped anterior cardinal 3a, and prominent long-tongue-shaped posterior cardinal 3b extending well above commissure, often for nearly half its length. Left valve hinge with distinct but small nodular cardinal tooth 2, situated anterior to median of shell, at anterior end of oblique hinge plate emerging from body cavity. Socket 3b’ wide and deep, extending behind cardinal tooth 2. Socket 3a’ not developed. Adductor muscle scars shallow. Left valve anterior adductor muscle scar placed on sloping inner shell wall. Left valve posterior adductor muscle scar supported by weak to distinct buttress. Right valve anterior adductor muscle scar placed on shell wall; faintly rimmed posterior-ventrally. Right valve posterior adductor muscle scar situated on shell wall, distinctly below onset of cardinal 3b.

Hypelasma fritzlangi sp. nov. most significantly differs from both Hypelasma colloti Paquier, 1898 and Hypelasma salevensis (Favre in Joukowsky and Favre 1913) in the absence of acute carinae. In its left valve, a broad, rounded, rather central bulge is developed, which separates a rather poorly rounded anterior shell portion from a better-rounded posterior one. Correspondingly, the angle between the coiling axis and the commissural plane in H. fritzlangi is often larger than in the other two species, and its growth habit is less pronouncedly requieniid. Its right valve is non-carinate.

Cardinal tooth 2 is less prominent in H. fritzlangi than in the other two species and socket 3a’ is not developed in adults. In the juvenile left valve of Fig. 14A, a minute recess directly behind tooth 2 potentially represents 3a’ (not labeled). As a result, tooth 3a must either have been reduced in adults, or its ventral side must have directly bordered the body cavity. Unfortunately, no adult isolated right valves are available to assess this condition.

From Plesiodiceras valfinense, Hypelasma fritzlangi is distinguished by its much more slowly expanding whorls and its generally requieniid growth habit. Radial ornament, when preserved, is finer in H. fritzlangi than in P. valfinense. Tooth 2 is relatively prominent and arcuate in P. valfinense, while rather small and nodular in H. fritzlangi. Tooth 3b is obliquely triangular and not or barely projecting beyond the commissure in P. valfinense; in contrast, it is elongate tongue-shaped in H. fritzlangi and extends well above the commissure.

Both adductor muscles in P. valfinense insert on downward extensions of the hinge plate in the right valve and on raised, inwardly inclined buttresses in the left valve. In H. fritzlangi, the separation of hinge plate and inner shell wall in the right valve is less distinct than in P. valfinense, and while both adductor muscles insert in a similar position, the posterior one is in its entirety positioned below tooth 3b. In the left valve of H. fritzlangi, the posterior adductor muscle inserts on a more or less prominent buttress, while the anterior one sits rather directly on the inner shell wall.

Only known from the type locality.