Raphistomatidae Koken, 1896

Ebbestad, Jan Ove R. & Cope, John C. W., 2021, A low diversity Sinuites gastropod community from the Floian, Early Ordovician, of South Wales, Acta Palaeontologica Polonica 66 (2), pp. 319-335 : 326-331

Family Raphistomatidae Koken, 1896

Remarks.—In his clade analyses, Wagner (2002) recognized “ceratopeoids” as one of three major subclades within the “euomphalinaes” (note that these names were used informally, see Wagner 2002: 16). Wagner (2002) remarked that the “euomphalinaes”, were close to the superfamily Euomphaloidea White, 1877 , as defined by Morris and Cleevely (1981, as Euomphalaceae), who included the families Ophiletidae Knight, 1956 , Helicotomidae Wenz, 1938 , Euomphalidae Koninck, 1881 , and Omphalotrochidae Knight, 1945 .

The “ceratopeoids” of Wagner (2002) encompassed two subclades, the “raphistomatids” (with the subgroups “lesueurillines”, “scalitines” and “holopeines”) and the “helicotomids” (with the subgroups “ophiletinines” and “euomphalopterines”) (see Wagner 2002: table 3). Species with the genus Ceratopea Ulrich, 1911 , were included by Wagner among the early “ceratopeoids” recognized by the strong basal growth lines, a strong flange­like peripheral carina, an upper whorl surface that is rounder than the lower, and a basal carina. The sigmoidal shape of the umbilical growth lines was considered a feature of several early “ceratopeoids” encompassing species in several genera.

Traditionally, the Euomphaloidea encompassed three families in Knight et al. (1960) later increased by two in Bouchet et al. (2005, 2017), the Helicotomidae Wenz, 1938 , Euomphalidae Koninck, 1881 , and Omphalotrochidae Knight, 1945 , respectively, and the Omphalocirridae Wenz, 1938 , and Lesueurillidae Wagner, 2002 . In the taxonomic revision of Wagner (2002) Ceratopea was placed with the family Raphistomatidae in the superfamily Eumophaloidea . On the other hand, Bouchet et al. (2005, 2017) placed the family with basal gastropods unassigned to a superfamily, and recognized Ceratopeidae Yochelson and Bridge, 1957 , as a subjective junior synonym of the Raphistomatidae . Originally Ceratopeidae was erected to encompass taxa with a Ceratopea ­like operculum ( Yochelson and Bridge 1957).

Bandel and Frýda (1998) proposed the Subclass Euomphalomorpha for Palaeozoic taxa within the Euomphaloidea . The group was recognized mainly by the small egg­shaped and openly coiled protoconch, which is known primarily in Late Palaeozoic members. Morphologically the euomphalomorphid protonchs are more complicated and differ markedly from the simple protoconch morphology of the Archaeogastropods”. Protoconchs of the early “ceratopeoids” are unknown and the placement of Ceratopea in the Raphistomatidae relies solely on the teleoconch analysis by Wagner (2002). Classification above family level is uncertain (see Wagner 2002: 21), and therefore left tentative. See also discussions in Frýda and Rohr (2004), Frýda et al. 2008), and Frýda (2012).

Genus Ceratopea Ulrich, 1911

Type species: Ceratopea keithi Ulrich, 1911: 665 , by original designation, figured in Bassler (1909: 157, pl. 20: 3) as operculum of undetermined gastropod, from the Lower Ordovician Cassinian (Floian, Fl2?, Lower Ordovician) Beekmantown Group at Wytheville, Wythe County, Virginia, USA.

Remarks.— Ceratopea was originally established by Ulrich 1911) based on the horn­shaped calcareous operculum, which is often silicified ( Yochelson 1979). Only three associations of partial or complete shells with an operculum in places are known, all attributed to C. unguis Yochelson and Bridge, 1957 ( Yochelson and Bridge 1957; Yochelson and Wise 1972; Rohr et al. 2004), while isolated opercula are common ( Yochelson 1979). Yochelson and Bridge (1957) pointed out that the question of monophyly of Ceratopea remains problematic as long as the type species is only known from the operculum. The wider scope of the genus presented by Wagner (2002) was also acknowledged as paraphyletic. Wagner (2002) suggested that taxa older than early Middle Ordovician that have been attributed to Raphistomina Ulrich and Scofield, 1897 , should belong with Ceratopea .

The Middle to Late Ordovician genus Pararaphistoma Vostokova, 1955 , has a somewhat Ceratopea ­like adult shell, which lead Yochelson and Copeland (1974) to suggest that Pararaphistoma was a junior synonym of Ceratopea . Wagner 2002) showed that Pararaphistoma is a derived lesueurillid genus, with a marked ontogenetic change in morphology from a lesueurillid morphology in early ontogeny to a Ceratopea ­ like morphology in later ontogeny, mainly due to clockwise rotation of the aperture. The genus also possesses a slit at the apex of the sinus, which is lacking in Ceratopea .

Species of Pararaphistoma have been reported from the Tremadocian–Floian (Lower Ordovician) of the Montagne Noire in France (Yochelson in Babin et al. 1982) and the Canning Basin of Australia ( Yü 1993), all attributed to the Darriwilian (Middle Ordovician) P. qualteriata ( Schlotheim, 1820) or P. vaginata Koken in Koken and Perner, 1925, of Baltica. The latter is named from France and Australia, and the material from these two areas appears quite similar. The shells are flattened lenticular and widely phaneromphalus, with a rounded base, depressed whorls with upper whorl surface sloping adaxially and being gradate, and as such they differ from a Ceratopea ­ type morphology. A second species from Australia is named P. qualteriata , and is known from internal moulds ( Yü 1993). The shell appears much more Ceratopea ­like in the overall shape and convexity, which is of course also applicable to the shape of P. qualteriata from Baltica. In general, the internal moulds do not allow any closer comparison.

At least 40 species of Ceratopea have been recognized, ranging from the Tremadocian (Early Ordovician) to the Darriwilian (Middle Ordovician; Cullison 1944; Yochelson 1964; Yochelson and Bridge 1957; Yochelson and Copeland 1974; Yochelson and Peel 1975; Wagner 2002; Rohr et al. 2015 and references in these). It is mainly a Laurentian genus, and opercula are common in many Lower Ordovician deposits of USA, Canada, and North and East Greenland in particular but the genus is also recognized in Svalbard and Scotland ( Yochelson 1964), the Darriwilian of the Farewell Terrane of Alaska ( Rohr and Blodgett 1988; Wagner 2002), the Dapingian and Darriwilian (Middle Ordovician) of the Argentinian Precordillera ( Bertero 2007; Ebbestad et al. 2013), and the Darriwilian of North China ( Ebbestad et al. 2013; data from the Paleobiology Database compiled by Peter Wagner).

Ceratopea ? moridunensis sp. nov. Figs. 6 , 7 .

ZoobankLSID: urn:lsid:zoobank.org:act:564B114F­A213­458A­9423­17585217B21F

Etymology: From Moridunum the Latin name for Carmarthen, South Wales, where the Llangynog Inlier is situated.

Type material: Holotype ( NMW 2017.15 G.71, Fig. 6A ), large specimen with dorsal side preserved and preserving the convexity . Paratypes (45 specimens, NMW 2017.15 G.72–113), internal and external moulds, all from the type locality. One slab has four specimens ( Fig. 7 ). Type locality: Dan­lan­y­Castell quarry, Carmarthen, Wales, UK .

Type horizon: Merlinia selwynii Trilobite Zone, Moridunian regional Stage (lower Floian Stage, Fl 1) .

Material.— Type material only.

Diagnosis.—A medium­sized species of Ceratopea with a prominent peripheral flange­like carina, base without circumbilical carina, and a steep inner margin without thickening.

Description.—Shell lenticular, phaneromphalus with 5–6 whorls of uniform expansion. Aperture tangential. Initial whorls probably abandoned and plugged. Largest shell close to 30 mm across. Spire low with large pleural angle, estimated to be>120°. Whorls overlap at or just below periphery, giving a slightly gradate shell with incised sutures. Upper whorl surface arched at suture, evenly convex but becoming concave near periphery which is carinated, flange­like, rounded, and prominent but narrow. Inner margin of aperture straight, seemingly without thickening, steep with an angle of about 30–35° relative to axis of coiling. Base evenly but sharply rounded, without a carina. Lower surface gently convex, sloping at about 45° relative to axis of coiling. Transition to peripheral carina abrupt. Peripheral sinus symmetrical, V­shaped and wide. Depth unknown. Transition to peripheral carina and presence/absence of notch unclear. Growth lines on upper surface gently prosocline. Growth lines on lower whorl surface gently prosocyrt with a slightly stronger abapertural curvature near base, continuing with a concave curvature on the inner margin giving an overall sigmoidal shape from the lower whorl surface to the umbilicus and an excavated shallow sinus at the innermost part of the umbilicus. Shell thin, seemingly without any thickening even at the base. Ornamentation consists of fine comarginal growth lines on upper and lower whorls,

but that form slightly uneven comarginal crenulations on the lower surface.

Remarks.—The apex whorls are not preserved in any specimen of Ceratopea ? moridunensis sp. nov., and it is likely that these were abandoned and possibly septate or filled with (now dissolved) shell material, a condition that is found in many Palaeozoic gastropod groups ( Yochelson 1971; Wagner 2002; Cook et al. 2015). Because of this condition, estimation of the number of whorls is difficult but certainly exceeds five.

Ornamentation and the shape of the comarginal growth lines are seen in several specimens ( Fig. 6A, B, F–I ), but the sigmoidal shape across the lower whorl surface and umbilicus is best seen in Fig. 6G 2 , H 1 . Several specimens have a markedly wrinkled surface, seen on both the dorsal and ventral sides of the shell (for instance Fig. 6D, G, I ). The wrinkling of the upper surfaces is chaotic and unstructured while that of the lower surface forms prominent comarginal bands. The overall wrinkling may be partly an artefact of preservation and its effect on a very thin shell, but seems to be amplified on the lower surface by the already underlying comarginal irregularities of the ornamentation. The growth lines on the base are fine.

The rounded peripheral carina extends prominently outwards, and it seems to be present already in early ontogeny ( Fig. 6C ), although the initial spire is not preserved in any specimen. No growth lines are observed crossing the periphery, but it is assumed that they follow the path of the growth lines above and below the keel. There are no bordering lirae on the keel, so that a lunuate selenizone is not formed, but preservation makes it difficult to see whether there is a notch or slit present or not. Both the holotype ( Fig. 6A ) and NMW 2017.15G.72 ( Fig. 6B ) preserve the outer lip and demonstrate the gentle prosocyrt curvature, but show no indication of a notch/slit at the periphery. In the holotype the carina seems to project slightly upwards ( Fig. 6A 2 ) but owing to compaction it is unclear whether this is a preservational artefact or not. The specimen in Fig. 7A 1 , A 3 shows a ventral view, where the peripheral carina is visible. There seem to be an abrupt transition to the carina, without any change in convexity of the lower whorl surface.

Wagner (2002) commented that Ceratopea previously was diagnosed only on the operculum morphology, although Yochelson and Copeland (1974) provided a diagnosis combining shell and operculum characters. The broader and more detailed redefinition of the genus by Wagner (2002) suggested that in addition to early “ceratopeoid” synapomorphies (strongly curved sinuses, a peripheral band that points adapically, sigmoidal innermost margin and a thick basal carina), the genus is defined by a lenticular aperture, flange­like periphery, and a rounded (convex) upper whorl surface.

The Llangynog species is best placed with Ceratopea due to the rounded upper whorl surface, the apparently deep V­shaped peripheral sinus leading into the extended peripheral flange­like carina, the steep inner margin and the sigmoidal shape of growth lines creating a shallow sinus at the innermost part of the inner margin. The lack of a basal carina is perhaps the main difference of C.? moridunensis sp. nov. relative to other species of Ceratopea . Species like C. canadensis ( Billings, 1865) and C. buttsi Yochelson and Bridge, 1957 , have coarser growth lines, a basal carina, less convexity of the upper whorl surface, and a less pronounced peripheral carina (see also Yochelson and Copeland 1974). C. unguis Yochelson and Bridge, 1957 , have fine growth lines similar to those of C.? moridunensis sp. nov., but differs in having a basal carina, a less pronounced peripheral carina, a more gradate shell and lacking the inner margin excavation (see Yochelson and Bridge 1957; Rohr et al. 2004). C. lemonei ( Flower, 1968) is similar to C.? moridunensis sp. nov. in lacking a basal carina and in the crenulation on the lower surface, but the angle of the inner margin is smaller. C. praevium ( Whitfield, 1889) differs mostly from C.? moridunensis sp. nov. by having low convexity of the upper whorl surface.

Geographic and stratigraphic range.—Ordovician; Argentina?, Canada, Greenland, Scotland, and USA.

Figures

Fig. 6. Raphistomatid gastropod Ceratopea? moridunensis sp. nov. from the Floian, Lower Ordovician Bolahaul Member of the Ogof Hên Formation, Dan­lan­y­Castell quarry, Wales, UK. A. NMW 2017.15G.71, holotype in dorsal (A1) and near apertural (A2) views. B. NMW 2017.15G.72, silicon cast in dorsal view, showing the curvature of the aperture. C. NMW 2017.15G.73, internal mould in dorsal view, showing the flange­like peripheral carina in the early whorls. D. NMW 2017.15G.74, internal mould in dorsal view, with partially preserved distorted shell. E. NMW 2017.15G.76, natural cross section, showing the shape of the lower parts and base of the whorls. F. NMW 2017.15G.75, initial whorls in dorsal view, showing their convexity. G. NMW 2017.15G.77, uncompressed partial specimen in ventral (G1) and ventral oblique (G2) views, showing shape of ornamentation and the wrinkled appearance of comarginal bands. H. NMW 2017.15G.78, silicon cast of ventral side in ventral view (H1), ornamentation and details (H2, H3), umbilical morphology (H4). I. NMW 2017.15G.79, large partial specimen in ventral view, showing the wrinkled appearance of the ornamentation.The arrow points to a bivalve. Scale bars 5 mm.

Fig.7. Raphistomatid gastropod Ceratopea? moridunensis sp.nov. from the Floian, Lower Ordovician Bolahaul Member of the Ogof Hên Formation, Dan­lan­y­Castell quarry, Wales, UK. A. NMW 2017.15G.80; A1, overview of slab with four specimens in ventral views; A2, specimen with preserved wrinkled shell (white arrow in A1); A3, specimen (black arrow inA1) showing the flange­like peripheral carina (arrowhead). Scale bars 5 mm.

Fig. 2. Carcassonellid tergomyan Carcassonnella cf. vizcainoi Horný and Peel, 1996 from the Floian, Lower Ordovician Bolahaul Member of the Ogof Hên Formation, Dan­lan­y­Castell quarry, Wales, UK. A. NMW 2017.15G.1, silicon cast, in lateral (A1), posterior oblique (A2), and ventral (A3) views. B. NMW 2017.15G.2, internal mould in lateral view. Scale bars 5 mm.

Fig. 1. A. Moridunian stratigraphy (after Fortey and Owens 1978). B–D. Simplified maps of Wales (B), the southeastern part of the country (C), and the area south of Carmarthen (D) showing the location of the study area and outcrop. E. Simplified geological map with position of the Dan­lan­y­Castell quarry (modified from Cope 1982).

Fig. 3. Sinuitid bellerophontid Sinuites ramseyensis (Hicks, 1873) from the Floian, Lower Ordovician Bolahaul Member of the Ogof Hên Formation, Danlan­y­Castell quarry, Wales, UK. A. NMW 2017.15G.3, deformed shell in left lateral (A1), posterior oblique (A2), and anterior oblique (A3) views. Note the twisted and compacted median sinus seen as a line inA1 and A3. B. NMW 2017.15G.4, anterolateral shield in right lateral view.C. NMW 2017.15G.5, large, slightly dorso­ventrally flattened specimen in left lateral (C1) and dorsal (C2) views, showing median anterior sinus well. D. NMW 2017.15G.6, natural cross section in ventral view, showing the shape of the whorls. E. NMW 2017.15G.7, specimen in left lateral view (E1), showing the external mould of the initial whorls and the internal mould of the last whorl; details of the ornamentation as seen on the external mould (E2). Scale bars 5 mm.