Leptagonia franca, Mottequin & Simon, 2017

Mottequin, Bernard & Simon, Eric, 2017, New insights on Tournaisian-Visean (Carboniferous, Mississippian) athyridide, orthotetide, rhynchonellide, and strophomenide brachiopods from southern Belgium, Palaeontologia Electronica 37 (3), pp. 1-45 : 6-11

Leptagonia franca sp. nov.

Figures 4-8 , Table 1

zoobank.org/ 27FA86CF-03F3-456B-9667-C89DB981FE3E

p 1843 Leptaena depressa ; de Koninck, p. 215, pl. 13, fig. 6a-c (labelled Leptaena depressa var. analoga Sowerby in the plate legend).

p 1851 Leptaena analoga ; de Koninck, p. 653.

p 1868 Strophomena (Producta) analoga ; Dewalque, p. 333.

p 1881 Strophomenes (Producta) analoga ; Mourlon, p. 47.

v 1933 Leptaena analoga ; Maillieux, p. 92.

v 1934 Leptaena analoga (Phillips) ; Demanet, p. 61, text-figs. 11-14, pl. 5, figs. 1-14.

1950 Leptaena analoga Wilckens (sic); Termier and Termier, pl. 82, figs. 19-20.

p 1958 Leptaena analoga (Phillips) ; Demanet, p. 123, 126, 130.

p 1969 Leptaena analoga (Phill.) ; Mortelmans, p. 31 (only those from the Tournai Formation).

v 2017 Leptagonia sp. indet.; Mottequin and Simon, fig. 12d.

Etymology. From francus, a, um (Latin: of or belonging to the Franks, Frank), in reference to the rich Frank history of the town of Tournai.

Holotype. RBINS a5891 ( Demanet, 1934, pl. 5, fig. 1) from Tournai (Tournai Formation) ( Figure 5.1-6 ).

Paratypes. RBINS specimens from Tournai (Tournai Formation) illustrated by Demanet (1934) a5829 (pl. 1, fig. 30) ( Figure 6.1-6 ), a5892 (pl. 5, fig. 2) ( Figure 5.7-12 ), a5893 (pl. 5, fig. 3), a5894 (pl. 5, fig. 4), a5895 (pl. 5, fig. 5) ( Figure. 6.7-12 ), a5896 (pl. 5, fig. 6) ( Figure 7.1 ), a5897 (pl. 5, fig. 7) ( Figure 7.10-13 ), a5898 (pl. 5, fig. 8), a5899 (pl. 5, fig. 9) ( Figure 7.9 ), a5902 (pl. 5, fig. 12) ( Figure 7.4-5 ), a5903 (pl. 5, fig. 13) ( Figure 7.8 ), a5904 (pl. 5, fig. 14), a5905 (text-fig. 11), a5906 (text-fig. 13), a5907 (text-fig. 14) and herein a13099 ( Figure 5.13-18 ), a13100 ( Figure 6.13-17 ), 13101 ( Figure 6.18-22 ), a13102 ( Figure 6.23-27 ), a13103 ( Figure 7.2 ), a13104 ( Figure 7.6-7 ) .

Additional material. RBINS: two ventral valves and 27 articulated specimens.

Type locality. Tournai area ( Figures 2-3 ), southern Belgium.

Type stratum. Tournai Formation, Tournaisian ( Figure 4 ).

Diagnosis. Shell large (up to 70 mm in width), with a subtrapezoidal to subrectangular outline in juveniles, but transversally subrectangular in mature specimens; cardinal extremities slightly mucronate in juveniles, but becoming acute to subrounded (due to development of lamellose shelly expansions) in mature specimens. Shell parvicostellate and rugate, with 11–15 costae in 5 mm at 10 mm from the dorsal umbo; 15–22 rugae on ventral disc. Ventral muscle field subdiamond-shaped, raised anterolaterally, and anteriorly on a low pseudospondylium. Dorsal muscle platform raised above valve floor extending to about half length of the disc; dental sockets deep, well-defined; socket plates strongly divergent; dorsal marginal ridge well developed, with numerous mantle canals developed anteriorly and antero-laterally to it.

Description. Shell large (up to 70 mm in width), markedly wider than long, widest at hinge line in juveniles or posteriorly to hinge line in mature specimens, concavoconvex in lateral profile with sharp, dorsally directed geniculation, with a subtrapezoidal to subrectangular outline in juveniles, but transversally subrectangular in mature specimens; anterior commissure rectimarginate; anterior margin slightly rounded or emarginate; cardinal extremities slightly mucronate in juveniles, but becoming acute to subrounded (due to development of lamellose shelly expansions) in mature specimens.

Ventral visceral disc shorter than trail in mature specimens, moderately convex longitudinally and transversally, subtrapezoidal in outline; umbo small; beak short; interarea low, striated parallel to hinge, apsacline to almost orthocline, weakly concave; apical foramen small ( Figure 7.1- 3 ); delthyrium broad, triangular, closed apically by a convex, striated pseudodeltidium; median depression developed anteriorly to geniculation in mature specimens.

Dorsal visceral disc flat longitudinally and transversally; trail shorter than disc; interarea clearly less developed than the ventral one, flat, apsacline, divided by convex, striated, medially grooved chilidium ( Figure 7.1-3 ).

Shell parvicostellate and rugate, with 11–15 costae (n=8) in 5 mm at 10 mm from the dorsal umbo. Between 15 and 22 rugae on ventral disc; prominent on both discs and absent on trails.

Ventral interior ( Figure 7.4-8 ) with small teeth, supported by short dental plates; muscle field subdiamond-shaped, raised anterolaterally and anteriorly on a low pseudospondylium; diductor scars large, fan-shaped, radially striated; adductor scars separating the diductors, raised on a prominent median ridge widening anteriorly and divided by a thin myophragm; base of the adductor ridge extends slightly anterior to the pseudospondylium as a rounded buttress onto valve floor; mantle canals not observed.

Dorsal interior ( Figure 7.9-13 ) with stout, ventrolaterally oriented cardinal process lobes; muscle platform raised above the valve floor; cardinal process pit developed between the cardinal process lobes and the posterior margin of the muscle platform; posterior adductor scars more or less circular and separated by a median ridge terminating close to their anterior end, but variably developed among the examined material; anterior adductor scars markedly smaller and separated by a slender septum continuing anteriorly for a variable distance in front of the muscle platform and reaching its maximum height at the junction of the lateral ridges bounding the former, then decreasing rapidly towards the anterior margin; dental sockets deep, well-defined; socket plates diverging at an angle of 142–159 degrees (n=2); outer surface of the marginal ridge including numerous traces of mantle canals; tubercles abundant, but variably developed following the specimens (compare Figure 7.9 and 7.11 ).

Dimensions ( Figure 8 , Table 1). The number of specimens measured is rather low due to lack of well-preserved material. For this reason, the mean value is given without standard error. Nonetheless, the unrolled ventral trail length/unrolled ventral disc length and unrolled dorsal trail length/unrolled dorsal disc length ratios analysed here show that an interrelation is probable.

interarea.

Remarks. Until recently, Tournaisian leptaenines from Western Europe, but not only (see Bahrammanesh et al. (2011) for a non-exhaustive list of references), were identified as Leptagonia analoga ( Phillips, 1836) , whose lectotype was selected and illustrated by Brunton (1968) as subsequently did Brand (1972) and Cocks and Rong (2000). Phillips’ species was described with great detail by Brunton (1968), Brand (1972), and Harper and Jeffrey (1996). As rightly proposed by Bassett and Bryant (2006), several features such as the outline of the muscular field could be used to discriminate all the forms identified as L. analoga , an opinion subsequently followed by Mottequin (2010) and Bahrammanesh et al. (2011). In order to concretise this point of view, a new large-sized species is here erected on the basis of the well-preserved material from the Tournai area, formerly illustrated by Demanet (1934). Leptagonia franca sp. nov. differs from L. analoga by its less elongate ventral disc, its more developed ventral trail, and its more inflated shell. Furthermore, its ornamentation is slightly coarser than in the latter and its ventral muscle field is subdiamond-shaped. Except the radial ornamentation, which is similar in both species, the same features plus a markedly distinct dorsal internal morphology are used to discriminate L. franca and L. caledonica Brand, 1972 . Due to its distinctive external and internal features, L. franca cannot be confused with L. simorini (Sokolskaya in Sarytcheva et al., 1963) and L. smithi Brand, 1972 . The new species differs from L. convexa ( Weller, 1914) notably in its less elongate ventral disc and its more developed ventral trails. Leptagonia franca is clearly larger and has a coarser radial ornamentation than L. missouriensis Carter, 1968 . Sanders (in Easton et al., 1958) erected Leptaena cooperi from the Represso Limestone of Sonora ( Mexico), on the basis of a few incomplete specimens (see Carter, 1987), that he compared with the Belgian material illustrated by Demanet (1934). Indubitably, the internal morphology of both species is identical except some differences noted by Sanders (in Easton et al., 1958), such as the outline of the visceral disc (slightly more than semi-elliptical in L. franca vs. nearly completely elliptical in L. cooperi ). Further material is however required to better describe the poorly known Mexican species.

Occurrence. Until now, Leptagonia franca is mainly known from the Tournai Formation in the eponymous area but also occurs in the Tournaisian succession of the Dinant Synclinorium, as evidenced by the specimens illustrated by Demanet (1934, pl. 5, figs. 10, 11) from the Hastarian Pont d’Arcole Formation. Its precise distribution in the Tournaisian succession would require a detailed survey of the complete Tournaisian succession, which is well beyond the scope of this paper. However, Demanet (1923, pl. 5, fig. 8) illustrated a ventral valve identified as Strophomena analoga from the Waulsort Formation at Sosoye in the Dinant sedimentation area ( Figures 2-4 ). This specimen (RBINS a11747) characterized by its flat visceral disc and thin radial ornamentation (17 ribs per 5 mm at 10 mm from the umbo; 22 rugae) is re-illustrated herein ( Figure 9.1-4 ) and left provisionally in open nomenclature due to the absence of additional material, and is thus identified as Leptagonia sp. indet.

Figures

FIGURE 2. Schematic geological map of southern Belgium with location of the fossiliferous localities (modified from de Béthune, 1954). Abbreviations: D, Dréhance; F, Furfooz; G, Germany; HSMOTS, Haine-Sambre-Meuse Overturned Thrust Sheets; L, Grand Duchy of Luxemburg; N, The Netherlands; S, Sosoye.

FIGURE 3. Late Tournaisian sedimentation areas in the Namur-Dinant Basin (not palinspastically restored; modified from Hance et al., 2001; Poty et al., 2006; Poty, 2016) with location of the fossiliferous localities. Abbreviations: ASA, Avesnois sedimentation area; CSA, Condroz sedimentation area; D, Dréhance; DSA, Dinant sedimentation area; F, Furfooz; G, Germany; HSA, Hainaut sedimentation area; L, Grand Duchy of Luxemburg; N, The Netherlands; NSA, Namur sedimentation area; S, Sosoye; VSA, Visé–Maastricht sedimentation area.

FIGURE 4. Distribution of the athyridides, orthotetides and strophomenides described and/or discussed herein within the essentially carbonate Tournaisian and Visean succession of southern Belgium (see text). Stratigraphy, lithostratigraphy (incompletely represented here for the Hainaut and Visé–Maastricht sedimentation areas; formations and members are in roman and italic letters, respectively), and biostratigraphy are adapted from Poty et al. (2002, 2006, 2014). The lenticular Vignobles Member is not represented here as it is rarely developed (it is comprised between the Vaulx Member of the Tournai Formation and the Antoing Formation). Note that the “Grande Brèche” is an informal stratigraphic unit resulting from the dissolution of evaporitic levels in the Grands Malades Formation; other brecciated horizons occur in the Lives Formation.

FIGURE 5. Leptagonia franca sp. nov. from the Tournai area, Tournai Formation (Tournaisian). 1-6, RBINS a5891 (holotype), articulated specimen, almost complete, in ventral (perpendicular to the surface of the alae), dorsal, lateral, posterior, anterior, and ventral (perpendicular to the disc) views. 7-12, RBINS a5892, articulated specimen, almost complete, in ventral (perpendicular to the surface of the alae), dorsal, lateral, posterior, anterior, and ventral (perpendicular to the disc) views. 13-18, RBINS a13099, articulated specimen (with an auloporid tabulate fixed to its right ventral flank), almost complete, in ventral (perpendicular to the surface of the alae), dorsal, lateral, posterior, anterior, and ventral (perpendicular to the disc) views. Scale bar equals 10 mm.

FIGURE 6. Leptagonia franca sp. nov. from the Tournai area, Tournai Formation (Tournaisian). 1-6, RBINS a5829, complete articulated specimen (with Petrocrania? ryckholtiana (de Koninck, 1843) attached to ventral and dorsal valves) in ventral (orientation as in Figure 5.1), ventral (perpendicular to the visceral disc), dorsal, lateral, posterior, and anterior views. 7-12, RBINS a5895, complete articulated specimen (with Petrocrania? ryckholtiana and microconchid attached to ventral valve), in ventral (orientation as in Figure 5.1), lateral, ventral (perpendicular to the visceral disc), dorsal, posterior and anterior views. 13-17, RBINS a13100, articulated specimen, almost complete, in ventral (orientation as in Figure 5.1), dorsal, lateral, posterior, and anterior views. 18-22, RBINS a13101, articulated specimen, almost complete, in ventral (orientation as in Figure 5.1), dorsal, lateral, posterior and anterior views. 23- 27, RBINS a13102, articulated specimen, almost complete, in ventral (orientation as in Figure 5.1), dorsal, lateral, posterior, and anterior views. Scale bar equals 10 mm.

FIGURE 7. Leptagonia franca sp. nov. from the Tournai area, Tournai Formation (Tournaisian). 1-3, Detail of the small pseudodeltidium and apical foramen of three articulated specimens with the ventral valve on top: RBINS a5896 (1), RBINS a13103 (2), and RBINS a5895 (3). 4-5, RBINS a5902, incomplete ventral valve showing the posterior morphology. 6-7, RBINS a a13104, incomplete ventral valve interior showing the posterior morphology and posterior view notably displaying the prominent teeth. 8, RBINS a5903, incomplete ventral valve interior showing the posterior morphology. 9, RBINS a5899, almost complete dorsal valve interior in ventral view. 10-13, RBINS a5897, almost complete dorsal valve interior in posterior and ventral views, detail of the lateral vascular canals, and close-up of the cardinal process and the muscular field. Scale bars equal 2.5 mm (1-3), 5 mm (4-8, 12-13), 10 mm (9-11).

FIGURE 8. Scatter diagrams of Leptagonia franca sp. nov. N: number of specimens measured. 1, Relation between shell width and length. 2, Relation between shell width and shell length/shell width ratio. 3, Relation beween shell width and thickness. 4, Relation between shell width and shell thickness/shell witdth ratio. 5, Relation between unrolled ventral valve length and ratio between unrolled ventral trail length (Vtl) and unrolled ventral disc length (Vdl). 6, Relation between unrolled dorsal valve length and ratio between unrolled dorsal trail length (Dtl) and unrolled dorsal disc length (Ddl).

FIGURE 9. 1-4, Leptagonia sp. indet. from Sosoye, Waulsort Formation (Tournaisian, Ivorian), RBINS a11747, almost complete ventral valve in ventral, ventral (perpendicular to the disc), lateral and anterior views. 5-26, Leptagonia cf. caledonica Brand, 1972 from Visé, Visé Formation (Visean, Warnantian). 5-9, RBINS a5915, incomplete articulated specimen in ventral (perpendicular to the disc), dorsal, lateral, posterior, and anterior views. 10-14, RBINS a5914, incomplete articulated specimen in ventral (perpendicular to the disc), dorsal, lateral, posterior, and anterior views. 15- 20, ULg.PA. 2016.12.25/1, articulated specimen, almost complete, in ventral, ventral (perpendicular to the disc), dorsal, lateral, posterior and anterior views. 21-26, ULg.PA.2016.12.25/2, articulated specimen, almost complete, in ventral, lateral, ventral (perpendicular to the disc), dorsal, posterior, and anterior views. Scale bar equals 10 mm.

FIGURE 10. Leptagonia cf. caledonica Brand, 1972 from Visé, Visé Formation (Visean, Warnantian). 1-5, 19-20, ULg.PA.2016.12.25/3, internal mould in ventral, dorsal, lateral, posterior and anterior views, and detail (SEM) of the muscular platform (latex cast). 6-10, 21-22, ULg.PA.2016.12.25/4, internal mould in ventral, dorsal, lateral, posterior, and anterior views, and detail (SEM) of the muscular platform (latex cast). 11-15, 23-24, ULg.PA.2016.12.25/5, internal mould in ventral, dorsal, lateral, posterior, and anterior views, and detail (SEM) of the muscular platform (latex cast). 16-18, ULg.PA.2016.12.25/6, distorted internal mould in dorsal view and close-up (SEM) of the muscular platform (latex cast). Scale bars equal 5 mm.

FIGURE 12. Schellwienella radialiformis Demanet, 1934 from the Tournai area, Tournai Formation (Tournaisian). 1-5, RBINS a5924, articulated specimen, almost complete, in ventral, dorsal, lateral, posterior, and anterior views. 6-10, RBINS a5928, articulated specimen, almost complete, in ventral, dorsal, lateral, posterior, and anterior views. 11-15, RBINS a5929, articulated specimen, almost complete, in ventral, dorsal, lateral, posterior and anterior views. 16-20, RBINS a5930 (lectotype), articulated specimen, almost complete, in ventral, dorsal, lateral, posterior, and anterior views. 21-25, RBINS a5926, almost complete articulated specimen (with a microconchid attached to the ventral valve, close to the anterior margin) in ventral, dorsal, lateral, posterior, and anterior views. Scale bars equal 10 mm.

FIGURE 13. Schellwienella radialiformis Demanet, 1934 from the Tournai area, Tournai Formation (Tournaisian). 1-5, RBINS a5927, articulated specimen, almost complete, in ventral, dorsal, lateral, posterior and anterior views. 6-10, RBINS a13105, articulated specimen, almost complete, in ventral, dorsal, lateral, posterior, and anterior views. 11-12, RBINS a5924, detail (SEM) of the pseudodeltidium and of the koskinoid perforations penetrating in the umbonal part of the ventral valve. 13, RBINS a5928, detail of the microornament on dorsal valve (SEM). 14-17, RBINS a5925, incomplete ventral valve in external and internal views and detail (SEM) of the posterior internal and external morphology. 18-24, RBINS a13106, almost complete dorsal valve in dorsal, lateral, posterior, and internal views and detail (SEM) of the posterior morphology (cardinal process). Scale bars equal 10 mm (1-10, 14, 18-22), 1 mm (11), 0.25 mm (12), 0.5 mm (13), 2 mm (17, 23), 2.5 mm (24), 5 mm (15).

FIGURE 14. Schellwienella radialiformis Demanet, 1934 from the Tournai area, Tournai Formation (Tournaisian). 1-4, RBINS a13107, almost complete dorsal valve in dorsal, lateral and posterior views and detail of the chilidium and cardinal process lobes. 5-10, RBINS a5921, articulated internal mould with shelly remains in ventral, dorsal, lateral, posterior and anterior views and close-up of the ventral interarea (5). Scale bars equal 10 mm (1-2, 4-6), 1 mm (4, 5).

FIGURE 15. Schellwienella ornata Demanet, 1934 from Visé, Visé Formation (Visean, Warnantian). 1-8, RBINS a5922 (lectotype), articulated internal mould in ventral, enlarged ventral (showing the extent of the traces left by the dissolved dental plates), dorsal, lateral, posterior, and anterior views, and internal views of a latex cast (SEM) showing the internal morphology of the posterior part of the specimen (7-8). 9-14, RBINS a5923 (paralectotype), articulated specimen (partly sectioned, probably by Demanet) in ventral, dorsal, lateral, posterior, and anterior views, and close-up (SEM) of the parvicostellate ornamentation in the central part of the dorsal valve. Scale bars equal 10 mm (1, 3-6, 9-13), 5 mm (2), 2.5 mm (7-8), 1.25 mm (14).

FIGURE 16. Schellwienella cylindrica (M’Coy, 1844), from the Visean (Brigantian) of Castle Espie (Comber, Co. Down, Ireland). 1-3, NMING:F6267 (lectotype), almost complete dorsal valve in dorsal, lateral, and posterior views. Scale bar equals 10 mm.

FIGURE 17. Serratocrista scaldisensis sp. nov. from the Tournai area, Tournai Formation (Tournaisian) (all SEM). 1-6, RBINS a13108, articulated specimen, almost complete, in ventral, dorsal, lateral, posterior, and anterior views, and close-up of the umbo showing the absence of dental plates. 7-11, RBINS a13109, articulated specimen in ventral, dorsal, lateral, posterior, and anterior views. 12-16, RBINS 13110 (holotype), articulated specimen in ventral, dorsal, lateral, posterior, and anterior views. 17-21, RBINS a13111, articulated specimen in ventral, dorsal, lateral, posterior, and anterior views. 22-25, RBINS a13112, almost complete ventral valve in ventral, lateral, posterior, and anterior views. Scale bars equal 1 mm (6), 5 mm (1-5, 7-25).

FIGURE 18. Serratocrista scaldisensis sp. nov. from the Tournai area, Tournai Formation (Tournaisian). 1, 6, RBINS a13110 (holotype), close-up of the strongly convex pseudodeltidium, and detail of the microornament (central part of the dorsal valve). 2-3, 19-21, RBINS a13111, detail of the strongly convex pseudodeltidium and of the cardinal process, and close-up of the koskinoid perforations developed in the ventral umbonal area and on the interarea. 4-5, RBINS a13113, incomplete articulated specimen in dorsal view and close-up of the posteriorly grooved lobes of the cardinal process. 7-13, RBINS a13114, dorsal valve in external, internal and posterior (10) views, detail of the chilidium (9), the cardinal process (11), and oblique views of the internal posterior region (12-13). 14-18, RBINS a13115, almost complete dorsal valve in external, internal and posterior views and oblique views of the internal posterior region. Scale bars equal 1 mm (1-3, 5-6, 9, 11, and 19-20), 2.5 mm (4), 5 mm (7-8, 10, 14-16, 12-13, and 17-18), 0.1 mm (21).