Loxolenichnus stellatocinctus Breton and Wisshak, 2017

Breton, Gérard, Wisshak, Max, Néraudeau, Didier & Morel, Nicolas, 2017, Parasitic gastropod bioerosion trace fossil on Cenomanian oysters from Le Mans, France and its ichnologic and taphonomic context, Acta Palaeontologica Polonica 62 (1), pp. 45-57 : 51-54

Loxolenichnus stellatocinctus Breton and Wisshak isp. nov.

Figs. 5 , 6 .

Etymology: From the Latin stellatus, star-shaped, and cinctus, encircled; allusion to the stellate groove surrounding the central perforation(s).

Type material: Holotype: MHNLM 2015.2.244 ( Fig. 5 ) . Paratypes: MHNLM 2015.2.346 and MHNLM 2015.2 .347, ex collection Peter Girod. Two oyster valves from the lower Campanian Inoceramus lingua – Goniotheuthis quadrata Zone, found in a quarry near Höver, Germany ( Fig. 6 ) .

Type locality: Excavation of the earth working site of the Lycée Bellevue, town of Le Mans, France (48°00’53” N; 0°12’21” E Google Earth ©) .

Type horizon: Cretaceous, Upper Cenomanian, Calycoceras guerangeri Biozone, Marnes à Pycnodonte biauriculata Formation.

Diagnosis.—One or several, vertical or oblique penetrations, irregularly cylindrical, surrounded on the outer substrate surface by an asymmetrical depression with a set of stellate grooves increasingly distinct towards the margin of the trace.

Description.— Holotype: Substrate: An incomplete left valve of the oyster R. suborbiculatum (preserved length ca. 6 cm) with ventral and ventro-posterior margins broken. The colour pattern (longitudinal brown flames) is preserved near the umbo. Entobian perforations near the posterior margin and on the centre of the shell. The holotype is located near the ventro-anterior margin ( Fig. 5A ).

Trace: The penetration hole is 8 mm long, roughly cylindrical, with a diameter of ca. 1 mm. It plunges very obliquely towards the centre of the valve into the shell which is 2.5 mm thick at this place. Its course is slightly sinuous ( Fig. 5A ) and the opening on the inner side of the valve is a trench of ca. 4 × 1 mm ( Fig. 5B ). The aperture on the outer side of the valve is an irregular depression ca. 3 × 1.5 mm. It is located in the middle of a roughly circular attachment scar on the shell, but slightly eccentric. Except for a recent partial mechanical etching, the surface of the shell is intact between the penetration hole and the peripheral stellate groove. The attachment scar is delineated by a stellate groove. There are 13 main rays, longer in the ventro-anterior direction (of the oyster shell) where they are up to 2.2 mm long, than in the opposite direction where they are reduced to small notches 0.5 mm deep. The transition between the shortest notch and the longest ray is progressive. The longest rays are sharply triangular, but their extremity is blunt and rounded. The groove is deepest along the side of the rays, so that there is an axial ridge in the axis of each ray. Between the eight main rays on the ventro-anterior side come in seven accessory rays, 3–5 times shorter than the main rays but with an axial ridge too. The accessory rays become progressively shorter—as do the main rays—in the dorso-posterior direction, where the groove becomes deeper, obliquely sunk into the shell, and the rays are there less clearly marked. Dorso-posteriorly, the inner side of the groove is a smooth surface, oblique, joining the outer surface of the shell with an angle (estimated) of 45°, 1.5 mm high which becomes progressively lower and fades, mainly in the anterior direction ( Fig. 5C–E ).

Paratypes: The paratype MHNLM 2015.2.346 ( Fig. 6A–C ) accommodates two Loxolenichnus stellatocinctus igen. et isp. nov., located directly next to each other, each surrounded by a stellate groove, the diameter of which are 8 mm in maximum. The first trace shows two penetration holes, one perpendicular and one oblique. The second trace shows, within the marginal stellate groove, two partial (1/4 circumference) grooves and there are two approximately central and one marginal penetration holes, one of them being superposed to one of the partial grooves. At least two penetration holes reach the adductor muscle scar. Callus formations as a host reaction are visible on the inner side of the valve and indicate a penetration during the life of the oyster . The second paratype MHNLM 2015.2.347 ( Fig. 6D–F ) is a right valve with two semi-circular concentric stellate grooves centred on the ventral margin of the valve. The grooves display alternately long and short rays. A possible penetrative way is a notch on the ventral margin of the valve, located at the centre of the grooves .

Remarks.—By including two former Oichnus ichnospecies, Oichnus is now constrained to pure predation traces Praedichnia). The new ichnogenus, in contrast, comprises three ichnospecies that are all combinations of complete penetrations (parasitic in nature and thus unlike Praedichnia sensu stricto) and an attachment scar (Fixichnia), and this combination of two different modes of bioerosion is characteristic of the new ichnogenus.

Stratigraphic and geographic range.—Upper Cretaceous, France and Germany. Known only from oysters.

Figures

Fig. 5. Parasitic gastropod bioerosion and perforation trace Loxolenichnus stellatocinctus igen. et isp. nov., MHNLM 2015.2.244, holotype, Marnes à Pycnodonte biauriculata Formation, Upper Cenomanian, Lycée Bellevue earthmoving works, Le Mans, Sarthe Department, France; on LV of Rhynchostreon suborbiculatum (Lamarck, 1801). A. Entire LV shell with the arrow showing the perforation. B. LV (viewed from inside), the arrow shows the opening of the perforation on the inner side of the shell, diascopic illumination. Outer (C) and inner (D) sides of the shell, close-ups of the perforation, the dashed line delimitates approximately the course of the perforation through the shell, diascopic illumination. E. Positive X-ray print of the perforation.

Fig. 6. Parasitic gastropod bioerosion trace Loxolenichnus stellatocinctus igen. et isp. nov., MHNLM 2015.2.346 and MHNLM 2015.2.347, paratypes; lower Campanian Inoceramus lingua–Goniotheuthis quadrata Zone, quarry near Höver, Germany. A. Outer side of an oyster valve, accommodating two specimens of L. stellatocinctus (arrows). B. Close-up of the two specimens and the multiple perforations. C. Inner side of the oyster valve showing two of the perforations reaching the adductor muscle pad. Outer (D) and inner (E) sides of an oyster valve with a marginal L. stellatocinctus. F. Close-up of D, note the two concentric stellate rims and the marginal notch.