Mesosaccella toddi, Hryniewicz & Little & Nakrem, 2014

Hryniewicz, Krzysztof, Little, Crispin T. S. & Nakrem, Hans Arne, 2014, Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard, Zootaxa 3859 (1), pp. 1-66 : 19-22

Mesosaccella toddi sp. nov.

( Figure 7 G–P )

2011 Malletiid sp. 2—Hammer et al., tab. 2.

Etymology. After Jonathan A. Todd, curator of the Mesozoic and Cenozoic mollusc collection in the Natural History Museum, London. Study of this collection helped the work on the fossils described in this paper.

Type locality. Seep 9, Knorringfjellet, Spitsbergen, 78°18’49.9”N 16°10’58.9”E.

Type material. Holotype: PMO 224.861 ; a large, complete shell . Paratypes: PMO 225.031 Fragment of the cardinal area showing taxodont dentition . PMO 224.862 ; an internal mould showing weak anterior adductor muscle scar and posterior adductor muscle scar and pallial line with shallow pallial sinus . PMO 217.609 ; a small, complete shell . PMO 217.610 ; a small, complete shell . PMO 217.616 ; a large, well preserved shell showing external commarginal and rosette-shaped ornament .

Material examined. 35 specimens, articulated valves and internal or external moulds. See Appendix 1 for the list of specimens.

Diagnosis. Inflated and thick-shelled with posterior part approximately twice the length of the anterior part, and with three strong posterior carinae. External ornament of strong commarginal ridges fading towards posterior and anterior and crossed obliquely by commarginal growth lines. Interridge spaces occupied by rows of rosetteshaped ornamentation, fading anteriorly and posteriorly.

Dimensions. 6–12.5 mm in length, 4–8.2 mm in height, 3–7.1 mm in width. See Figure 9 A–D and Appendix 2E for details.

Description. Shell small to medium in size, up to 12.5 mm long, 8.2 mm high, and 7.1 mm wide, elongated (H/ L≈0.72), and inflated (W/L≈0.52, W/H≈0.81), with more inflated specimens more frequent than less inflated. Moderately inequilateral (Pl/L≈0.72) throughout observed ontogeny. Orthogyrate beaks, passing into broadly curved anterodorsal margin through shallow concavity directly in front of beaks. Anterodorsal margin bends down and gets more tightly rounded to form broadly arcuate anterior margin, which gets progressively less curved and passes smoothly into ventral margin. Ventral margin gently rounded, deepest approximately below beaks. Posterior to first carina ventral margin oblique, with shallow sinus developed on larger specimens, connected to shallow, but well defined sulcus. Posterior shell extremity pointed, supported on second carina. Posterodorsal shell margin straight, joining beak through very small and shallow concavity directly behind it. External ornament of weak commarginal growth lines, obscured in shell mid-flank by strong, commarginal ridges transecting growth lines obliquely. Interridge spaces occupied by rows of rosette-shaped ornament arranged in regular intervals. Posterior area ornamented only by growth lines and rosettes; delineated by two strong carinae. First carina forms angle of ca. 55° with anteroposterior axis and visible throughout observed ontogeny. Second carina forms approximate angle of 27° with anteroposterior axis and also visible throughout ontogeny. Escutcheon wide, shallow, bounded by third carina. Dentition taxodont with long and straight teeth arranged in rows along hinge line. Specimen approximately 9 mm in length has nine teeth in anterior row and 16 teeth in posterior row. No resilifer; teeth joining below beak to form single teeth row. Anterior adductor muscle scar circular and well impressed; rather small. In specimen ca. 9 mm long anterior adductor muscle scar is 1 mm in diameter. Anterior adductor muscle scar positioned slightly above anteroposterior axis. Posterior adductor muscle scar poorly visible, small, elongated, positioned above pallial sinus. Five pairs of elongated muscle scars present directly underneath hinge plate: two in anterior part of hinge plate, one underneath cardinal area and two pairs of longer muscle scars present underneath posterior part of hinge plate. Some of these may represent pedal muscle attachments. Pallial line well impressed; pallial sinus shallow.

Remarks. Protobranch bivalves with a shell shape similar to M. toddi are known from several other Cretaceous localities. Woods (1899) listed several carinated species from the Cretaceous of England which he assigned to the genus Nuculana Link, 1807 . In our opinion these belong to both Nuculana and Mesosaccella . For example, no resilifer is visible on the drawings of N. lineata in Woods (1899, pl. 1, fig. 31d), which excludes this species from Nuculana . Nuculana speetoniensis ( Woods 1899, p. 3. pl. 1, figs. 6–7) from the Lower Cretaceous Speeton Clay has a very similar shell shape to M. toddi , the main difference between them being the shorter and less pointed posterior margin of N. speetoniensis . Nuculana lineata J. de Sowerby, 1836 ( Woods 1899, p. 7, pl. 1, figs. 28–32), from the Lower Cretaceous of Southern England has a similar shape and ornament to M. toddi , having commarginal ridges fading on the anterior and posterior shell parts, but can be distinguished from M. toddi by being less inflated and having a rounded posterior with less pronounced carinae. In the Aptian of Spitsbergen Sokolov & Bodylevsky (1931, p. 66, pl. 12, figs. 4–6) recorded Leda angulatostriata , which has a very similar shell shape to M. toddi , the main difference being L. angulatostriata has a less pronounced posterior carination than M. toddi . In our opinion L. angulatostriata also belongs to Mesaccella.

Occurrence. Seeps 1, 3, 9 and 12 (Upper Volgian–latest Ryazanian), Slottsmøya Member, Svalbard ( Tab. 1 ).

Palaeoecology. Mesosaccella toddi was probably a shallow burrowing deposit feeder, like M. rogovi . The inflated shell of M. toddi , its relatively short posterior and shallow pallial sinus suggests, however, that this species was a slow burrower living underneath the sediment-water interface. The strong commarginal ornament and relatively thick shell may have been adaptations for burrowing in coarser sediment, like silt and sand (e.g. Marshall 1978), or it might have been an adaptation for living in acidic environment (e.g. Allen 1993). As for M. rogovi , M. toddi might have fed on chemosynthetically produced organic matter.

Subclass Autobranchia Grobben, 1894

Superorder Pteriomorphia Beurlen, 1944

Figures

FIGURE 7. (A–F) Mesosaccella rogovi sp. nov. (A–B) Lateral view of a internal mould showing pallial line. Fragment of the shell shows concentric growth lines. (C) Dorsal view of specimen with partially preserved shell showing weak carina, escutcheon, unidentified muscle scars and ridges formed by growth line deflections. (D–E) Lateral view of right valve internal mould showing weak posterior adductor muscle scar and weak pallial line with very shallow pallial sinus. (F) Internal view of the right valve showing anterior and posterior dental rows and cardinal dentition. (G–P) Mesosaccella toddi sp. nov. (G–K) Left valve lateral views of articulated specimens. (J) Details of external ornament showing characteristic commarginal ridges and rosette-shaped ornament in interridge spaces (arrowed). (K) Holotype showing strong posterior carinae. (L–M) Anterior and posterior views of holotype. Note three carinae and escutcheon visible in the posterior view. (N) Dorsal view of holotype showing three carinae and their relation to wide and shallow escutcheon. (O) Internal view of the dorsal margin of articulated specimen showing cardinal-most teeth and unidentified muscle scars. (P) Lateral view of right valve internal mould showing relatively strong, circular anterior adductor muscle scar and elongated posterior adductor muscle scar, as well as strong pallial line with shallow pallial sinus. (A–E) PMO 217.371, (F) PMO 217.539, (G) PMO 217.609, (H) PMO 217.610, (I–J) PMO 217.616, (K–N) PMO 224.861, (O) PMO 225.031, (P) PMO 224.862. Scale bars 5 mm (A–F, K–P), 2 mm (G–I).

FIGURE 9. Measurements of Mesosaccella toddi sp. nov. (in mm). (A) Height to length. (B) Width to length. (C) Width to height. (D) Posterior length to length.