Tehamatea rasmusseni Hryniewicz & Little & Nakrem 2014, sp. nov.

Tehamatea rasmusseni sp. nov.

(Figure 13 A–J)

2011 Lucinid sp.—Hammer et al., fig. 7d–e, tab. 2.

Etymology. After Jan Audun Rasmussen, curator of the Greenland collection in the Natural History Museum of Denmark, Copenhagen, study of which helped with taxonomic determinations in this paper.

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

Type material. Holotype: PMO 217.234; a partially articulated shell the vertically displaced valves; right valve shows cardinal and anterior lateral dentition of the right valve; left valve is an internal mould, with silicone rubber cast. Paratypes: PMO 217.169; an articulated, partially preserved shell showing outline and details of external ornament. PMO 217.173; an internal mould and silicone rubber cast showing part of the right valve cardinal dentition. PMO 217.227; an articulated specimen showing the ligament and posterior sulcation. PMO 217.243; an articulated, partially preserved internal mould and silicone rubber cast showing the right valve cardinal dentition. PMO 217.247; an articulated, partially preserved internal mould showing bioimmuration trace. PMO 225.101; an internal mould with silicone rubber cast showing anterior lateral dentition. PMO 225.104; an internal mould showing anterior adductor muscle scar and posterior adductor muscle scar and possible mantle gill scars. PMO 225.111; an internal mould with silicone rubber cast showing anterior and posterior hinge plate areas.

Material examined. 60 specimens, mostly articulated internal moulds, some with adhering shell, and a few single valves. See Appendix 1 for list of specimens.

Dimensions. 17–100 mm in length, 13–76 mm in height, 7.3–54 mm in width. See Figure 14 A–F and Appendix 2J for details.

Diagnosis. Shell oval to weakly hexagonal in shape, covered with dense commarginal growth lines. Posterodorsal margin gently convex, posterior area slightly flattened. Posterior margin slightly truncated. Anterior and posterior laterals short and weak. Cardinal tooth 3a very small, 3b large, not bifid. Anterior adductor muscle scar elongated, weakly incurved. Anterior pedal retractor scar well impressed, separated from anterior adductor muscle scar. Posterior adductor muscle scar large, deeply impressed, merged with posterior pedal retractor scar.

Description. Shell large, up to 100 mm long, 76 mm high, and 54 mm wide. Average H/L ≈ 0.78, relatively constant throughout ontogeny. W/H ratio ≈ 0.67, also relatively constant throughout ontogeny. Shell oval to weakly hexagonal in outline, inequilateral with umbones positioned closer towards anterior; average Pl/L ratio of ≈ 0.64. Umbones prosogyrate, not very prominent. Anterodorsal margin short and relatively straight. Lunule asymmetric, larger in left valve than in right; large, lancet-shaped and deep, occupying on average ≈ 0.46 of anterior shell length. Anterior margin arcuate, with dorsally tightening curvature. Ventral margin broadly rounded, smoothly passing into moderately tight posteroventral margin. Posterior margin inclined, truncated, posterodorsal margin long, weakly and evenly convex; some specimens develop weakly flattened posterior area. Ligament opisthodethic, external, long. Hinge plate thick. AI short, thick but not prominent, positioned at anterior end of hinge plate. PI short, with socket corresponding to right valve anterior lateral tooth. Posterior laterals weak. PII unknown, presumably shallow. PIII very weak and short, oval. Cardinal dentition present. Right valve: 3a very small, opisthocline; 3b prosocline, strong, not bifid, weakly curved; at base 3b supported by a thickening of hinge plate. Left valve: 2 triangular, deepest at base, becoming weaker dorsally; 4b arcuate, prosocline. Anterior adductor muscle scar deep, striated and detached from pallial line around two thirds to three quarters of length. Ventral margin of anterior adductor muscle scar sharp, dorsal margin irregular and jagged. Anterior pedal retractor scar circular, positioned below anterior lateral teeth, weakly separated from anterior adductor muscle scar. Deep groove connecting anterior adductor muscle scar with umbonal cavity represents trace of descent during growth. Posterior adductor muscle scar deep, striated, pointed ventrally, heart-shaped, projecting beyond the pallial line for about half of diameter. Trace of descending posterior adductor muscle visible as deep grooves connecting it with umbonal cavity. Posterior pedal retractor scar not seen, presumably merged with posterior adductor muscle scar. Pallial line strong, entire; wavy below anterior adductor muscle scar. Some specimens develop dispersed pustules on internal shell surface.

Remarks. Tehamatea rasmusseni sp. nov. differs from T. ovalis (Stanton, 1895), from Tithonian to Albian seeps from California by the smaller size of the 3a anterior cardinal in the former (Stanton 1895; Kiel 2013). Otherwise, the two species are very similar. Tehamatea rasmusseni differs from another Californian seep lucinid, the Tithonian–Hauterivian T. colusaensis (Stanton, 1895), by the more rounded posterior margin and more equilaterally positioned beaks in T. rasmusseni. Tehamatea rasmusseni has similar dentition to T. agirrezabalai Kiel, 2013, from Albian seeps of northern Spain, but in comparison with that species T. rasmusseni has a longer and more detached anterior adductor muscle scar and less projecting beaks. Tehamatea vocontiana from Hauterivian seep carbonates of southern France (Lemoine et al. 1982) and Crimea (Kiel & Peckmann 2008) has a cardinal dentition with only an orthocline 3b developed, unlike T. rasmusseni, which has both 3a and 3b developed, with the 3b weakly curved and prosocline.

Occurrence. Seep 9 (uppermost Ryazanian), Slottsmøya Member, Svalbard (Tab. 1). Known only from the type locality.

Palaeoecology. The seep-restricted distribution of the genus Tehamatea (Kiel 2013), together with the clustering of T. rasmusseni at seep 9 and the presumed antiquity of chemosymbiosis among lucinids (Taylor & Glover 2010) strongly suggests that T. rasmusseni was chemosymbiotic and took advantage from the reduced compounds available in the seep environment. Lucinids are a diverse group of burrowing bivalves having obligate chemosymbiotic relationships with sulfide-oxidizing bacteria (e.g. Dando et al. 1986; Reid & Brand 1986; Herry et al. 1989; Taylor & Glover 2000; Glover et al. 2004). They inhabit a variety of environments, being especially diverse in tropical and temperate shallow water environments with high redox potential, like seagrass beds (Mikkelsen & Bieler 2008), mangrove swamps (Frenkiel et al. 1996), coral sands (Glover & Taylor 1997), reducing sediments (Dando et al. 1986) and sewage outfalls (Herry et al. 1989). Lucinids are also common at hydrocarbon seeps, from both shallow and deep water (e.g. Salas & Woodside 2002; Holmes et al. 2005; Taylor & Glover 2009; Oliver et al. 2011). An increasing number of modern lucinid genera are being reported from deep water sites, and have been recovered from organic-rich sediments in water as deep as 2570 m (Cosel 2006).

To reach sulfide-rich pore waters necessary to feed their symbionts, most lucinids burrow down to the oxic/ dysoxic interface, where they remain stationary with their umbones facing upwards. Their muscular foot is then used to construct ventral tunnels to supply sulfide-rich water to the symbionts in the gills (Taylor & Glover 2010). Oxygenated seawater is supplied via a subvertical mucus-lined tube entering the body parallel to the anterior adductor muscle (Stanley 1970). The elongated and detached anterior adductor muscle acts as a partition separating symbiont-bearing gills from the respiratory surface of the mantle around the anterior opening (Taylor & Glover 2000).

Similar anatomical features as shown by internal shell features are found in the lucinid fossil record back to the Silurian (Boyd & Newell 1979; Fürsich 1982; Kelly 1992; Liljedahl 1992). The wavy pallial line below the anterior adductor muscle scar suggests that T. rasmusseni developed mantle lobes with possible respiratory function (e.g. Taylor & Glover 2000). One specimen shows traces in both valves of tubular structures that represent either shell boring activity or an organism living between the mantle and the shell (bioimmuration traces). Very similar structures have been found in other fossil seep bivalves (Kiel & Peckmann 2008; Jenkins et al. 2013), and have been attributed to possible polychaete worms. All specimens found are articulated or semi-articulated shells filled with carbonate micrite and were enclosed in in matrix composed of cracked and worn Buchia shells with some rare and disarticulated Pseudolimea arctica, Oxytoma octavia and Camptonectes spp. As Svalbard seeps developed in low-depositional rate environment and seabed omission was frequently the case (Hryniewicz et al. 2012), Tehamatea rasmusseni must have been a relatively deep burrower. After death, specimens remained buried for some time and were not exposed by the bottom currents until early carbonate cementation in the seep environment kept the valves in an articulated state.