Gigantidas coseli Saether, Little, Campbell, Marshall , Collins and Alfaro

Saether, Kristian P., Little, Crispin T. S., Campbell, Kathleen A., Marshall, Bruce A., Collins, Mike & Alfaro, Andrea C., 2010, New fossil mussels (Bivalvia: Mytilidae) from Miocene hydrocarbon seep deposits, North Island, New Zealand, with general remarks on vent and seep mussels, Zootaxa 2577, pp. 1-45 : 21-26

Species Gigantidas coseli Saether, Little, Campbell, Marshall, Collins and Alfaro sp. nov.

( Figs. 4 B, C, 8–10, 17)

?Mytilid closely resembling Idasola Beu & Maxwell, 1990 .? Bathymodiolus n. sp. B Collins, 1999.

Group 3 (G3) Collins, 1999.

Large, curved, elongate fossil form similar to Gigantidas Campbell et al., 2008 . Bathymodioline bivalves Saether et al., 2010.

Holotype. Specimen L4203 ( Figs. 8 A.6, B.1–4), adult, UOA.

Type locality. Moonlight North (Y16/f1054), Hawke’s Bay, North Island, New Zealand; Waiauan (Late Middle Miocene–Earliest Late Miocene) hydrocarbon seep carbonate.

Paratypes. Eight moderately to well preserved small to large specimens. One small, TM8750 ( Fig. 8 A.2), one medium sized, TM8749 ( Figs. 9 C, D), from Bexhaven (Y16/f0566); three large, L4560, L4210, L4213, from Moonlight North (Y16/f1033, Y16/f1054); one small, L4204 ( Fig. 8 A.1), one medium sized, L4209 ( Fig. 9 E), from Rocky Knob; one medium sized, L4215 ( Fig. 8 A.4), from Tauwhareparae. All specimens at UOA, two borrowed from GNS. See Table 6 for dimensions.

Other material. 22 poorly to moderately preserved small to large specimens: one from Bexhaven (Y16/ f1032); 14 from Moonlight North (Y16/f1033, Y16/f1054); one (tentatively assigned) from Puketawa (Y16/ f0580), four from Rocky Knob (Y16/f1036, Y16/f1043, Y16/f1047), two from Turihaua (Y18/f0372). All specimens at UOA, three borrowed from GNS. See Appendix I for dimensions and specimen data.

Etymology. Named for Dr. Rudo von Cosel, a leading researcher in vent and seep mussel taxonomy.

Diagnosis. Shell small for genus (L up to 99.0 mm); umbones subterminal, anterior, at 6–18% along shell length; anterior portion rather protrusive anteriorly; ligament ending in gentle taper in adult; subligamental ridge visible from lateral and ventral perspectives; posterior adductor scar moderate in size.

Description. Shell small for genus (L up to 99.0 mm, H up to 34.0 mm*, I up to 27.0 mm), aduliform, rather thin (usually not preserved or expressed only in flaky patches), slender, becoming more elongate with growth, juvenile (L <40 mm) H/L = 0.32–0.39, I/L = 0.26–0.34, adult H/L reaching 0.23, I/L reaching 0.18, equivalve, some specimens with valves slightly twisted against one another with left–right sense along anteroposterior axis ( Fig. 8 B.2). Umbones subterminal, prosogyrate, anterior, in most specimens at 11–13% shell length throughout growth, in some becoming further anterior with increasing size, position more variable in smaller specimens than in larger specimens; umbonal region moderately elongated, moderately pronounced in adult, more angulated and distinct in juveniles, covering ca. 20–25% of dorsal length in adult, less in juveniles. Posterior angulation running from umbones to posterior end of ventral margin, becoming stronger with growth, somewhat sharply defined internally, more rounded externally, forming widest point of shell at about mid-length; in dorsal view, posterior angulation development causes increasing central bulging of shell with growth, tapering to posterior margin; much smaller anterior angulation runs from umbones to anterior margin, forming secondary anterior bulge in dorsal view, area between bulges slightly constricted in some of largest specimens; bulges less apparent in smaller specimens, which appear more uniformly fusiform in dorsal profile. Anterior portion of shell rather protrusive anteriorly. Anterior margin somewhat narrowly and evenly rounded; dorsal margin nearly straight in very small specimens, becoming gently convex in halfgrown specimens to rather convex in fully grown specimens; posterodorsal corner at point of greatest valve height, broadly rounded, indistinct to weakly angulated; posterior margin broadly and unevenly rounded, sloping ventrally; ventral margin nearly straight to weakly and more or less evenly convex in juvenile, specimens), strongly concave in adults, rather strongly convex along posterior third, straight (typically) or weakly convex in anterior quarter below umbo. External surface with irregular, well developed commarginal growth lines and waves; growth waves weakly reflected on interior, especially towards margins, strongest in posteroventral portion. Hinge and ligament plate unknown. Ligament extending over ca. 80–90% of dorsal margin, running from beaks to in front of posterodorsal corner, terminating posteriorly in gentle taper. Subligamental ridge well developed, running from behind beaks, becoming obsolete at 70–100% ligament length in direction diverging from dorsal margin towards point about two-thirds up on posterior margin, clearly visible from lateral perspective. Muscle scars faint (generally poorly preserved, known principally from three specimens, Figs. 9 A–C); anterior adductor and anterior byssal retractor scars unknown; posterior adductor scar of moderate size, situated at two-thirds of valve height in dorsal half, posterior limb rounded, united anterodorsally with posterior scar of posterior byssal retractor; anterior scar of posterior byssal retractor well separated, preserved in only one incomplete specimen with eroded dorsal margin, making relative position unclear, but roughly beneath third and last quarter of ligament; pallial line more or less parallel with ventral margin, at least in posterior half, anterior portion not preserved in any specimens. Larval shell unknown.

* Measured in an incomplete internal mould comprising the largest posterior portion of any specimen in the collections, which, if assumed to be from a shell of typical overall shape, would reach a length of at least 120–150 mm, especially with consideration of the trend of increasing elongation with growth.

Remarks. Gigantidas is a recently erected genus with only two species (both modern) so far described. Although the group is largely based upon anatomical features, several shell characteristics can be used to justify inclusion of the new species there, including shell elongation and overall shape, umbonal placement, position of the posterior adductor muscle scar, fusion of the posterior adductor muscle scar with the posterior scar of the posterior byssal retractor, and the widely separated anterior part of the posterior byssal retractor scar. Of the two modern species, G. c o s e l i resembles G. gladius in lateral view and G. horikoshii in dorsal view. Gigantidas coseli is less elongate than G. gladius and more elongate than G. horikoshii , and the largest specimens of G. c o s e l i are significantly smaller than both. The umbones are somewhat more anteriorly placed than in G. gladius and G. horikoshii . The posterior angulation runs at a steeper angle over the shell than G. gladius and G. horikoshii , its termination occurring in a more anteroventral shell location. The anterior angulation is less pronounced than G. gladius , the dorsal profile with a weaker secondary anterior bulge similar to that of G. horikoshii . The anterior part of the shell is less protrusive anteriorly than in G. gladius and G. horikoshii because of the more anteriorly situated umbones, and the anterior margin is less narrowly rounded than in both G. gladius and G. horikoshii . The ligament ends in a gentle taper posteriorly in adult G. coseli , unlike G. g l a d i u s in which the termination is abrupt or has a short taper, and G. h o r i k o s h i i in which it is abrupt, although it is weakly tapered in smaller specimens of G. horikoshii . The subligamental ridge of G. coseli is strong like in G. g l a d i u s and G. horikoshii , but oriented more divergently from the dorsal margin, allowing it to be seen from the lateral perspective, unlike in G. gladius and G. horikoshii , in which it is only visible from the ventral perspective. The posterior adductor scar is smaller relative to the shell compared with that of G. gladius and G. horikoshii . Morphometric analysis supports a closer relation of the new species to G. gladius than G. horikoshii , with the ontogenetic height vs. length curve of the smaller G. c o s e l i appearing to align with that of the larger G. gladius ( Fig. 11 ). The growth curve of G. horikoshii ( Fig. 11 ) is quite distinct from those of the other two species, clearly diverging from that of G. c o s e l i already at the stage of quartergrown specimens (L = 40–50 mm), and showing consistently less tendency toward elongation with growth through to the fully grown stage.

Gigantidas coseli represents the first record of a formally described and named species in this genus from a hydrocarbon seep environment, with all reports of modern named species having so far been from hydrothermal vent sites in the Pacific Ocean. However, up to three undescribed species have been reported from seep environments off Japan and New Zealand ( Fujita et al. 2009; Baco et al. 2010; Table 2), and there are several other species within the B. childressi clade that are known only from hydrocarbon seep sites, including those from geographically close regions in the SW Pacific, and two species in the clade ( B. japonicus and B. platifrons ) are recorded from both hydrocarbon seep and hydrothermal vent environments (Table 1).

Figures

FIGURE 4. Images showing typical hydrocarbon seep carbonate deposit and appearance of New Zealand hydrocarbon seep mussels in field samples before curating. A. Several float boulders (arrowed) originating upslope at Moonlight North locality. Person for scale. B. Clustered specimens of Gigantidas coseli sp. nov. occurring in situ from Bexhaven locality. Scale bar is 20 mm. C. Ex situ occurrence of specimens of G. c o s e l i (arrowed) from Moonlight North. Scale bar is 30 mm.

FIGURE 8. Growth series and holotype of Gigantidas coseli sp. nov. A. Diagnostic lateral views of left and right valves to show growth series. Scale bar is 20 mm. A. 1. Paratype L 4204. A. 2. Paratype TM 8750. A. 3. Specimen L 4563. A. 4. Paratype L 4215. A. 5. Specimen L 4561. A. 6. Holotype L 4203. A. 7. Specimen L 4211. B. Diagnostic views of holotype L 4203. Scale bar is 10 mm. B. 1. Anterior view. B. 2. Dorsal view. B. 3. Lateral view of left valve. B. 4. Lateral view of right valve.

FIGURE 11. Scatter plot with lines of best fit showing ontogenetic changes in shell height vs. shell length (A) and shell height / length ratio vs. shell length (B) in specimens of Gigantidas coseli sp. nov. and its modern congeners.: G. c os e li;: G. gladius Cosel and B. Marshall, 2003;: G. horikoshii Hashimoto and Yamane, 2005.