Semeloidea bexhavenensis, Saether, Kristian P., Jingeng, Sha, Little, Crispin T. S. & Campbell, Kathleen A., 2016

Saether, Kristian P., Jingeng, Sha, Little, Crispin T. S. & Campbell, Kathleen A., 2016, New records and a new species of bivalve (Mollusca: Bivalvia) from Miocene hydrocarbon seep deposits, North Island, New Zealand, Zootaxa 4154 (1), pp. 1-26 : 14-16

Semeloidea (s. l.) bexhavenensis sp. nov.

( Fig. 8 )

Holotype. Specimen L4666, ( Fig. 8 ),?adult, UOA.

Type locality. Bexhaven (Y16/f0566A), Hawke’s Bay, North Island, New Zealand; middle Miocene (Lillburnian) hydrocarbon seep carbonate.

Paratype. Specimen L4665 ( Fig. 8 ),?adult, UOA.

Etymology. Named for the type locality Bexhaven, where both specimens were collected.

Diagnosis. Shells large (H up to ca. 10 mm); anterior and posterior margins fairly broadly rounded; radial ribs fine, concentrated ventrolaterally; umbones central, angulate, slightly prosogyrate.

Description. Shells large for genus (H up to ca. 10 mm; I up to ca. 6 mm; L up to ca. 12 mm), thin, squat (H/L = 0.84–0.88), trigonal to subquadrangular; ca. 30–34 fine radial ribs on each valve, especially strong towards anterior and posterior flanks, alternating between thicker, stronger ribs and finer, weaker ones, fading in medial area and in medioventral region, stronger internally than externally, apparently absent from umbonal slopes and central umbonal region, first ribs to appear reaching about halfway down on anterior and posterior margins; weaker, closely and evenly spaced, concentric growth lines cross ribs, at least on internal shell, of equal strength at least in umbonal and posteroventral regions, probably over whole shell where fully preserved; umbones small and narrow, tapering to distinct angulation roughly in centre of dorsal margin, weakly prosogyrate; anterodorsal and posterodorsal margins straight, anterior and posterior margins evenly rounded, a little more narrowly in anterior margin, ventral margin gently convex; hinge details, ligament, muscle scars and prodissoconch unknown.

Remarks. Semeloidea abdiplex , from the Miocene Ihungia Series, Gisborne District, is the closest in age and geographical location to the fossil specimens of this study. However, the two specimens of S. bexhavenensis sp. nov. have similar heights (9.2 mm and 9.9 mm, respectively), which is considerably larger than any previously reported New Zealand fossil Semeloidea (the next largest, S. miocenica , has a maximum reported height of 6.5 mm). Semeloidea abdiplex also has more broadly rounded anterior and posterior margins than S. bexhavenensis .

Semeloidea miocenica has a distinctly more trigonal lateral shell outline with considerably more narrowly rounded anterior and posterior margins and relatively longer antero- and posterodorsal margins compared to overall shell size. Semeloidea donaciformis is more strongly trigonal in shape than S. bexhavenensis , the anterior and posterior margins of the former species being rather more narrowly rounded and extending over a greater portion of the overall shell margin, although to a lesser extent than S. miocenica .

These differences make the new species intermediate in shell shape between S. abdiplex and S. donaciformis , with S. miocenica the most distinct from it. The new species differs further from each of the other three New Zealand species in having finer radial ribs, although their distribution is similar in each species, as they all appear to fade out in the medial, medioventral and umbonal regions, being particularly strong towards the antero- and posteroventral portions of the shell. The umbones in S. abdiplex and S. donaciformis appear to be slightly more prosogyrate than those of S. bexhavenensis , although they are similar in placement and narrow tapering to form a distinct angulation in the centre of the dorsal margin. The umbones of S. miocenica appear to be less prosogyrate than those of S. bexhavenensis , this condition being almost indistinguishable in the former species, and the umbones are more broadly rounded in S. miocenica . Given the absence of reports of this genus, family and superfamily from fossil seeps and modern New Zealand seep environments, and the occurrence of only two specimens of S. bexhavenensis from a single northern New Zealand fossil seep site, it is probable that the specimens belong to a species that was not commonly associated with seep environments in life.

Northern localities Southern localities Taxon BX KK MN PK RK TH TT TW WP HA NG UH WA Protobranchia ord. indet. ×

Solemyidae gen. indet. ×

Bathymodiolus (s. l.) heretaunga × × × × × × × × ×? × × Gigantidas coseli ×? × × ×? cf. Pulvinites sp.

Acesta cf. saginata ×

Parvamussium cf. maorium ×

cf. Meganodontia sp. × × × × Pratulum quinarium × ×? × Notocalyptogena neozelandica × × × × × × cf. Pliocardia sp. × ×

Semeloidea bexhavenensis sp. nov. ×

No. of identified bivalve taxa: 5 1 7 2 4 2 1 3 1 2 1 3 3 Average diversity: 2.89 2.25

Figures

FIGURE 8. Semeloidea (s. l.) bexhavenensis sp. nov. A. Specimen L 4665. Scale bar is 3 mm. A 1. Dorsal view. A 2. Lateral view of left valve. A 3. Anterior view. A 4. Lateral view of right valve. B. Specimen L 4666 (holotype). Scale bar is 3 mm. B 1. Lateral view of right valve. B 2. Dorsal view. B 3. Lateral view of left valve. B 4. Anterior view. C. Shell fragment from anteroventral portion of left valve of specimen L 4666, showing corrugated nature. Scale bar is 1 mm. C 1. Internal surface. C 2. External surface. C 3. View from the shell edge.