Provanna marshalli, Saether & Little & Campbell, 2010

Saether, Kristian P., Little, Crispin T. S. & Campbell, Kathleen A., 2010, A new fossil provannid gastropod from Miocene hydrocarbon seep deposits, East Coast Basin, North Island, New Zealand, Acta Palaeontologica Polonica 55 (3), pp. 507-517 : 510-514

Provanna marshalli sp. nov.

Figs. 4, 5.

Etymology: Named for Bruce A. Marshall, malacologist and collections manager of Mollusca at NMNZ, who has published extensively on New

SAETHER ET AL.—MIOCENE PROVANNID GASTROPOD FROM NEW ZEALAND 511

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Zealand molluscan taxonomy, and has been involved in the identification of the modern New Zealand seep faunas since the first official seep collections were made in 1996.

Type material: Holotype: G7103 ( Fig. 4H), well preserved shell lacking protoconch and earliest whorls. Paratypes: one moderately preserved specimen, TM8704 , from Puketawa (Y16/f0580); nine moderately to doi:10.4202/app.2009.1112

well preserved specimens from Rocky Knob: Y16/f1027, AU19605, seven specimens (G7105–7109, G7113, G7126); Y16/f1029, AU15834, one specimen (G7110); Y16/f1031, AU19610, one specimen (G7111). All specimens at UOA, TM8704 borrowed from palaeontology collection, GNS.

Type locality and horizon: Rocky Knob (Y16/f1027), northern Hawke’s Bay area (38 ° 30.58’S, 177 ° 93.10’E). Fossil hydrocarbon seep deposit of the Bexhaven Limestone Formation, Tolaga Group, Middle Miocene .

Material.— 19 poorly to well preserved specimens from Rocky Knob: Y16/f1027, AU19605, 14 specimens (G112, G114–125); Y16/f1028, AU19606, two specimens (G128, one unnumbered specimen); Y16/f1030, AU19609, three specimens (G7104, G7129, one unnumbered specimen). All specimens at UOA .

Dimensions.—See Table 1.

Diagnosis.—Shell medium−sized, ovate, fusiform; sculpture highly variable from nearly smooth to strongly cancellate; body whorl with up to 11 spiral ribs and 30 axial riblets, often forming strong nodes at their intersections, especially along tabulate shoulder; suture inclined at roughly 5 ° from horizontal; at least three teleoconch whorls present.

Description.—Shell medium−sized (D up to 6.7 mm; H up to ca. 10.8 mm), ovate (D/H = 0.60–0.82), fusiform; shell microstructure consists of at least one (outer) simple prismatic layer (ca. 5–10 µm) above thicker crossed lamellar layer (ca. 30–40 µm); periostracum not preserved. Sculpture highly variable, ranging from nearly smooth, with only very weak spiral ribs or very weak axial riblets formed by rugose growth lines, to strongly cancellate with several spiral ribs, widely but evenly spaced. At least three teleoconch whorls present, protoconch and very earliest whorls not preserved (see remarks below); whorls rather convex with tabulate shoulder delineated by spiral rib, often strongly noded; suture distinct, inclined at angle roughly 5 ° from horizontal (shell axis vertical); last whorl moderately globose with distinct neck having up to three spiral ribs, moderate to weak; 0–5 spiral ribs on penultimate whorl, moderate to weak; on last whorl number of ribs varies from none (or possibly two very weak, one on shoulder, one on neck) to ca. 11, with increased strength towards neck and shoul− der; 0–30 axial riblets on last whorl, less on earlier whorls, which often form strong nodes where crossing spiral elements, especially prominent on tabulate shoulders; strong sculpture can be consistent from aperture through to early whorls, or fade out in middle whorls, being expressed only in antepenultimate (possibly earlier) whorl and in latest shell, but usually with some weak sculpture in between. Aperture higher than wide (d/h = 0.61–0.94, probably less variable in life before taphonomic alteration), usually just over half height of shell; basal and outer lips often eroded, where intact forming almost circular continuation; basal lip comprises lowest point of shell from apertural perspective, passing to distinct siphonal canal, visible in a few specimens where not eroded. Operculum unknown.

Remarks.—A single, variable species is herein recognised based upon the following points: (i) the similarity in general shell shape between the specimens, with variations being observed chiefly in ornamentation; (ii) a continuum of ornamental strength, with smooth and strongly cancellate end−member specimens separated by intermediate specimens of incremental ornamental strength that grades from one end to the other; (iii) the frequency of observed morphological variation of a similar nature in other described species of the genus; (iv) the collection of the majority of specimens from a single assemblage within one site. Modern species of this genus are commonly rather simple and variable in shell shape and form, which can make it difficult to compare them across localities, although there is usually some obvious shell morphological difference where two or more species are found at a given site ( Warén and Bouchet 1993).

Many of the Provanna marshalli specimens are well preserved, shown by the retention of original shell microstructure ( Fig. 5), although the protoconchs and very earliest whorls are never present. It is rare to collect the fully intact larval shell of modern species of Provanna , because it is thought that the apical whorls of the protoconch are discarded after the veliger phase ( Kaim et al. 2008a; Warén and Bouchet 2009). Furthermore, the corrosive condition of their environment often leads to loss of further early whorls in life, and secondary growth of a calcareous plug to replace the protoconch is common ( Kaim et al. 2008a; Warén and Bouchet 2009). These environmental factors, along with preservational constraints during fossilisation, make observations of this key taxonomic shell feature in fossil material highly unlikely. Indeed, only once has the protoconch I been reported in fossil material, in two specimens of Provanna antiqua (Kiel 2006: fig. 5.1–4). A decollate protoconch II also was reported in a Late Cretaceous−age juvenile Provanna specimen by Kaim et al. (2008a), where exquisite preservation occurred by early diagenetic silicification. Moreover, of 18 described modern species, only four species have been figured with an intact protoconch I, and these always from juvenile specimens ( Warén and Ponder 1991; Warén and Bouchet 1993, 2009; Gustafson and Lutz 1994; Kaim et al. 2008a).

The smooth specimens of Provanna marshalli resemble the species Provanna chevalieri Warén and Bouchet, 2009 from off West Africa, Provanna glabra Okutani, Tsuchida, and Fujikura, 1992 from Sagami Bay, Japan, and Provanna laevis Warén and Ponder, 1991 from the Gulf of California, but none of these species ever develops the strong ornamentation seen in other specimens of P. marshalli . Two further species may have smooth−shelled individuals: Provanna antiqua and Provanna variabilis , and strong morphological variation is seen in both species. However, morphological variation in P. marshalli is even greater than that seen in P. antiqua and P. variabilis , with a similar smooth end−member but ranging up to a far greater number of axial riblets. The suture in P. marshalli is different from P. antiqua in its greater inclination from the horizontal (where shell axis is vertical).

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Moreover, the shells of Provanna marshalli are considerably larger, reaching nearly twice the height of Provanna antiqua , are wider but less squat overall, and also with a larger maximum height than Provanna variabilis . The new species has similarities to the “Shosanbetsu Provanna sp. ” from a Miocene whale fall community in Japan, which was figured and briefly, but not formally, described by Amano and Little (2005). However, the Japanese species has a maximum measured height of 6.1 mm, which is significantly less than P. marshalli (ca. 10.8 mm). It also has weaker sculpture that exhibits less variability (22–26 axial ribs on the last whorl), and a higher maximum number of spiral cords (16) than is seen in any specimen of P. marshalli . Provanna admetoides is the only species of the genus to possess more axial riblets (35–45 on last whorl) than the maximum observed in specimens of P. marshalli . Provanna admetoides differs in this character, as well as in its lesser variability, and far stronger and less numerous spiral ribs, which gives it a much more distinctly spirally keeled appearance than P. marshalli . Furthermore, the reticulations in the ornament of P. admetoides , formed by the intermeshing of axial and spiral elements, are elongated axially, as opposed to more or less equilateral in P. marshalli . Specimens of P. marshalli with stronger, more cancellate sculpture can resemble Provanna nassariaeformis Okutani, 1990 , but have a larger maximum number of axial and spiral ribs, a less globose shell, and more variability in ornamentation overall than this species. Specimens of P. marshalli with intermediate ornamentation often resemble Provanna lomana (Dall, 1918) , but the latter does not display the variation in its sculpture, and spiral ornamentation disappears in whorls earlier than the last whorl, unlike in P. marshalli , in which it persists, albeit more weakly, in some specimens.

The shell microstructure of P. marshalli , where preserved, is shown to be similar to that observed in some modern and fossil specimens of the genus. Kiel (2004) described the microstructure of the shell material of two P. variabilis specimens from the Juan de Fuca Ridge, which consists of an outer, organic periostracum slightly separated (probably during preparation) from an outer simple prismatic layer, beneath which is a thicker layer with a complex crossed lamellar structure. Another simple prismatic layer forms the innermost portion of the shell, but this layer is variably present or absent depending upon location on the shell. Kiel (2006) reported a similar shell microstructure in P. antiqua and Kaim et al. (2009) also showed this pattern in the shell of a related gastropod, Hokkaidoconcha hikidai Kaim, Jenkins, and Warén, 2008a . The periostracum is never preserved in the New Zealand fossil specimens, but there are several places in the shell of one specimen ( Fig. 5A) where the outer simple prismatic layer is visible above a thicker crossed lamellar layer. Since the shell microstructure has only been observed in commarginal exposures, it cannot be confirmed whether the nature of the crossed lamellar layer is simple (i.e., two non−vertical dip directions) or complex (i.e., three or more non−vertical dip directions) (cf. Carter 1990). In another specimen from this study ( Fig. 5B), a distinct simple prismatic layer is preserved above layers that recrystallised during diagenesis. The boundary is not particularly sharp between the outer simple prismatic and complex crossed lamellar layers in modern specimens (see Kiel 2004: figs. 30–32). In the fossil material, the transition between these layers also is indistinct ( Fig. 5A), although there is a rather distinct boundary between the preserved simple prismatic and lower recrystallised layer in one specimen ( Fig. 5B). The inner simple prismatic layer, which can form a sharp boundary with the complex crossed lamellar layer in modern specimens (see Kiel 2004: fig. 30) is never preserved in the fossil specimens. This is what might be expected, where the organic periostracum would have protected the outer simple prismatic layer until it naturally decayed after burial, the inner simple prismatic layer afforded no such protection. Weathering of internal shell layers likely occurred on the sea floor or during early burial, leaving the two inner shell layers relatively unscathed where conditions conducive to such preservation prevailed thereafter.

Stratigraphic and geographical range.— Early to Middle Miocene seep carbonates of Hawke’s Bay, North Island, New Zealand. Known mostly from the type locality at Rocky Knob (28 specimens), with one confirmed specimen from Puketawa and one possible juvenile specimen from Ugly Hill .