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Research Article
A new species of genus Provanna (Gastropoda, Abyssochrysoidea) from gas hydrate-bearing sediments of the northern South China Sea
expand article infoCong Wu§|, Fang Chen§|, Ying Tian#, Kazutaka Amano¤, Xin Su«
‡ National Engineering Research Center of Gas Hydrate Exploration and Development, Guangzhou, China
§ Key Laboratory of Marine Mineral Resources, Ministry of Land and Resources, Guangzhou, China
| Guangzhou Marine Geological Survey, Guangzhou, China
¶ Dalian Shell Museum, Dalian, China
# Dalian Ocean University, Dalian, China
¤ Department of Geology and Paleontology, National Museum of Nature and Science, Ibaraki City, Japan
« China University of Geosciences, Beijing, China
Open Access

Abstract

The genus Provanna (belonging to Superfamily Abyssochrysoidea) is a group of globally distributed gastropods commonly discovered in the deep-sea chemosynthetic environments. To date it is composed of 29 extant and nine fossil species and all of them seem to be endemic to vents, seeps or organic falls. Despite the increasing interest in cold seeps in South China Sea in recent decades, how this genus is distributed in this area is largely unknown. A new fossil species, Provanna dongshaensis sp. nov., collected from the hydrate-bearing sediments in the cold seep area of northern SCS, was studied. A basic morphological description together with the observation of shell microstructures was made, and a brief comparison to known Provanna species was summarized. The occurrence depths of this new species are consistent with the remarkable negative carbon isotope of carbonate layers, which mutually supports the relationship to the ancient seep. Such sedimentary samples from drilling cores may be potentially better materials for systematic research of deep-sea gastropods and other chemosynthesis fauna.

Key Words

Ancient seep, fossil, Provanna, South China Sea

Introduction

Provanna is a genus of small gastropods found in deep-sea chemosynthetic communities such as hydrothermal vents, hydrocarbon seeps, and organic falls like sunken wood and whale falls. They are known to primarily feed on bacterial mats and detritus, functioning as both deposit feeders and grazers (Warén and Bouchet 1986, 1993; Sibuet and Olu 1998; Sasaki et al. 2010).

The genus Provanna is considered the most diverse among the superfamily Abyssochrysoidea (Johnson et al. 2010), encompassing 29 extant species (Chen et al. 2016a, 2019; Betters and Cordes 2023) and nine fossil species (Amano and Jenkins 2013; Amano and Little 2014). The extant species have been identified in various deep-sea environments globally, including West Pacific (Okutani 1990; Okutani et al. 1992; Okutani and Fujikura 2002; Sasaki et al. 2016; Chen et al. 2016a, 2019; Ogura et al. 2018; Chen and Watanabe 2022), Fiji (Warén and Bouchet 1993), the East Pacific (Warén and Bouchet 1986, 1989), the Gulf of Mexico (Warén and Ponder 1991), the Caribbean Sea, the Southern Ocean (Linse et al. 2019), and West Africa (Warén and Bouchet 2009). On the other hand, fossil species have primarily been found in hydrocarbon seep deposits in Japan (Kaim et al. 2008, 2009; Amano and Jenkins 2013; Amano and Little 2014), the USA (Kiel 2006; Hybertsen and Kiel 2018), Peru (Kiel et al. 2020), Trinidad (Gill et al. 2005; Kiel and Hansen 2015) and New Zealand (Saether et al. 2010). Notably, there appears to be a prevalent peri-Pacific distribution pattern for the genus Provanna, with a concentration around Japan and the east coast of the USA, except for seven specific localities in the Gulf of Mexico, West Africa, the Caribbean Sea, Trinidad, and the East Scotia Ridge (Fig. 1). Nevertheless, recent years have witnessed a rise in more definitive discoveries of Provanna species in additional typical seep regions like West Africa and the Southern Ocean (Saether et al. 2010; Amano and Jenkins 2013; Amano and Little 2014; Sasaki et al. 2016; Hybertsen and Kiel 2018; Linse et al. 2019; Kiel et al. 2020). Since 2010, 15 Provanna species, comprising nine extant and six fossil species, have been formally described, representing one-third and two-thirds of the extant and fossil groups, respectively. Betters and Cordes (2023) extends the distribution range of P. laevis, P. ios, and P. pacifica, and presents a comprehensive review of the genus Provanna, integrating the geographic distribution, genetics, and morphological characteristics of each extant species. Despite these developments, the global biodiversity of the genus Provanna remains inadequately documented.

Figure 1. 

Global distribution of genus Provanna displayed as type locations of known species and location of this study (GMGS2-07B and GMGS2-09B). orange circle: active hydrothermal vents, hydrocarbon seeps where extant Provanna species are discovered; blue circle: seep deposits or organic falls that yield fossil Provanna.

In the South China Sea (SCS), a marginal sea of the Pacific Ocean, seep carbonates were first discovered in the northern SCS in 2004 (Chen et al. 2005). Since then, several sites of hydrocarbon seepage have been identified, indicating the presence of cold seep systems in the region (Feng et al. 2018). However, limited attention has been given to the gastropods related to the chemosynthetic system in this area, with scant information available on the detailed taxonomy and distribution of the genus Provanna in the South China Sea.

During China’s second gas hydrate drilling expedition (GMGS-2) in the east of the Pearl River Mouth Basin, SCS, long drilling core sediments containing carbonate deposits were obtained. This acquisition gave a chance to investigate the local chemosynthetic community with a special emphasis on animals that have carbonate shells. Several specimens of the provannid genus were fortunately retrieved from the hydrate-bearing layers and have been identified as a new type within the genus Provanna. In this study, we give a formal description of the specimens, together with observations of their shell microstructures. Additionally, we undertake some brief comparisons with other Provanna species, and explain the geological significance of this newly found species.

Material and methods

Specimens of the new fossil Provanna were discovered in two drilling sites, GMGS2-07B and GMGS2-09B, at a water depth of 791 m and 664 m respectively, located in Dongsha area, northern part of SCS (Fig. 1 with a window partial magnified).

The individual gastropods were collected after the treatment of sediment sample by soaking and screening, a similar laboratorial processing of the coarse fraction analysis. 16 to 40 grams of dried sediment for each sample was soaked for 2 days in the beaker without any chemical dispersing agent until completely dispersed. It was then washed through a 250-mesh copper screen (pore size = 0.063mm), until each sample was guaranteed to be clean. For each specimen, the sampling procedure is as follows: (1) Sectioning is performed along the axis from the apex to the umbilicus of the shell. This results in two cross-sections of the shell; (2) Sampling is conducted at intervals of 1cm from the apex, with each sample measuring approximately 1 cm×1 cm; (3) The sectioned shells are polished on sandpapers with grit sizes of 220, 600, 800, 1200, 2000, and 3000 for a duration of 3 minutes each; (4) After each polishing step, the specimens are cleaned in an ultrasonic cleaner for 15 seconds; (5) The polished samples are then immersed in an EDTA solution buffered with ammonia to a pH of approximately 7–8, with a small amount of formaldehyde added, for a period of 10 minutes; (6) The samples, after immersion, are cleaned in an ultrasonic cleaner for two cycles of 20 seconds each; (7) The samples are then air-dried in a cool, dry place for 48 hours; (8) The shell quality structure is initially observed under a microscope to assess clarity; if the structure is clearly visible, the process proceeds to the next step, otherwise, steps (3)-(8) are repeated. Morphological observation and morphometric measurements of the shell were carried out under a Zeiss Discovery V20 stereo microscope. Scanning electron microscopy (SEM) of these specimens were carried out uncoated at 15 kV in a Hitachi TM3030 SEM provided by Guangzhou Marine Geological Survey (GMGS).

Systematic description

Clade Caenogastropoda Cox, 1960

Superfamily Abyssochrysoidea Tomlin, 1927

Family Provannidae Warén & Ponder, 1991

Provanna Dall, 1918

Type species

Trichotropis (Provanna) lomana Dall, 1918; Magdalena Bay, USA, Recent.

Provanna dongshaensis sp. nov.

Fig. 2A–O

Etymology

This new species is the only appearance of genus Provanna in SCS to date . We use the specific epithet “dongsha” to present the place where it is discovered.

Type material

Holotype : GMGS2-09B-C15-2. Paratype: GMGS2-07B-A1.

Type locality

Site GMGS2-07B, GMGS2-09B, Dongsha area, northern SCS (Fig. 1).

Buried depth and type horizon

3.50, 3.60, 6.07 mbsf (meter below sea floor) at core GMGS2-09B, 33.85 mbsf at core GMGS2-07B; Carbonate layers, Late Pleistocene (Table 1).

Table 1.

General information of specimens found in the Site GMGS2-07B and GMGS2-09B.

Specimens No. Type Measurements (mm) Protoconch Surface preservation Depth (mbsf) Sampling core Ages (a.B.P.)
GMGS2-09B-C15-2 (Fig. 2A–C) holotype shell height: 7.80; diameter of the last whorl: 5.02; apertural height: 3.66; shell thickness of the last whorl: 0.58 broken slightly eroded 6.07 09B-4 16230 ± 50 *
GMGS2-07B-A1 (Fig. 2D–F) paratype shell height: >10.89; diameter of the last whorl: 7.13; apertural height: 6.77; shell thickness of the last whorl: 0.41 unknown strongly eroded 33.85 07B-2H-2A 91693**
GMGS2-09B-C15-1 (Fig. 2G–I) (fragment) broken eroded 3.50 09B-2M-1A 15120 ± 50*
GMGS2-09B-C14 (Fig. 2J–L) (fragment) unknown strongly eroded 6.07 09B-4 16230 ± 50*
GMGS2-09B-C13 (Fig. 2M–O) (fragment) broken eroded 3.60 09B-2M-1A 15120 ± 50*

Material

Type material and other fragment specimens. All specimens are deposited in China Deep Sea Drilling Core Repository, Guangzhou City, China.

Dimensions

Holotype is approximately 7.80 mm in height and 5.02 mm in width of the whole whorl, 3.66 mm in apertural height and up to 0.58 mm thick at the apertural wall. Apical angle of approximately 54° reaching at least 3.66 mm in height and 3.02 mm in width.

Diagnosis

A medium-sized Provanna, sturdy, shell with prominent, angular whorls with nodes at intersection of the axial and spiral ribs.

Description

Protoconch: at least one whorl, but poor preservation in all specimens; maximum diameter about 0.58 mm (Fig. 2C); sculptured pattern not clear; transition to teleoconch unknown. Teleoconch: shell height 7.8 to more than 10 mm. Diameter of the last whorl 5.02 to 7.13 mm. Shell thick, 0.41 to 0.58 mm in the thickness of last whorl. Shell four to five whorls, sculptured by reticulate pattern. 15 to 16 strongly orthocline axial ribs (Fig. 2C, H, L) on both the penultimate and last whorls. Three prominent and equally strong spiral ribs with blunt, strong and swollen tubercles at intersections on the penultimate whorl, and up to 7 on the last whorl. Tubercles more prominent on lower whorls, like strings of pearls. Basal margin marked by the distinct with strength-decreasing spiral rib. Axial ribs start at the upper suture and fade below the lower spiral rib. Regularly and closely spaced, fine spiral treads on whorl flank, fine axial growth increments. Aperture broadly oval, siphonal canal bordered by low ridge.

Figure 2. 

Provanna dongshaensis sp. nov., from a late Pleistocene seep site in the northern SCS. A–C. (Holotype, GMGS2-09B-C15-2). Specimen from core 09B-4, shell height: 7.8mm; D–F. (Paratype, GMGS2-07B-A1). Specimen from core 07B-2H-2A with the protoconch almost lost, shell height: 10.89mm; G–I. Incomplete specimen GMGS2-09B-C15-1 from core 09B-2M-1A; J–L. Incomplete specimen GMGS2-09B-C14 from core 09B-4. M-O. Incomplete specimen GMGS2-09B-C13 from core 09B-2M-1A. Scale bar: 5 mm.

Remarks

(Table 2) P. dongshaensis sp. nov. is differentiated from those provannids with smooth shell surface and simple sculpture by its prominent angulate appearance, particularly including the extant species living in Japan seeps and vents (e.g., P. abyssalis, P. glabra, also see "Shell microstructures of Provanna"). It exhibits a smaller number of the spiral ribs but more pronounced axial structure compared to P. cingulata and P. macleani. P. dongshaensis sp. nov. shares with P. sculpta the strong and inflated tubercles, which makes them different from those spiny species like P. goniata, P. ios, P. stephanos, etc, but differs in its shelf-structure. Compared to another similar species, P. fortis, it has more robust axial structures on the body-whorl, which are generating the inflated tubercles and bead-like appearance.

Table 2.

Spatial distribution, geological time, and survival depth (extant species) of all known species of genus Provanna, associated with their morphological comparisons to the Provanna dongshaensis sp. nov. described in this study.

Species of Provanna Distribution and Environment Buried/survival depth (m) Ocean/Area* Age Morphological comparison to P. dongshaensis sp. nov.
P. alexi Amano & Little, 2014 Shosanbetsu Village, northwestern Hokkaido, Japan, Chikubetsu Formation, whale-fall. WP Miocene lack the strong angulation
P. antiqua Squires, 1995 Washington State, USA, seep and wood-fall. EP Eocene to Oligocene rounded whorls and lacks spines/tubercles
P. fortis Hybertsen & Kiel, 2018 Satsop Weatherwax seep deposit, Washington State, USA. EP Eocene the most similar,4–5 whorls, distinct angulation, 2 spiral ribs on the spiral base and up to 5 on the lowermost whorl, the same shelf-structure with new type, but less distinct strong axial structure, which make the new type a new species
P. hirokoae Amano & Little, 2014 Joetsu City, Niigata, Japan, Ogaya Formation, seep. WP Miocene lack the strong angulation
P. marshalli Saether, Little & Campbell, 2010 East Coast Basin, North Island, New Zealand, Bexhaven and Ihungia Limestone Formation, seep. WP Miocene similar to P. antiqua, lacks strong angulation but more spiral ribs on the whorls than P. dongshaensis sp. nov.
P. nakagawaensis Kaim, Jenkins & Hikida, 2009 Nakagawa area, northwestern Hokkaido, Yezo Group, seep and wood-fall. WP Cretaceous a distinct reticulate pattern, lacks the spines and does not have the shelf-structure made up by strong angulation
P. pelada Kiel et al., 2020 Cerros El Pelado block 2, Talara Basin, Peru. SA Oligocene
P. tappuensis Kaim, Jenkins & Waren, 2008 Tappu area, northwestern Hokkaido, Japan, Yezo Group, seep. WP Cretaceous strong axial and spiral sculpture, but less spiral ribs on the body-whorl
P. urahoroensis Amano & Jenkins, 2013 Urahoro, eastern Hokkaido, Japan, Nuibetsu Formation, seep. WP Oligocene does not have the distinct reticulate pattern or anything resembling the spines
P. dongshaensis sp. nov. (this study) Dongsha area, northern South China Sea, seep. WP Late Pleistocene
P. abyssalis Okutani & Fujikura, 2002 Japan Trench, seep. 5379 WP Recent rounded whorls and lacks spines/tubercles
P. admetoides Warén & Ponder, 1991 Florida Escarpment, seep. 624–631 GM Recent more spiral keels on body-whorl, and also notably large number of axial ribs, up to 34–45 according to Warén and Ponder (1991), which make it most distinct from all Provanna species
P. annae Nekhaev, 2023 Plip Volcano, Bering Sea, vent. 387–342 NP Recent lack the strong angulation
P. beebei Linse et al., 2019 Beebe Chimlets, Beebe Vent Field, Mid-Cayman Spreading Centre, vent. 4956–4972 CS Recent more number of spiral ribs and less strong axial ribs
P. buccinoides Waren & Bouchet, 1993 Hine Hina, Lau Basin; North Fiji Basin, vent. 1900–2765 WP Recent more angulation
P. chevalieri Warén & Bouchet, 2009 Regab site, West Africa, seep. 3150 WA Recent lack the shelf-structure
P. cingulata Chen, Watanabe & Ohara, 2016 the Shinkai Seep Field, Southern Mariana Forearc. 5687 WP Recent lacks angulation and axial sculpture, has an increasing number of spiral ribs
P. clathrata Sasaki et al., 2016 Irabu Knoll and Hatoma Knoll, Okinawa Trough, Japan, vent. 1647–1743 WP Recent more angulation and spiny, no tubercles, and lacks the shelf-structure
P. cooki Linse et al., 2019 East Scotia Ridge segment E9, Southern Ocean, vent. 2394–2641 SO Recent higher whorls and lacks tubercles
P. exquisita Chen & Watanabe, 2022 Eifuku Volcano, Mariana Arc, 1606 WP Recent more angulation to form spiral keels
P. fenestrata Chen, Watanabe & Sasaki, 2019 Crane site, Tarama Hill, Okinawa Trough, Japan, vent. 1559 WP Recent strong axial and spiral sculptures, raised and equally spaced
P. glabra Okutani, Tsuchida & Fujikura, 1992 off Hatsushima, Sagami Bay, Japan, seep. 1110–1200 WP Recent lack the strong angulation
P. goniata Warén & Bouchet, 1986 Guaymans Basin, seep. 2000–2020 EP Recent more angulation and spiny
P. ios Warén & Bouchet, 1986 East Pacific Rise 21°N–17°S, Galapagos Spreading Center, vent. 2450–2620 EP Recent more angulation, spiny, and higher spiral shell
P. kuroshimensis Sasaki et al., 2016 Kuroshima Knoll, Okinawa, Japan, seep. 644 WP Recent lack the strong angulation
P. laevis Warén & Ponder, 1991 Gulf of California, Guaymas Basin, Oregon Margin, Juan de Fuca Ridge, vent and seep. 500–2000 EP Recent lack the strong angulation
P. lomana Warén & Bouchet, 1986 Oregon Margin, seep. 450–1200 EP Recent lack the strong angulation, and the spiral ribs on the body-whorl
P. lucida Sasaki et al., 2016 Minami-Ensei Knoll, Okinawa Trough, Japan, vent. 701 WP Recent lack the strong angulation
P. macleani Warén & Bouchet, 1989 Oregon Margin, seep and sunken drift wood. 2713–2750 EP Recent more number of spiral ribs and less strong axial ribs
P. muricata Warén & Bouchet, 1986 East Pacific Rise 21°N, Galapagos Spreading Center, and North Fiji and Lau Back-Arc Basins, vent. 2450–2615 EP Recent more angulation and spiny
P. nassariaeformis Okutani, 1990 Mariana Back-Arc Basin, Manus Back-Arc Basins, vent. 3670–3680 WP Recent more number of axial ribs and lack the shelf-structure
P. pacifica Warén & Bouchet, 1986 Gulf of Panama; Oregon Margin; Costa Rica Margin 1017–2750 EP Recent more angulation and spiny
P. reticulata Warén & Bouchet, 2009 Regab, Guiness and MPS 1-Congo sites, West Africa, seep. 750–3150 WA Recent more angulation and spiny, more number of spiral ribs
P. sculpta Warén & Ponder, 1991 Louisiana Slope, seep. 550 GM Recent similar strong and swollen tubercles at the intersections between spiral and axial ribs, but lack the shelf-structure; and higher spiral shell
P. segonzaci Warén & Ponder, 1991 Lau Back-Arc Basin, vent. 1750–1900 WP Recent more angulation, spiny, and lack the shelf-structure
P. shinkaiae Okutani & Fujikura, 2002 Japan Trench, seep. 5343 WP Recent more angulation and spiny, higher spiral shell
P. stephanos Chen, Watanabe & Sasaki, 2019 ‘Off Hatsushima’ seep site, Sagami Bay, central Honshu, Japan, seep. 908 WP Recent major spines on the spiral ribs and more angulation to form spiral keels
P. subglabra Sasaki et al., 2016 Okinawa Trough, Japan, vent. 710–1632 WP Recent lack the strong angulation
P. variabilis Warén & Bouchet, 1986 Juan de Fuca Ridge, Endeavour Segment, Gorda Ridge, Oregon Margin, vent, seep. 675–2200 EP Recent less strong axial ribs and lack the shelf-structure

Stratigraphic and geographic range

Only from the type locality and horizon.

Discussion

Morphological comparison within genus Provanna

The shell-shape characteristics can be used for the rapid identification of Provanna, particularly in fossil materials that lack organic body. Certain characteristics unify all Provanna species and distinguish the genus. Their specimens are never wider than tall. Regardless of their size, the specimens typically feature no more than 2 to 3 unbroken shell whorls, thin periostracum, no umbilicus, and small, turbinate, dextral shells. Their apertures have a characteristic shape, rarely being circular or ovate. Betters and Cordes (2023) conducted a comprehensive taxonomic review of extant species and a new identification key for this genus based on their shell and radular morphology, providing an effective method to streamline and standardize species identifications.

Despite the absence of a radula in our material, the polytomous key for species identification by Betters and Cordes (2023) holds significant reference value for distinguishing extant species. Applying Beeters’ method, P. dongshaensis sp. nov. appears to have “both axial and spiral sculpturing” and “sculptural elements present,” making it distinctly different from those Provannids with 1) a smooth shell surface, such as P. abyssalis, P. glabra, P. kuroshimensis, P. laevis, P. lucida, P. subglabra, and P. urahoroensis (fossil species); 2) only axial or spiral sculpturing, such as P. chevalieri, P. lomana, and P. variabilis.

To facilitate a more in-depth discussion of other complex structural varieties, particularly the differentiation of fossil species, additional essential points regarding the shell, including angulation, tubercles, spiral ribs, and shelf-structure, are consequently elaborated upon here as supplementary remarks to “Remarks.”

Two Miocene fossil species, P. alexi and P. hirokoae, inhabited the Japanese seep location described by Amano and Little (2014), which are lacking the angulation, strong axial and spiral sculpture on the body-whorl, although the spiral base has an increasing number of spiral ribs. P. cingulata and P. macleani have the comparable sculpture with numerous spiral ribs, whereas P. dongshaensis sp. nov. has a lesser number of spiral ribs but more prominent axial structure. The shelf-structure, characterized by significant angulation, appears in the P. dongshaensis sp. nov., but is absent in P. buccinoides, P. clathrata, P. nakagawaensis.

Especially the emergence of bead-like tubercles in P. dongshaensis sp. nov. seems to be a unique trait only shared with one extant species (P. sculpta) living in the Gulf of Mexico and Caribbean Sea, far from SCS. The new fossil species P. dongshaensis sp. nov. can therefore be quickly distinguished from other species that are angulate and spiny, such as P. goniata, P. ios, P. pacifica, P. muricata, P. reticulata, P. segonzaci, P. shinkaiae, and P. stephanos. Another similar species, Provanna fortis, shares some features with P. dongshaensis sp. nov., including pronounced angulation, 2 spiral ribs on the spiral base and up to 5 on the lowermost whorl, and a shelf-structure, etc. However, it features less robust axial structures on the body whorl with no indication of inflated tubercles or bead-like strings.

Shell microstructures of Provanna

The observation of shell microstructures was used as supplemental evidences for elucidation of the fossil vent and seep gastropods since they lost the organic body. According to the important report of microstructure related to provannid gastropods by Kiel (2004), the shell of modern species Provanna variabilis Warén & Bouchet, 1986 was composed of three layers of microstructure: an outer thin simple prismatic layer (oSPL), a thicker central complex crossed lamellar layer (CCL) and a thin inner simple prismatic layer (iSPL). Then more fossil material was subsequently investigated by Kiel (2006), Saether et al. (2010), and Amano and Little (2014). Based on the similarity of the shell microstructure between modern and fossil materials, together with observation of fossil abyssochrysoidean gastropod Hokkaidoconcha hikidai Kaim, Jenkins & Warén, 2008 from the Cretaceous seep sites in Hokkaido, the presence of an outer prismatic microstructural layer and an underlying crossed lamellar microstructural layer is regarded as a common feature in the shells of all Provanna species, as well as in other provannid genera and in the Superfamily Abyssochrysoidea (Amano and Little 2014).

In P. dongshaensis sp. nov., the central complex crossed lamellar layer is markedly found in the SEM and present at all sections of the shell (Fig. 3A–D). The outer prismatic layer can be also found above the thicker crossed lamellar layer in P. dongshaensis sp. nov. (Fig. 3A, B, D), but seems to be thinner than that reported in other Provanna species (less than 20 μm surrounding the broken aperture). Similar to P. marshalli, the boundary or the transition between the outer simple prismatic and complex crossed lamellar layers of P. dongshaensis sp. nov. could be blurred and indistinct (Fig. 3A, D, also see Saether et al. 2010). However, there is a relatively obvious border between the inner simple prismatic layer and complex crossed lamellar layer at all observed positions of the shell (Fig. 3C, D). According to Saether et al. (2010) and Amano and Little (2014), the inner simple prismatic layer could be absent owing to the diagenesis processes, but it is present in P. dongshaensis sp. nov., similar to the case in P. variabilis, and look to be thicker than those in P. reticulata and P. alexi.

Figure 3. 

Shell microstructures of Provanna dongshaensis sp. nov. observed in SEM. A. Marginal exposure of specimen GMGS2-09B-C13; B. Eroded surface of specimen GMGS2-09B-C13; C, D. Marginal exposure of specimen GMGS2-09B-C14. Abbreviations: oSPL = outer simple prismatic layer; CCL = complex crossed lamellar layer; iSPL = inner simple prismatic layer. Scale bar: 200 μm (A), 100 μm (B), 50 μm (C), 30 μm (D).

Buried depth, age and geological significance of P. dongshaensis sp. nov. in SCS

P. dongshaensis sp. nov. were yielded at the depth of 3.50, 3.60, 6.07 mbsf in GMGS2-09B and 33.85 mbsf in GMGS2-07B (Table 1, Fig. 4). At all these depths carbonate nodules were also yielded (Fig. 4). The stable carbon isotope signature (δ13Ccarbonate) of the carbonate reveals significant negative values varied from about -48‰ to -62‰ (unpublished data, partly given in Fig. 4). The δ13Ccarbonate signature along with the petrography imply that these carbonate blocks developed at an ancient methane seep (Campbell 2006; Zhuang et al. 2016; Chen et al. 2016b). Such authigenic carbonate is considered as the product of methane hydrate dissociation via microbial anaerobic oxidation of methane in this area (Chen et al. 2016b, 2017). The age of two carbonate layers of GMGS2-09B which yield P. dongshaensis sp. nov. are late Pleistocene according to the AMS 14C dating data of related coral skeleton (unpublished, also see Table 1). The P. dongshaensis sp. nov. buried at 33.85 mbsf in GMGS2-07B is also attributed to Pleistocene based on the last occurrence (LO) of foraminifera Globigerinoides ruber (pink) at the depth of 44.3 mbsf.

Figure 4. 

Three buried depths of Provanna dongshaensis sp. nov. distributed in the GMGS2-07B and GMGS2-09B with their lithology and remarkable negative carbon isotope values.

At the burial depths of the genus Provanna, a diverse assemblage of other gastropod species has also been identified. These species belong to the genus Pusia (Family Costellariidae), Canidia (Superfamily Buccinoidea), Epitonium (Family Epitoniidae) and Lissotesta (Family Skeneidae) (data to be published). The occurrence of these gastropod fossils is closely correlated to the distribution of carbonate nodules, validating the inference that the beddings containing carbonate nodules in the study area are indicative of paleo-seepage activity across different phases/epochs. These episodic paleo-seepage events had created environments that were conducive to the growth and development of benthic organisms, particularly gastropod species.

Considering the adaption and endemic distribution, the discovery of provannid gastropods and accurate identification of P. dongshaensis sp. nov. could be the biological evidence, and play an essential role in explaining the existence of the ancient seep in the northern SCS during the Pleistocene. To date, P. dongshaensis sp. nov. is the only known Provanna fossil record in this hydrate-yielded location and in the Pleistocene epoch, expanding the geological range of genus Provanna geographically and temporally. To further understand and interpret the evolutionary history of genus Provanna, additional real fossil specimens should be gathered and examined in the future.

Furthermore, the carbonate nodules creating carbonate layers have widespread appearance in the neighboring drilling cores of this area, (e.g., at least 13 unique carbonate layers were recognized in GMGS2-08, Chen et al. 2016b). Some other gastropod specimens like Neolepetopsidae have also been collected among these products of methane hydrate dissociation, and urgently need further accurate investigations in the near future. We suspect that there might be a significantly higher diversity of gastropods that persisted around the ancient hydrocarbon seep in the northern SCS. Such buried-seep sedimentary samples of GMGS2 series drilling cores could represent enlarged materials for systematic research of deep-sea gastropods and other chemosynthesis fauna.

Acknowledgments

I would like to thank Steffen Kiel, for discussion of shell structures and shell formation in gastropods, and two anonymous reviewers for their critical reading of the manuscript. This study was financially supported by the National Natural Science Foundation of China (grant No. U2344222).

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