RESEARCH ARTICLE
A new Pseudovertagus (Cerithiidae; Caenogastropoda; Mollusca) from the Lower Pleistocene of the Philippines, with notes on Sabia-hermit crab associations
2025 年 29 巻 p. 286-291
詳細
2025 年 29 巻 p. 286-291
Pseudovertagus canudai sp. nov. is described on the basis of a single specimen from the Pleistocene Tuktuk Formation in Leyte, Philippines. This represents the second extinct species of Pseudovertagus. Pseudovertagus canudai sp. nov. most closely resembles the rare living species Pseudovertagus phylarchus but can be distinguished by its smaller shell, the presence of finely incised spiral grooves on the last three whorls, and the slightly prosoclinal axial ribs on the early spire whorls. The shell bears rounded pits, which were most likely excavated by the co-occurring hipponicid gastropod Sabia while it was occupied by a hermit crab. This is the second fossil example of a Sabia-hermit crab association from an offshore soft-bottom environment.
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Pseudovertagus is a genus in the family Cerithiidae (Caenogastropoda) that is only known from the tropical Indo–West Pacific (IWP) region. It is characterized by its large shell with an upturned siphonal canal, a glossy surface, and fascinating color patterns. To date, five living and one extinct species are known from this genus (Houbrick, 1978; Kase et al., 2024). The living species include Pseudovertagus aluco (Linnaeus, 1758) (intertidal to 9 m in the IWP), P. clava (Gmelin, 1791) (intertidal to 40 m from Australia to the Pitcairn Islands), P. nobilis (Reeve, 1848) (subtidal to considerable depth in the West Pacific), P. peroni Wilson, 1975 (40 to 80 m in Western Australia), and P. elegans Bozzetti, 2006 (Madagascar) (Salvat and Rives, 1975; Wilson, 1975; Hourbrick, 1978). With the exception of P. aluco, modern occurrences of these species are rare, and fossil occurrences are even rarer, with only P. aluco being reported from the Pliocene and Pleistocene of Indonesia. The extinct species Pseudovertagus kondoi was recently reported from the upper Pliocene interval of the Ananai Formation in Kochi, Japan by Kase et al. (2024). Here, we report the occurrence of a specimen of Pseudovertagus from the Pleistocene of Leyte, Philippines. This is the only specimen of Pseudovertagus discovered by the first author and his colleagues in the Philippines and Java, Indonesia over the past 20 years of collecting Cenozoic mollusks in the region. Although this specimen is not well preserved, its shell characteristics distinguish it from other species in the genus, and it is described herein.
The specimen reported here (NIGSPAL-040) was collected by the second author on November 7, 2010 at locality PLP-1, a new exposure revealed by road widening along Libungan–Matag-ob–Palompon Road in Barangay Tinubdan, Palompon, Leyte, Philippines (Figure 1). The area is the eastern corner of the Visayan back-arc basin, where Miocene to Pleistocene clastic and carbonate sediments crop out. The bed exposed at PLP-1was mapped as the Tuktuk Formation by Corby et al. (1951), and this stratigraphic division and formation name have been followed by subsequent authors (Porth et al., 1989; Mines and Geosciences Bureau, 2010). The type section of this formation is in Barrio Tuktuk (probably Barangay Toctoc) along the northeastern coast of the Calubian Peninsula, where it is more widely exposed than in the Palompon area. Early Pleistocene nannofossils (NN 19 Nannofossil Zone) and planktonic foraminifers were recorded here by Porth et al. (1989) and Mines and Geosciences Bureau (2010).
The bed exposed at PLP-1 is approximately 10 m thick and is composed of gently-dipping unconsolidated bioturbated silty sandstone with hummocky cross-stratification in the lower part. There are two prominent conglomerate layers (25 cm and 30 cm thick) with cobble-sized clasts in the middle of the exposure. The clasts include coral rubble and poorly preserved fragmentary shells of intertidal mollusks (neritids, patellogastropods, etc.) and land snails transported by occasional debris flows. The silty sandstone contains sporadic but highly diverse mollusks. This assemblage contains many conids and turrids identical to modern species known mostly from depths of several tens to hundreds of meters, suggesting that the bed was deposited in an open sea, inner shelf environment.
Superfamily Cerithioidea Fleming, 1822
Family Cerithiidae Fleming, 1822
Genus Pseudovertagus Vignal, 1904
Type species.—Murex aluco Linnaeus, 1758, living species in the tropical East Indian and West Pacific oceans, by original designation.
Pseudovertagus canudai sp. nov.
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Etymology.—In honour of the late Francisco Canuda, former employee of the Mines and Geosciences Bureau Region 8 Office, Palo, Leyte, Philippines, for his invaluable assistance with fieldwork.
Type.—Holotype, in the collection of the National Institute of Geological Sciences, University of the Philippines, Diliman, Quezon City, registered as NIGSPAL-040, collected by Allan Gil S. Fernando on November 7, 2010.
Type locality.—Roadcut outcrop exposed in 2009 along Libungan–Matag-ob–Palompon Road in Barangay Tinubdan, Palompon, Leyte, Philippines (11°03′58″N, 124°25′06″E).
Type horizon.—Tuktuk Formation (lower Pleistocene).
Distribution.—Known only from the type locality.
Diagnosis.—Shell moderate in size for genus with nodulous, slightly prosoclinal axial ribs on successive early spire whorls and only spiral ribbons that are finely incised axially on last three whorls.
Description.—Shell moderately elongate, measuring 63.3 mm in length (inferred to be approximately 67 mm if complete) and 20.6 mm in width, consisting of at least nine whorls and having a pleural angle of 25°. Protoconch missing. Suture weakly impressed in early four whorls and gradually becoming linear abapically. First four preserved teleoconch whorls obtusely convex in middle, with steeply inclined narrow subsutural ramp; whorl surface sculptured with eleven thick, slightly prosoclinal axial ribs and two thick spiral cords and becoming nodulous where they intersect. Axial ribs juxtaposed on successive whorls becoming gradually smaller from the 7th whorl onward and only barely perceptible below upper suture. Sculpture of last three whorls different from those of earlier ones and consisting of five spiral ribbons separated by shallow and fine spiral grooves; adapical two spiral ribbons thicker and nodulous, and abapical three others thinner and finely incised by regularly spaced, growth increments. Last whorl nearly flat ventrally with ventrolateral varix composed of obtuse axial swelling. Siphonal canal short, reflexed dorsally. Anal canal forming deep sulcus, constricted by prominent parietal fold. Parietal area concave and covered with thin callus. Outer lip totally missing. Columella vertical, concave, thickened with callus, forming flangelike extension. Two spiral ribbons present behind on base. Color markings barely visible, pale reddish-brown spots on nodes and very small rectangles on abapical three incised spiral ribbons.
Comparisons.—Although the holotype and only available specimen is incomplete, its shell characteristics are distinct enough to distinguish it from the other known species of the genus. Pseudovertagus canudai sp. nov. looks most similar to Pseudovertagus phylarchus (Iredale, 1929), which has been recorded at depths of 5 to 30 m in the Philippines and Queensland, Australia but is the least common living species of the genus (Houbrick, 1978; Figure 2B). Both species show similar changes in the surface sculpture during shell growth. In the early whorls, both have strong longitudinal ribs and two spiral cords. Then, just before the penultimate whorl, one spiral rib and three spiral ribbons are added to the subsutural ramp and lower part of whorl, respectively. However, in P. phylarchus, these sculptures almost disappear in the last two whorls, whereas in P. canudai sp. nov., the longitudinal ribs weaken but the spiral ribbons remain throughout. In other words, in P. canudai sp. nov., the shell growth stops just before the penultimate whorl of P. phylarchus. In addition, the alignment of axial ribs on successive whorls is prosoclinal in P. canudai sp. nov., whereas it is opisthoclinal in P. phylarchus. Moreover, the spiral ribbons in P. canudai sp. nov. are incised by axial increments, whereas in P. phylarchus they are smooth (compare Figure 2A, B). P. canudai sp. nov. is easily distinguished from P. clava by its shorter shell length and lack of axial sculpture on the last two whorls.
The holotype specimen of Pseudovertagus canudai sp. nov. has six pits that are unequivocally the result of the excavation by the commensal gastropod limpet Sabia judging from their characteristic structure (Vermeij, 1998; Kase et al., 2024). These pits are distorted and circular to elliptical in shape, and the largest is 9.7 mm long and 9.1 mm wide. They are deep and bear a horseshoe-shaped central swelling and a steep wall with a row of small pits. These pits are thought to be the fitting points of the crenulated shell margin of the guest shell (Figure 3B). This genus is well known from Sabia conica in the Indo–West Pacific and Sabia australis in Australian waters (Cernohorsky, 1968; Laws, 1970; Knudsen, 1991; Vermeij, 1998). We also found a shell of Sabia cf. conica from the same site as the holotype specimen of P. canudai sp. nov. (Figure 3B), and it is likely that this species of Sabia made the pits in the shell. According to Knudsen (1991), the type locality of Sabia conica is unknown, as Schumacher (1817) did not provide locality information when he described the species. Knudsen (1991) suggested that populations in the IWP identified as S. conica are actually a mixture of at least four species. Therefore, the referral of this specimen to Sabia conica (Schumacher, 1817) is tentative.
Sabia conica and S. australis have been suggested to be protandrous consecutive hermaphrodites that are commensal with snails and hermit crabs. They utilize a variety of host shells, but host preference, incidence of occupation, attachment site on the host, and number of individuals attached to a host vary by locality (Matsunaga, 1964; Cernohorsky, 1968; Knudsen, 1991; Morton, 1998; Yamahira and Yano, 2000; Morton and Jones, 2001; Takeda and Washio, 2013). In Japan and Vietnam, S. conica exhibits a site preference for positions on host shells from which feces and/or other organic material from host shells are easily accessible using a long, extensible proboscis (Matsunaga, 1964; Morton, 1998). However, in other areas, S. conica and S. australis also feed on nutrients unrelated to the host (Knudsen, 1991; Morton, 1998; Morton and Jones, 2001). Vermeij (1998) examined the spacio-temporal distribution of Sabia-gastropod associations and extensively argued this unique commensalism from an evolutionary point of view. Previous reports of Sabia-gastropod/hermit crab associations are mostly from intertidal to shallow subtidal hard bottom environments, and this report represents only the second example of a Sabia-hermit crab association from an offshore, soft-bottom environment. Recently, Kase et al. (2024) reported that despite the co-occurrence of many gastropods in the Pliocene open sea, inner shelf soft sediments of Shikoku, Japan, Sabia pits were specifically found in shells of the cerithiid gastropod Pseudovertagus kondoi, which is notably the same genus as the shell reported herein. They suggested that these pits were excavated when the host shells were occupied by hermit crabs rather than live snails. In addition, the pits on the holotype of P. canudai sp. nov. are suggested to have been excavated when the shell was carried by a hermit crab. The four large pits in this specimen are located on the ventral surfaces of the last three whorls. As discussed by Kase et al. (2024), Pseudovertagus is thought to have been a semi-infaunal snail, so it is highly unlikely that Sabia would have occupied this location on a shell buried in sediment while its host was alive. Rather, the pits were likely made after the host shell was carried above the sediment by a hermit crab (Ishikawa and Kase, 2007). NIGSPAL-034 has a jagged spiral cut extending half volution back from the outer lip (Figure 2A2, 3), which is most likely a peeled break caused by a predatory shell-destroying crab (Vermeij, 1978). We suggest that the Sabia pits were excavated when a hermit crab occupied the host shell after it had already been broken (Ishikawa et al., 2004). In this case, the hermit crab then carried the host shell on its back using a newly formed shell aperture.
Questions remain as to why Sabia selected Pseudovertagus among other snails and why this association is uncommon in offshore soft bottom environments in both the present and the past. We speculate that the current lack of reports on this unique association from these environments is due to the rarity of the event and that records have therefore been overlooked.
The specimen examined in this work was obtained during a fieldwork in Leyte conducted in November of 2010 under the leadership of R. Majima (Yokohama National University, Yokohama). We would like to thank R. Majima who gave us the opportunity to join this fieldwork. We also would like to thank L. L. Jasareno (then Director, Mines and Geosciences Bureau, Quezon City, Philippines) for the research permit. We are grateful to W. Mago, E. Azurin and the late F. Canuda (Mines and Geosciences Bureau, Philippines) for their generous support during the fieldwork. We also thank two anonymous reviewers for their valuable comments and suggestions that improved this paper.
T. K. initiated the study and was primarily responsible for the taxonomic aspects. All the authors took fieldwork together and contributed to the writing of the paper.
