Southeast Asia as the birthplace of unusual traits: the Melongenidae (Gastropoda) of northwest Borneo

In this paper the Miocene to recent melongenid species of northwest Borneo are discussed. The recent fauna is poor with three genera represented by one species each. In the Miocene three (possibly four) genera occur with eight (possibly nine) species of which three are described as new species: Melongena murifactor , uniquely characterized by the formation of a septum walling off the adapical sector of the aperture; Pugilina erecta , characterized (along with its close Miocene relative from Java, P. ickei ) by a free-edged, erect inner lip; Volema goliath , large for the genus with later whorls progressively covering the upper row of spines of earlier whorls. The apertural septum of M. murifactor is unique within Gastropoda, whereas the erect inner lip of P. erecta and P. ickei is unique within Melongenidae. That these extralimital traits occur exclusively in the Miocene of southeast Asia is consistent with the hypothesis that adaptive innovations are most likely to arise in diverse, productive, shallow-water ecosystems.


Introduction
The neogastropod family Melongenidae Fischer, 1884, is a relatively small, compact group of 22 recent and at least 42 post-Eocene fossil species and subspecies of medium-sized to very large predators. All species have a tropical to subtropical distribution in shallow waters, and all occur on the shores of continents and continen-tal islands. In northwest Borneo the recent fauna is poor with three genera represented by one species each. In the Early Miocene three genera occurred with three species, in the Middle-Late Miocene three (possibly four) genera with six (possibly seven) species (Table 1). Three of these Miocene species are described as new species. A short introduction to the area and sampling performed is given in Raven (2002: 3-7).
Although the taxonomic composition of pre-Oligocene members of this family remains unclear, the Oligocene to recent members appear to have ancestors among Late Eocene species of the genus Pugilina Schumacher, 1817. Most shells of the group are quite simple, but at least two phenotypes representing significant departures from the standard morphology have evolved, one of which may be unique within Gastropoda. Both of these phenotypes are known only from fossil melongenids in southeast Asia. In the course of a review of melongenid species from the Miocene of Borneo presented in this paper, we document these unusual phenotypes, one of which is associated with a new species and the other with both a new and a previously described Indonesian species. We place these odd morphologies into the broader context of evolution in the highly competitive environment of Neogene marine southeast Asia.
Abbreviations: coll., collection; Fm, Formation(s), fr., fragment(s); H, height; juv., juvenile(s); sp., specimen(s); W, width.   Zoology, 78 (3) -2009 Systematics Family Melongenidae Fischer, 1884 Remarks. As Vermeij and Wesselingh (2002) have pointed out, distinctions among the various named genera and subgenera of post-Eocene Melongenidae are not always clear. Although a formal phylogenetic analysis for the family has not yet been attempted, our comparative study of fossil and living melongenids indicates that two morphological groups, perhaps representing sister clades, can be recognized. One is represented in the Recent fauna by Pugilina, with species in the Atlantic and Indo-West Pacific regions, and Hemifusus Swainson, 1840, known only from southeast Asia. This group is characterized by a relatively high spire, a relatively long siphonal protuberance, the absence of an adapical notch in the outer lip of the adult shell, and usually by the presence of a single row of shoulder nodes or spines (but see below under P. erecta n. sp.). The inner side of the outer lip is lirate (spirally ridged) in P. morio (Linnaeus, 1758), the type species of Pugilina, but smooth in most other species including all Hemifusus.
The second group comprises the taxa Melongena Schumacher, 1817; Volema Röding, 1798;Rexmela Olsson and Harbison, 1953;and Torquifer Roth, 1981. In this group, the spire is generally low, the siphonal protuberance is short, the outer lip bears a distinct adapical notch in the adult stage, and the fully developed sculpture consists of a row of shoulder tubercles or spines, one or two additional rows of tubercles on the central sector of the last whorl, and a basal row of spines or tubercles immediately adapical to the siphonal protuberance. Any or all of these rows of tubercles may be absent. In the American Rexmela (Pleistocene to Recent of Florida) and Torquifer (Early to Late Miocene of Atlantic tropical America) there is a pronounced tendency to form imbrications just below the suture, marking previous positions of the adapical end of the shell's outer lip. Subadult shells in the Melongena group closely resemble Pugilina in lacking basal spines and in the absence of an adapical notch. The inner side of the outer lip is lirate in Volema and in some subadult specimens of American Melongena.
All living species of Melongena and Rexmela, as well as the Miocene genus Torquifer, are known only from the New World. The living Melongena melongena (Linnaeus, 1758) of the Caribbean region and the coast of Atlantic Central America and northern South America to Suriname, and M. patula (Broderip and Sowerby, 1829) from the eastern Pacific (Gulf of California to Ecuador) (Clench and Turner, 1956), are characterized by low-spired shells in which the last whorl of the adult becomes highly expanded and covers part of the spire. Nodes and spines, when developed, are relatively blunt and short. This condition also applies to Miocene and Pliocene representatives of Melongena from tropical America. Rexmela often has highly elongated, adapically curved spines on the shoulder angulation and a less expanded last whorl and higher spire. In the Old World, all species that have been assigned to Melongena are extinct. When spines are developed, they tend to be very long, especially on the shoulder angulation, and the last whorl is distinctly conical rather than broadly inflated as in the tropical American species. The oldest species of Melongena, M. crassicornuta Conrad, 1848, from the Byram Fm (Early Oligocene) of Mississippi, and M. laxecarinata (Bellardi, 1882) from the Early Oligocene of Italy (see MacNeil and Dockery, 1984;Dockery and Lozouet, 2003) may have given rise to the New and Old World groups, respectively. Further work may show that the Oligocene and Miocene species of the Old World that have been assigned to Melongena, including M. cornuta (Agassiz, 1843) andM. lainei (de Basterot, 1825) from Europe, and M. gigas (Martin, 1883) from Indonesia, should be separated as a separate genus or subgenus. For now, however, we continue to use the generic name Melongena in the broad sense for both the New World lineage, including the type species M. melongena, and for the fossil species from Europe and Asia.
Genus Melongena Schumacher, 1817 Type species: Melongena fasciata Schumacher, 1817(= Murex melongena Linnaeus, 1758 Melongena gigas (Martin, 1883) Description. A large species (up to at least 25 cm high) with a very large aperture, a low spire with large spines and a row of smaller spines lower on the neck. Remarks. This species is rare in the studied deposits. Only one fully grown specimen was collected, besides some fragments and a juvenile. The fully grown specimen is 25 cm high, whereas the largest (not fully grown) specimen recorded by Martin (1883: 90) is only 17.5 cm high.
Melongena murifactor n. sp. = Melongena sp. 3, Martin, 1928: 20 Diagnosis. A small species characterized by a strongly convex subsutural region, very low spire, an extremely deep adapical notch on the outer lip, and the presence of periodic septa walling off the adapical sector of the aperture.
Description. Shell thick, relatively small for genus, maximum length 65.7 mm, globose, with a very low spire and markedly convex subsutural ramp; abapical part of last whorl weakly and gradually constricted; sculpture variable: holotype ( Fig. 1) smooth, without spines, other specimens from the same sample with three discontinuous spiral rows of nodes (Fig. 2), one at shoulder, one just below shoulder, and one at base of last whorl just above keel-like siphonal fasciole; aperture smooth within; heavy ventral callus; adapical end of outer lip with very deep notch; adapical sector of aperture walled off from part of adapical notch by a septum, whose adapertural edge lies some distance within the aperture; at least one earlier septum also present. There is no evidence of a color pattern, also not in UV light.
Remarks. This species is highly distinctive not only because of its globose form and convex subsutural ramp, but especially because of its extremely deep adapical (anal) notch, which is periodically walled off by a septum forming the adapical side of the aperture. In the specimens from Borneo, the outer lip of the shell has grown beyond the outer edge of the septum, so that the adapical notch remains continuous. In these specimens, there is a second septum, set about 70° to 90° back from the outer lip, indicating that the septum forms periodically. Martin (1928: 20) referred to an unnamed species of Melongena from the Late Miocene of Atjeh (= Aceh), northern Sumatra, as Melongena sp. 3. Inspection of this specimen (RGM 675) indicates that it belongs to M. murifactor. Although its aperture is not well preserved, the remains of a septum are clearly evident. In his brief discussion of this specimen, Martin (1928) noted that the globose form, especially of the adapical sector of the last whorl, was distinctive; but he did not mention the septum.
As Martin (1928) pointed out, the present species resembles spiny members of the southeast Asian species Volema myristica Röding, 1798 (see below). We use this name for the taxon Martin (1928) and most other authors have called Melongena galeodes (Lamarck, 1822). Volema myristica differs from M. murifactor by lacking the adapical septum and by having intermittent spiral ridges on the inner side of the outer lip.
The only species that appears to resemble M. murifactor is Cassidea acanthina Dalton, 1908, which Vredenburg (1922 reassigned to Melongena. This species, from the Late Oligocene of Birma (= Myanmar), is globose, with a reflected inner-lip callus and a shallow umbilicus. The spines are described as large, irregular, and robust. There is no mention of an adapical notch or septum.
Melongena murifactor is not similar to any other Old World species of Melongena, but it does resemble some New World species in its very low spire and globose form. No American species of Melongena s.s., however, possesses an adapical septum of the kind seen in M. murifactor.
Our examination of nearly all living and fossil melongenid species reveals that adapical septa occur in highly reduced form in some individuals of only two taxa, Melongena (Rexmela) corona corona (Gmelin, 1791) from the west coast of Florida, and M. (R.) c. johnstonei Clench and Turner, 1956, from northwestern Florida. Specimens of these taxa in the Vermeij collection show the formation of a septum closing off the last-formed shoulder spine at the outer lip. Only one septum is present in any given specimen, and the edge of the septum is flush with the edge of the rest of the outer lip. This is unlike the situation in M. murifactor, in which the outer lip has grown beyond the edge of the most recently formed septum. To our knowledge, there are no previous descriptions of the adapical septum in the literature, likely because it is a subtle feature that could be easily interpreted as a gerontic aberration.
All kinds of apertural elaborations -notches, sinuses, canals, and spine-like extensions -are known in gastropods. When these form periodically during shell growth, the most recently formed structure is initially almost always hollow on its adapertural side. During the next growth cycle, or when the aperture remains in the same position for a long interval of time, its connection with the apertural cavity is usually severed by a general infilling of the hollow structure with shell material secreted by the mantle surface on the shell's interior. The extent of this infilling varies within and among species. The spines of many busyconine whelks, coralliophiline and muricine muricids, columbariids, and the turbinid Astraea, remain adaperturally hollow, and only the base of the structure is closed off by a thin layer of shell material. This layer is not, however, a septum, because its formation occurs within the broad entrance of the spine in the apertural cavity. The hollow entrance is itself gradually filled in with additional shell material. In many thick-shelled muricids, as well as in the spinose strombids Lambis and Harpago, the long-spined turbinid Guildfordia, and spiny species of Vasum in the Vasidae, the open edges of the spine are often closed in conjunction with the basal infilling of the hollow spine (see also Savazzi and Sasaki, 2004). In members of the Bursidae, whose adapical notch forms periodically when a varix is produced, the old adapical notch is succeeded in growth by a new thin lip that reaches to the penultimate whorl without forming a notch; only the formation of the next varix initiates a new notch. In most gastropods, old siphonal canals are typically left open to the aperture and are never sealed off by a septum or infilling.
Our discovery of septa in Melongena murifactor and our preliminary studies of how periodically formed apertural extensions are walled off or filled in has exposed an aspect of shell morphology and growth about which very little appears to be known. We hope the present paper draws attention to the topic of secondary shell secretion and resorption within the gas-tropod shell aperture.
In his note on the fossils from Brunei (Martin, 1932: 3) a 'Melongena 3' is reported from well 13 in the Seria oil field (340-350 m depth). On p. 6 Martin states that it concerns a very young specimen. In the RMNH collection the box with the label 'Melongena spec. 3' comprises a glass tube with another label inside stating 'Seria-13 340-350 m' and a single shell of a juvenile Eucithara (Clathurellinae, Conidae), unrelated to Melongena sp. 3 from Aceh. Diagnosis. Small Pugilina with a single row of shoulder tubercles and an erect inner lip.
Description. Shell moderately small for genus, maximum length 62.1 mm, fusiform; body whorl with eight to ten shoulder spines, extended abapically as narrowly rounded axial ribs, which reach to the constriction; spiral sculpture consisting of a continuous subsutural cord, ten cords on the steep shoulder slope, ten between the spines and the basal constriction, and 12 on the constriction and siphonal protuberance; aperture adapically broad, smooth within; outer lip adapically forming short narrow channel with parietal wall, but without notch; inner lip erect and expanded along most of its length.
Remarks. The new species Pugilina erecta displays some variation. The two largest specimens (lengths 57.5+ (Fig. 7) and 62.1 mm (Fig. 8)) differ from the holotype (Fig. 6) and a second specimen of identical length (47.5 mm) by having the inner lip adherent instead of erect. We interpret this difference as reflecting a subadult stage despite the larger size of these specimens. Another specimen, of length 51.8 mm (Fig. 9), has spiral ridges on the inner side of the outer lip. This specimen resembles the holotype and the other small specimen in having an erect inner lip. In UV light the shells have a pattern of fine radial lines with pale riblets and darker interspaces.
Pugilina erecta is most similar to P. ickei (Martin, 1906) from the Middle and Late Miocene of Java. The latter species is larger (maximum length 113 mm in a specimen with incomplete spire and siphonal protuberance), from Tji Angsana-Merang (= Ciangsana-Merang), and is characterized by either a row of tubercles or a continuous angulation at the abapical end of the axial ribs. This angulation or second row of tubercles is absent in P. erecta. Both species have an erect inner lip, a character otherwise unknown in Pugilina and indeed in the family Melongenidae.
A specimen identified by Martin (1928: 19) as Melongena (Pugilina) madjalengkensis (Martin, 1895) from the Late Miocene of Aceh in northern Sumatra has a discontinuous row of shoulder nodes and obsolete spiral sculpture on the central sector of the body whorl between the shoulder nodes and the basal constriction which is spirally threaded. Like most of our Borneo specimens, the Aceh specimen (H 94.5 mm) has an erect inner lip. The taxon Pyrula (Melongena) madjalengkensis Martin, 1895, was originally de-scribed from the Pliocene of Java. Oostingh (1935) and van Regteren Altena (1950) considered it a stratigraphic subspecies of the Pleistocene to Recent P. cochlidium (Linnaeus, 1758) (as P. pugilina (Born). True P. madjalengkensis resembles the living P. cochlidium in almost all characters, including the presence of a consistently adherent inner lip. We therefore tentatively assign Martin's (1928) specimen from Aceh as P. erecta even though his specimen is larger and smoother than our specimens from Borneo.
Juvenile specimens can be differentiated from those of P. cochlidium by the lower number, larger and more pointed shoulder spines in P. erecta.
To our knowledge, P. erecta and P. ickei are the only fossil or living melongenids with an erect inner lip. All other species have the inner lip fully attached to the abapertural side of the body whorl. The erect inner lip is therefore the second unique departure by a southeast Asian species from the standard shell morphology of melongenids.
Gastropods with an erect inner lip are found mainly on soft bottoms. Those rock-dwelling muricids that possess an erect inner lip tend to have a small, nearly round aperture with an effectively continuous rim that adheres closely to the substratum. Muricids as a whole are slow-moving snails, and it is possible (but unproven) that those with an erect aperture are sedentary, perhaps even parasitic, in habit.
Mud-dwelling and sand-dwelling gastropods with an erect inner lip do not burrow completely. Indeed, an inner-lip edge that is almost vertically oriented with respect to the sediment surface would seem to prevent active burrowing, and points instead to a relatively sedentary, mostly surface-dwelling habit. Compared to gastropods in which the inner lip merges gradually with the outer surface of the shell, gastropods with an erect inner lip have a larger volume between the shell's ventral side and the sediment surface. An erect inner lip could thus contribute to a prey-trapping or prey-enveloping method of predation, as proposed for long-spined species of Murex by Paul (1981). No observations have been made on any living gastropod to evaluate such a suggestion. Prey-trapping of crabs beneath the shell has been described for species of Harpa, which have a very large aperture and foot but an adherent inner lip (Rehder, 1973). It is conceivable that wide-apertured species of Melongena also use trapping to prey on bivalves, and that species of such melongenid genera as Pugilina and Volema, whose apertures are narrower, would have to resort to an erect inner lip to construct an adequate cage for trapping prey. Pugilina ickei (Martin, 1906) Material. MALAYSIA, Sarawak. Auban outcrop km 0.6, Middle Miocene, Lower Miri Fm, R F5140: 1, leg. CL (Fig. 10). BRUNEI, Tutong. Tutong sample 11B, Late Miocene, Miri Fm, RMNH: 1 juv., leg. BPM (Fig.  11); Tutong sample 11A, Middle-Late Miocene, Miri Fm, RMNH: 1, leg. BPM.
Remarks. Like P. erecta this species often, but not always, has an erect inner lip. It can be differentiated from P. erecta as it has fewer spines, which are also shorter and rounder.
Remarks. Two shells from the Sibuti Fm (Early Miocene) of Borneo, measuring 54.6 and 47.4 mm in length (Figs. 12-13), may represent a third new species of Me1ongenidae, but we hesitate to describe these formally as a distinct taxon. They are characterized by prominent shoulder spines oriented adapically, which may be absent over distances of less than a quarter to almost half a whorl. Axial ribs extend from the spines to the basal constriction. Spiral sculpture is weak, being wholly absent from the central sector of the last whorl from the spines to the constriction. The aperture is smooth within, and the inner lip is adherent.
This specimen looks like a miniature P. ponderosa (Martin, 1895), a large species (shell length 116 mm) from the Late Miocene of Java, which like our specimen has adapically oriented spines and a smooth inner lip. More material will be needed to clarify the status of the taxon from the Sibuti Fm.
Pugilina cochlidium (Linnaeus, 1758) Description. An intermediate sized species (up to 11 cm high) with solid shell, moderately high spire, shouldered whorls with short and rounded nodes (Fig. 14). Juveniles have spiral sculpture or variable strength all over the shell, whereas in adults this sculpture is only noted on the neck. Strong, brown periostracum.
Material. Recent. MALAYSIA, Sarawak. Bako National Park, Teluk Pandan Besar, beach, R T7133: 1, vii.2003, leg. V; Miri, Piasau & Miri beaches, found alive on objects low on beach, R T0615: 6 + 18 juv., 1992-1997, leg. R; Miri, Tanjing Lobang, intertidal area with sandstone, abundant alive on/under rocks in lower half of tidal zone, R T1732: 14 + 4 juv., 1992-1997, leg. R (Fig. 14) Remarks. This is the most abundant and widely spread recent species. It occurs from low in the intertidal zone to shallow subtidal, especially between rocks. After death of the mollusc the shells are often inhabited by large hermit crabs which may take the shells to different depositional environments, such as beaches and estuaries.
Remarks. Nuttall (1961: 75) recorded Pugilina (Hemifusus) cf. ternatana from the Lower Miri Fm at Penanjong, Tutong, Brunei, Miocene. As there is no figure nor a description, it is impossible to check the identification.
'Hemifusus ?' was recorded by Martin (1931: 3) from Tutong outcrop 8 (Seria Fm) in Brunei. No description or illustration was provided, and the specimen is not present in the collection of RMNH, therefore it is impossible to know which species this could be, or even whether the material really belongs to this genus.
Recent. MALAYSIA, Sabah. Kota Kinabalu, Tanjung Aru, mud flat with brackish water influx and some mangroves, few sp., not collected, 1995-1997, leg Remarks. We have one complete juvenile specimen and several larger incomplete specimens  from the Sibuti and Miri Fms of Borneo. These specimens have sculpture consisting of four spiral rows of spines, one situated immediately beneath the suture, the second on the shoulder angulation, and two others more basal. This pattern is identical to that of Recent specimens we have seen from southwestern Sulawesi, Borneo, and the Moluccas. As currently understood, V. myristica is a highly variable species. In some populations from southwestern Sulawesi there are no spines at all on the last whorl, whereas in others, such as those from north Java and Bohol (the Philippines), there is a single peripheral row of spines with sometimes a sub-  Fig. 2, H 32.7 mm. Fig. 3, H 14.1 mm. Fig. 22 300 m SE of Canada Hill, Miri, Holocene, R H1112: H 40.7 mm. Fig. 23 Kampung Pohon Batu, Labuan Island, Recent, R T3137: part of group of shells of various species eaten by men, 54.1 mm. Fig. 24. Volema n. sp., to be described by Kurihara & Kase (pers. comm. to GJV, Oct, 2008), Auban outcrop lower part of section, Lower Miri Fm, R F4109: H 54.3+ mm. sutural row. The species always possesses strong cords, unlike the nearly smooth type species of Volema, V. pyrum (Gmelin, 1791) (see also Cernohorsky, 1974).
The form described by Vredenburg (1925) as Melongena galeodes var. sindiensis from the Gaj beds (Early Miocene) of Pakistan closely resembles our spiny form. It has one subsutural row of projections, a row of widely spaced spines at the shoulder angulation, and another near the base. According to Vredenburg (1925), the spiral cords are weaker than in the Recent V. galeodes (Lamarck, 1822) (= V. myristica). More material is needed to confirm that V. g. sindiensis is really distinct from V. myristica.
Remarks. We have one damaged specimen (54.3 mm high, Fig. 24) from the Miri Fm of Borneo that we provisionally assign this species. The overall shape is similar, as are the narrow whorls of the spire and the sculpture.

Diagnosis.
A large species with a strongly convex subsutural region, very low spire, three rows of spines, with later whorls progressively covering the upper row of spines of earlier whorls.
Description. Shell thick, large for the genus (Fig. 17), maximum width 119 mm, globose, with a very low spire; abapical part of the whorl concave; sculpture comprises three rows of nodes, the most marked at the shoulder (about ten per whorl in juvenile (Fig. 18), up to 12 per whorl in adult shells), one just below the shoulder and row of weak nodes lower on the neck; the upper row of spines is very marked in juvenile shells, especially in adapical view, but the spines become less marked in adult shells, whereas later whorls progressively cover the upper row of spines of earlier whorls; the outline between the middle rows is markedly straight, especially in the juvenile paratype; spiral sculpture of stronger ridges with weaker ones in between extends over the entire shell and is intersected with closely spaced growth lines that interrupt the spi-rals; aperture relatively small, smooth within; heavy ventral callus; strong adapical apertural channel; no evidence of a color pattern, also not in UV light.
Paratype. Miocene. BRUNEI, Tutong. Tutong sample 11A, Middle-Late Miocene, Miri Fm, RMNH: 1, leg. BPM (Fig. 18). Remarks. This species is distinctive because of its size and globose outline. The holotype is by far the largest specimen of the genus (H 65.7+ mm, W 83.8 mm) that we have seen. Regrettably it is damaged, but additional characteristics are represented in the paratype, which is a juvenile specimen (H 25.2, W 19.4 mm). The juvenile specimen has similarities to spiny specimens of V. junghuhni but has a much lower spire. Martin (1931: 6) reviewed this specimen and listed it as 'Melongena sp. 1'. C. Beets studied the paratype and recognized it as a new species which he intended to call M. fascioplexiformis, but never published his description.
Both specimens have barnacles on the shell. Beets removed the one from the juvenile. The adult specimen has one barnacle, the base of another and a few juvenile oysters.

Discussion
The melongenids we have described in this paper exhibit two features that are otherwise unknown in the family, namely, an adapical septal wall in the aperture (Melongena murifactor) and an erect inner lip (Pugilina erecta and P. ickei). Although the latter feature is common in many gastropod families (see above under P. erecta), it is unknown in any other living or fossil melongenids. The septum of M. murifactor is unique among gastropods.
Phylogenetic and geological studies (Wilson and Rosen, 1998;Meyer, 2003;Wilson and Vecsei, 2005;Frey and Vermeij, 2008;Renema et al., 2008;Williams and Duda, 2008) indicate that the shallow-water marine faunas of southeast Asia underwent dramatic diversification during the early Neogene, as the area of shallow habitat increased by a factor ten and as productivity due to upwelling and runoff increased. Much of the distinctive character of shallow-water Indo-West Pacific faunas, including the high incidence of shell defenses in molluscs, dates from this expansive phase. These conditions -intense predation and competition, high productivity, and extensive habitats -are ideal for the evolution of adaptive novelties (Vermeij, 2003). Similar favourable conditions may have existed elsewhere during this time interval as well, but the adaptive departures and specializations seen in central Indo-West Pacific faunas of the Neogene are unrivaled in other regions. We therefore suggest that southeast Asia was exceptionally favourable to the establishment of traits that expanded the phenotypic range of the clades in which they arose. Vermeij (2001) already pointed to this time interval in southeast Asia as a time and place in which many gastropod lineages (at least four in greater southeast Asia) independently evolved a labral tooth, a structure on the outer shell lip that speeds up predation on hard-shelled prey.
Among species of the gastropod genus Nerita, a partially enveloped shell (subgenus Amphinerita) and the sand-dwelling habit (subgenus Linnerita) are unique to the Indo-West Pacific, with envelopment being confined to its inner portions (Frey and Vermeij, 2008). Preliminary data from a survey of Neogene clade-specific innovations of molluscs by one of us (GJV) indicate that 32 ecologically or morphologically unique innovations evolved in the Indo-West Pacific, of which at least 15 are restricted to greater southeast Asia. Only ten comparable innovations have been identified for the Neogene to Recent time interval in the tropical Atlantic and eastern Pacific. Further work on this topic is in progress and will be presented in a future paper.