MOLLUSCA 



141 



when the atoll stood above the sea as a high island. 

 Leopold's (1969) report of spores and pollen indicates a 

 richer and more varied flora on Enewetak in the Miocene 

 than now occurs. The record of a high island fossil land 

 shell, Vatusila (Ladd, 1958 as Pvtochodon; Soiem, 1982), 

 is evidence of conditions quite different from those today. 

 Leopold (1969) also demonstrated conclusively from 

 analysis of pollen grains that mangroves occurred at 

 Enewetak through the Miocene. The limestone sections 

 ". . . are all reef associated. Many of the fossiliferous beds, 

 especially those rich in mollusks, appear to have been 

 deposited in lagoons, but other environments — reef wall, 

 open shoal, and off-reef deposits — are also found" (Ladd, 

 1966). 



Miocene Record 



The Miocene record, the richest and most diverse of 

 the fossil records, includes 170 species, 26% archaeogas- 

 tropods, 58% mesogastropods (including heterogastro- 

 pods), and 17% neogastropods, representing 90 genera 

 and 42 families of mollusks. 



The Miocene fauna is very different from that found at 

 Enewetak today. The archaeogastropods arc represented 

 by eight families; today there are 13 families. Turbinids, 

 trochids, and neritids comprise 71% of the Miocene 

 species record; trochids, fissurellids, stomatellids, and tur- 

 binids represent 68% of the present archaeogastropod 

 fauna. Among the mesogastropods, the rissoids (29 

 species) are the most numerous family comprising 30% of 

 the record; the cerithiids (15 species), 19%; and vitrinellids 

 and dialids (12 species) about 13% of the record. Today 

 the cypraeids, cerithiids, rissoids, stombids, vitrinellids, and 

 cymatids dominate the mesogastropods. 



Some of the differences between the Miocene and 

 modern faunas undoubtedly reflect biases in the fossil 

 record as such. Other differences indicate the presence of 

 habitats such as sea grass beds, mangrove swamps, brack- 

 ish water lagoons, and high island shorelines which are not 

 now present on the atoll. Thus four genera present in the 

 Miocene record are extinct on Enewetak today but are 

 found elsewhere in the Indo-West Pacific in environments 

 which do not presently occur at Enewetak. The neritid 

 Srrtaragdia (three species in the Miocene record) is associ- 

 ated with sea grass beds at Kwajalein, Marshall Islands, in 

 the Caroline Islands, and as far east as the Hawaiian 

 Islands (Kay, 1979). The sessile limpet-like mesogastropod 

 Hipponix (two species in the Miocene record) is found on 

 the volcanic shores of the Hawaiian Islands, on Guam, and 

 in the northern Mariana Islands (Vermeij et al., 1984). The 

 cerithiids Terebralia and Potamides (five species in the 

 Miocene record) occur in mangrove associations in the 

 Philippines and elsewhere in the IndoMalayan archipelago. 

 In all, 12 species or 7% of the Miocene fauna recorded by 

 Ladd (1966, et seq.) are associated with habitats not now 

 represented at Enewetak. 



Other extinctions may represent range restrictions for 

 reasons other than habitat change. Nerita insculpta, 



Tectahus, Protobarleeia, Pugilina, and Eocithara axe not 

 recorded after the Miocene. Nerita insculpta is now found 

 in Indonesia and the Northern Marianas but apparently not 

 on islands to the east. Only one species of Tectahus, T. 

 grandinatus, now occurs on the Pacific Plate, and it is re- 

 stricted to southeast Polynesia (Rosewater, 1970). 

 Protobarleeia m^/ersi now survives in Fiji and Australia 

 (Ponder, 1983b). Although the Melongenidae do occur in 

 the Indo-West Pacific, no species of Pugilina is known from 

 the Pacific Plate. 



Still other extinctions are even more puzzling. A 

 number of species recorded from the Miocene of Enewetak 

 survive on other Pacific islands, including Kwajalein. 

 Among the species are Leptoth[;ra laeta, L. Candida, 

 L. verruca, Merelina pisinna, Rissoina concinna, R. sup- 

 racostata, Mitra turgida, and Vexillum approximatum. 



Post-Miocene Record 



The post-Miocene record consists of 105 species of 

 mollusks: 23% archaeogastropods, 61% mesogastropods, 

 and 16% neogastropods. Most of these mollusks are 

 extant. Twenty-three or 22% are extinct at Enewetak 

 today, although some survive elsewhere in the Indo-West 

 Pacific, among them Simaptocochlea rosacea, Leptothi/ra 

 picta, and Nannophora cingulifera. 



Faunal Relations 



The closest tie in the fossil record between Enewetak 

 and another locale is with Bikini. About 34% of the 

 species found in the drill holes at Enewetak were also 

 found in drill holes at Bikini. Apart from the Bikini rela- 

 tionship, the strongest tie for which there is a published 

 record is with Fiji with 13% of the Enewetak Miocene 

 species shared. The record is less strong with Guam and 

 Saipan (4% of the Miocene record) and Java (3%). 



Several possibly far-flung relationships were cited by 

 Ladd (1966). Among them he notes that Haplocochlias 

 (one species in the Miocene) is recognized as a western 

 American genus, and the turbinid Cvnisca (one species in 

 the Miocene) is known only from South Africa. Neither of 

 these relationships has been subsequently confirmed. How- 

 ever, the shells of Sansonia /cenneyi, described from the 

 Miocene of Enewetak, are indistinguishable from Miocene 

 shells from Italy and Pliocene shells from France (Le 

 Renard, personal communication). 



Species Survival 



Ladd (1966) pointed out early that many living mol- 

 lusks in the Marshall Islands have been there since late 

 Tertiary time. More recently Stanley (1979) and Stanley 

 et al. (1980) commented on the "great stability of the 

 Indo-West Pacific fauna" and suggest that species have 

 tended to survive for unusually long periods in the region. 

 The Enewetak fossil record together with the modern 

 record provides strong evidence for the generalization: 62 

 species from the Miocene record are identified as living 

 today at Enewetak. They represent 35% of that record. 



