Partula desolata, Bauman & Kerr, 2013

Bauman, Scott & Kerr, Usa And Alexander M., 2013, Partula desolata sp. nov. (Pulmonata: Partulidae), an extinct land snail from Rota, Mariana Islands, Micronesia, Micronesica 2013 (5), pp. 1-12 : 5-9

Partula desolata sp. nov.

( Figs. 2–3 )

Partula cf./ aff. gibba, Bauman 1996: 21 , fig. 32.

Material examined. Holotype: BPBM 252143 -A, complete shell, Payapai Cave (Test Pit 3, Layer III, level 4), Rota, Collector : S. Bauman, July 1994 . Paratypes: UF 449333 , partial shell, Alaguan Rock Shelter (Test Pit 1), Rota, Collector : S. Bauman, 21 July 1994 ; UF 449332 , partial shell, As Matmos Cliffside Cave (Test Pit 1), Rota, Collector : S. Bauman, 23 July 1994 . UGI 3001 , complete shell, Payapai Cave (Test Pit 3, Layer III, level 4), Rota, Collector : S. Bauman, July 1994 .

Comparative material. Partula gibba, BPBM 252143-B, complete shell, Payapai Cave (Test Pit 3, Layer III, level 4), Rota, Collector: S. Bauman, July 1994; UF 449334 , partial shell, Alaguan Rock Shelter (Test Pit 1), Rota, Collector : S. Bauman, 21 July 1994; UF 449335 , partial shell, As Matmos Cliffside Cave (Test Pit 1), Rota, Collector : S. Bauman, 23 July 1994 ; Partula thalia Garrett, 1884 UF 112145, complete shell, Raiatea , Society Islands, French Polynesia, Collector: W. J. Clench , no collection date.

Diagnosis. A Partula from the Mariana Islands with a robust shell and an expanded, reflexed and thickened peristome.

Description. Shells dextral, heavy, opaque, ovate conical, colour altered taphonomically in at least some specimens, but in the holotype both externally and internally light brown, darker near sutures, protoconch whorl and peristome lighter. The following measurement ranges are drawn from three to, when possible, all four specimens and always include the complete shell of the holotype: Shell height 17.2–19.5 mm (holotype 19.5 mm), width 13.8–14.8 mm (holotype 14.5 mm), aperture height 9.8–12.5 mm (holotype 12.5 mm), aperture width 8.5–10.1 mm (holotype 10.1 mm), peristome width at parietal margin 2.0– 2.3 mm (holotype 2.0 mm), peristome thickness at parietal margin 2.2–3.0 mm (holotype 2.2 mm), shell height/width ratio 1.10–1.43 (holotype 1.10), apertural height/width ratio 0.71–1.10 (holotype 0.74); about one protoconch whorl and 3.5 teleoconch whorls. Spire conical, apex somewhat obtuse, whorls evenly descending, basal whorl ca. 60% of shell height, protoconch whorl smooth, postembryonic whorls nearly shiny in some specimens, nearly flattened, numerous prosoclinic growth striae at irregular intervals checking or offsetting fine, evenly spaced appressed spiral striae, suture distinct, emarginate, impressed, aperture slightly oblique, squarish to auriculate, non-labiate, with a thick, reflexed, flattened and polished peristome, columellar margin straight and flared against the body whorl, almost circular in outline through the basal and mid-palatal margin, then angling adaxially while narrowing suddenly so as to form a low, rounded sub-denticulate inner border, the aperture's entire margin thick, from 2.5–3.0 mm (holotype 3.0 mm) at the peripheral palatal margin, with distinct multiple growth lamellae, parietal callus present, opaque, thin, of uniform thickness, undenticulated, umbilicus typically Partula -like: narrow, deep, and partially eclipsed by the reflexed columellar lip.

Etymology. This snail was discovered and is being described at a time when over half of the Partulidae are now extinct ( Cowie 1992), including two of the five species inhabiting the Mariana Islands ( Hopper & Smith 1992; Smith 2008a). The remaining three species, as well as most other partulids Pacific-wide, are seriously threatened with extinction ( IUCN 2012). Hence, the specific epithet desolata (the singular feminine adjectival form of the Latin present infinitive desolare = to render forsaken or, as in the sense intended here, to have been rendered forsaken) is chosen to indicate that the excavated shells have reemerged in a time quite apart from that in which they once coursed, to a world now desolate of most members of the formerly diverse family Partulidae .

Distribution. Known only from four shells recovered from Late Holocene (<1000 yr BP) deposits in caves from the Alaguan and As Matmos regions of Rota, Mariana Islands. The distance between these localities spans roughly half the length of the island (6.5 km), as well as straddles the northern and southern coasts, indicating that the species was probably widely distributed within Rota. Excavations in similarly aged deposits on the two adjacent and larger islands, Guam and Tinian, have as yet not turned up this Partula (S. Bauman, unpubl.; J.A. Starmer, pers. comm.; C. Christensen, pers. comm.). Further, extensive searches by us and others ( Crampton 1925; Kondo 1970; Kurozumi 1994; Smith 2008a – b, Smith et al. 2008; D.R. Hopper, pers. comm.; J.A. Starmer, pers. comm.) for living Partula in Rota and the other Mariana Islands have not discovered contemporary specimens with this morphology. We, therefore, consider this species an endemic of Rota, Mariana Islands, and one now extinct.

Remarks. Partula desolata sp. nov. differs from other Mariana Partula , including the sympatric P. gibba , in multiple respects. To more rigorously assess the distinctiveness of the new species' shell, we used two-sample t tests to compare shell height, shell width, apertural height, and apertural width to the respective measures taken from other Mariana Partula . All tests were performed against the measurements taken by Crampton (1925) of the Mariana partulid species most similar in size, the now extinct P. salifana . This comparison was preferable since it rendered our tests maximally conservative, i.e., it decreased the chance of finding a statistically significant difference among all the species, and for two important reasons. First, P. salifana , while somewhat smaller on average than the new species, is nevertheless most similar to the new species in all tested dimensions ( Table 1 ). Thus, a finding of a significant difference indicates that the other and even smaller species, including P. gibba sampled from Guam, Rota and Saipan, must also be smaller than expected by chance sampling. Second, P. salifana also had the smallest sample size (n = 17), which thus provided the least statistical power to uncover a genuine difference. Still, despite our conservative tack, P. desolata sp. nov. is on average significantly larger than all other Mariana Partula ( Table 1 ).

The development of the peristome also shows much more expansion and thickening, to 3 mm ( Table 1 ; Figs. 2–3 ). In fact, this appears unprecedented among the Partulidae Pacific-wide. Expansion, recurving and thickening of the lip approaching that in P. desolata sp. nov. has previously been seen only in a few of the geographically remote species, notably the Raiatean P. thalia in the Society Islands ( Fig. 3E ). However, even the latter species' lip tends to one-half to one-third as thick (to 1.2 mm), its parietal callus is thinner and transparent, prominently denticulate, and its shell less gibbous with stronger and more numerous growth striae. Crampton's (1925) plates of P. gibba in apertural view occasionally give the appearance of having a peristome approaching the breadth of the new species. However, Crampton (1925) does not mention that any specimens of P. gibba have an unusually thickened peristome. Instead, Crampton (p. 26) writes that the Marianas Partula most similar to the thicklipped forms of the Society Islands is Guam's P. salifana , which we show to be, nevertheless, statistically smaller ( Table 1 ). Finally, the new species is bigger than shells of P. gibba also recovered from the excavations (e.g., Fig. 3D ), thus ruling out the possibility that its more robust form is ancestral to the latter species.

We also considered the possibility that P. desolata sp. nov. was a hypercalcified ecophenotype of P. gibba . Shell thickening occurs among terrestrial pulmonate snails inhabiting environments with high-calcium soils ( Solem 1988, p. 522) or pathologically via infection with digenean trematodes ( Żbikowska 2003), but never to the degree required here of any Marianas partulid to emulate P. desolata sp. nov. We and others ( Crampton 1925; Kondo 1970; D.R. Hopper, pers. comm.; J.A. Starmer, pers. comm.) have collectively examined many thousands of living and subfossil P. gibba , but specimens approaching this morphology have not been observed to our knowledge. As well, P. desolata sp. nov. with its well-developed lip is known from multiple specimens only recovered from sediments of similar age at widely separated sites on one island, Rota, consistent with a wide intra- and singular-island distribution typical of Partula spp. ( Crampton 1916, 1925, 1932). Finally, the thickened, more reflexed lip covaries with other distinctive shell characters, such as a more circular peristomal margin, larger shell size, flatter whorls, more ovate outline, less gibbous body whorl, and thicker shell.

While P. desolata sp. nov. has likely been extinct for upwards of a millennium, tentative remarks on its biology are possible. All members of the family Partulidae , as far as is known, are long-lived, fungivorous, simultaneous hermaphrodites, bearing a few live young from lightly calcified egg capsules retained inside a brood chamber ( Barker 2001). Hence, P. desolata sp. nov. also likely displayed these traits. Indeed, a large protoconch implicates ovoviviparous reproduction in snails ( Barker 2001) and the holotype's protoconch appears to be at least as broad as those from modern P. gibba from Rota and Saipan. Finally, all Partula , whether arboreal or strictly terrestrial, inhabit the leafy, humid and shaded understorey of native forest. However, the size, shape and robustness of the Partula 's shell is, as far as is known, not unambiguously linked to a treeversus ground-dwelling lifestyle ( Cowie 1992).

Damage to two of the paratypes of the new species (e.g., Fig. 3B–C ), appears consistent with that of predation by rats ( Hadfield et al. 1993; Meyer & Shiels 2009), or more speculatively, the large native terrestrial crabs, Coenobita spp. and Birgus latro (Linné, 1767) . Rat damage to Partula gibba is seen in shells recently collected from Rota (A. Gawel, pers. comm.). The radiocarbon date (<1000 yr BP) of charcoal associated with the shells ( Steadman 1999), as well as the abundance of fish bones ( Pregill 1998) and at one site (Alaguan Rock Shelter), human burials, indicate that P. desolata sp. nov. was extant until at least late prehistoric human occupation of Rota. Initial human settlement of the Marianas occurred ca. 4000 BP (Athens & Ward 2004). Unlike the many bird bones recovered from the same stratum at Payapai Cave ( Steadman 1999), none of the shells show evidence of charring or other modification by extreme heat that would unambiguously implicate the shells' damage as being a consequence of human consumption.

In conclusion, Partula desolata sp. nov. represents the first member of the Partulidae known only from the sub-fossil record. Other identifiable zooarchaeological Partula material has been assignable to modern taxa (e.g., Sinoto & McCoy 1975; Kirch et al. 1995; Lee et al. 2007). Hence, it will be interesting to learn whether further investigations of prehistoric deposits turn up more extinct species of land snails, just as such work has many extinct vertebrates whose demise has been linked with early human occupation of the islands.

Figures

Figure 2. Partula desolata Bauman & Kerr, sp. nov. Holotype, BPBM 252143-A, Payapai Cave, Alaguan region, Rota Island, Mariana Islands. Scale bar = 10 mm. A.–C. Apertural, adapertural, apical views, respectively. D. Slightly oblique basal view exposing the umbilical pit and a thickened and reflexed, but damaged, basal peristome.

Figure 3. Comparison of Partula desolata sp. nov. to P. gibba and P. thalia, showing disparity in size and development of peristome. Scale bar = 10 mm. P. desolata sp. nov., Rota, Mariana Islands, paratypes: A. UGI 3001, B. UF 449332 and C. UF 449333. D. P. gibba, Rota, UF 449334. E. P. thalia, Raiatea, Society Islands, UF 112145.