In the late 1960s and early 1970s a series of anthropological excavations were carried out at Minatogawa Fissure on southern Okinawa Island, producing a number of Late Pleistocene Human bones, as well as associated bones of other terrestrial vertebrates. Whilst most of the bones were from animals still found on Okinawa, or neighbouring members of the central Ryukyu Islands, or at least extinct animals thought to have been endemic (native) to the islands, a number of Frog species were reported which are today absent from the Ryukyus, but found in other parts of Japan. This is puzzling, as the Ryukyu Islands are generally thought to be biogeographically distinct from the rest of Japan; eleven out of fourteen Amphibian species found in the Central Ryukyus today are found nowhere else, two species are also found in southern China, with only a single species, the wide ranging Marsh Frog, Fejervarya kawamurai, is shared with the rest of Japan, and this species is also found in southern China and is thought likely to have reached the Central Ryukus in a separate dispersal event from the mainland. A specimen of Polypedates leucomystax, a Southeast Asian Frog otherwise thought to have been introduced to the islands in the mid twentieth century was also reported.
In addition these excavations yielded a number of Frog species still found in the on the island, but restricted to the moister, more forested northern part of the island. This is problematic, as the Pleistocene climate of Okinawa is interpreted to have been cooler and drier, which would cause such frogs to have had a more limited range, rather than an expanded one, and also because pollen associated with the tree species now found in the woodland on the north of the island has not been found here, implying that the south of the island was as lacking in woodland in the Pleistocene as it is today. Furthermore, the fissure is in an area of limestone karst (soft, absorbent and highly porous limestone), which would make it hard for bodies of standing water to form here under any circumstances.
In a paper published in the journal Palaeontologica Electronica in January 2015, Yasuyuki Nakamura of the Tropical Biosphere Research Center at the University of the Ryukyus and Hidetoshi Ota of the Institute of Natural and Environmental Sciences at the University of Hyogo and the Museum of Nature and Human Activities in Hyogo, Frog and Newt bones recovered from a more recent study at Minatogawa Fissure, as well as new excavations at the nearby Sashiki Fissure.
Maps of the Ryukyu Archipelago (1, 2) and Okinawajima Island (3). The map of Okinawajima shows topography, distribution of the Pleistocene limestone, and study sites.Nakamura & Ota (2015).
The sediments in the Minatogawa Fissure have been dated to about 15 670 years ago, towards the end of the Last Glacial Maximum. It is located in the Minatogawa Formation above the Yuhi River on the southeastern part of the island. Nakamura and Ota were unable to access the material from the original 1960s and 1970s excavations, nor any details of how the material was analysed (the previous publications covering the material simply listed the species). They were, however, able to access a series of Amphibian bones obtained by water-screening sediments from the site during a more recent excavation at the fissure in 1998-2001. These were compared to specimens of Amphibians from the Ryukus, other areas of Japan, and adjacent areas such as the Korean Peninsula.
The 1998-2001 Minatogawa Fissure material yielded a single humerus assigned to a female Namiye’s Frog, Limnonectesnamiyei, a species today found on northern, but not southern Okinawa and two humeri and two ilia attributed to female Holst’s Frogs, Babinaholsti, another species found today on the northern part of the island (leg bones are the most robust in Frogs, and therefore the most commonly preserved; it is usually possible to tell the sex of a From from its humeri).
Left female humerusfrom Minatogawa Fissure, referred to Limnonectes namiyei, lacking the proximal and distal parts and the crista ventralis in ventral (1), medial (2) and dorsal (3) views. Abbreviations: cr.par, crista paraventralis; fo.div, fossula dividens; ol.sc, olecranon scar; stm, spina tuberculi medialis. Scale bar is 5 mm. Nakamura & Ota (2015).
Much more abundant were bones of the Okinawa Tip-nosed Frog, Odorrana narina, with 21 right and 24 left female humeri, 16 right and 16 left male humeri and 22 right and 31 left ilia found in the material. This species is also found on the northern part of Okinawa today.Another abundant species was the Ryukyu Brown Frog, Rana ulma, with 41 right and 48 left female humeri, one left male humerus and eight right and nine left ilia found in the sample. This species is also found on northern Okinawa today.
Also present was a right humerus attributed to a female Ryukyu Kajika Frog, Buergeria japonica, a species still found on the southern part of Okinawa, and a pair of humeri of undetermined sex assigned to the Okinawa Green Tree Frog, Rhacophorus viridis viridis, also found on the southern part of Okinawa today.
Right femalehumerus from Minatogawa Fissure, referred to Buergeria japonica, lacking the proximal part in ventral (1), medial (2), and dorsal (3) views. Abbreviations: cr.lat, crista lateralis; cr.med, crista medialis; e.cap, eminentia capitata; ep.ul, epicondylus ulnaris; ol.sc, olecranon scar. Scale bar is 1 mm. Nakamura & Ota (2015).
Two species of Newt were also recovered from the material. Eight atlantes (the first vertebra after the skull), at least postatlantal precaudal vertebrae, 20 right and 15 left humeri and 11 right and 12 left femora were assigned to the Sword-tailed Newt, and three postatlantal precaudal vertebrae and one right femur were attributed to Anderson’s Crocodile Newt, Echinotriton andersoni, both of with are still present on southern Okinawa.
The Sashiki Fissure lies 6 km to the northeast of the Minatogowa Fissure in an uplifted terrace of the Naha Formation on the Chinen Peninsula. The fissure is a sinkhole that was exposed by limestone miners, then excavated by archaeologists from 2004 till 2008. The fissure yielded two separate deposits, a lower bed dated to between 28 729 and 31 745 years ago, around the beginning of the last glacial maximum, and an upper bed dated to between 3904 and 5558 years ago, during the Middle Holocene; these beds resting in separate depressions and not overlying one-another, though there is a thin layer at the top of the older deposits dated to about 7445 years ago (which did not yield any vertebrate fossils).
Photograph of the Sashiki Fissure (left) and schematic figure showing the structure of the fissure (right). In the right figure, broken lines represent the outline of the fissure behind rock, shaded areas represent studied sediments, and open circles represent the locations of the dating samples. Nakamura & Ota (2015).
The lower deposits at Sashiki yielded a single left ilium referred to Namiye’s Frog, Limnonectes namiyei, one right and five left female humeri, oneright and one left male humeri and six right and three leftilia assigned to Holst’s Frog, Babina holsti, and five right and five left female humeri, two right and one left male humeri and four right and one left ilia assigned to Ishikawa’s Frog, Odorrana ishikawae, all species found on the north of the island today.
Right female humerus in ventral (5), medial (6) and dorsal (7) views; (8–10) right male humerus lacking the proximal part in ventral (8), medial (9), and dorsal (10) views; and (11), right ilium lacking the anterior part in lateral view. Abbreviations: acet, acetabulum; acet.m, acetabular margin; cr.dors, crista dorsalis; cr.lat, crista lateralis; cr.med, crista medialis; cr.par, crista paraventralis; cr.ven, crista ventralis; e.cap, eminentia capitata; ep.rad, epicondylus radialis; ep.ul, epicondylus ulnaris; fo.div, fossula dividens; il.sh,ilial shaft; p.asc, pars ascendens; stm, spina tuberculi medialis; supr.fo, supracetabular fossa; tub.sup, tuber superior. Scale bars equal 5 mm. Nakamura & Ota (2015).
The Okinawa Tip-nosed Frog, Odorrananarina, which was one of the most abundant Frogs in the Minatogowa Fissure, but which is only found on the north of the island today, is represented in the lower Sashiki deposits by only four right and four left female humeri, oneright male humerus, one pelvic girdle and one left ilium.
Pelvic girdle: fused right and left ilia (lacking anterior parts) with the ischium and the pubis, in right lateral view. Abbreviations: acet, acetabulum; acet.m, acetabular margin; cr.dors, crista dorsalis; il.sh, ilial shaft; p.asc, pars ascendens; supr.fo, supracetabular fossa; tub.sup, tuber superior. Scale bars equal 5 mm. Nakamura & Ota (2015).
The other abundant Frog in the Minatogowa Fissure, the Ryukyu Brown Frog, Rana ulma, which is also absent from the south of the island today, is even more abundant in the lower Sashiki deposits, with 108 right and 68 left female humeri, 44 right and 43 left male humeri and 46 right and 47 left ilia assigned to this species.
Fossils referred to Rana ulma. (1–3), right female humerus in ventral (1) medial (2) and dorsal (3) views; (4–6)right male humerus in ventral (4) medial (5) and dorsal (6) views; (7) right iliumin lateral view. Abbreviations: acet, acetabulum; acet.m, acetabular margin; cr.dors, crista dorsalis; cr.lat, crista lateralis; cr.med, crista medialis; cr.par, crista paraventralis; cr.ven, cristaventralis; e.cap, eminentia capitata; ep.ul, epicondylus ulnaris; fo.div, fossula dividens; il.sh, ilial shaft; ol.sc, olecranon scar; p.asc, pars ascendens; stm, spina tuberculi medialis; supr.fo, supracetabular fossa; tub.sup, tuber superior. Arrows indicate the proximal ends of the crista paraventralis. Scale bars equal 1mm. Nakamura & Ota (2015).
The Okinawa Green Tree Frog, Rhacophorus viridis viridis, which was present in the Minatogowa Fissure and which is still found on south Okinawa today, is represented in the lower Sashiki Fissure by two right and five left female humeri, one left humeri of indeterminate sex, one right ilium and one pelvic girdle.
Pelvic girdle of Rhacophorus viridis viridis (fused right and left ilia lacking anterior parts withthe ischium) in left lateral view. Abbreviations: acet, acetabulum; acet.m, acetabular margin; cr.dors, crista dorsalis; il.sh, ilial shaft; p.asc, pars ascendens; pre.acet, preace tabular zone; supr.fo, supracetabular fossa; tub.sup, tuber superior. Scale bars equal 1mm. Nakamura & Ota (2015).
The Sword-tailed Newt, Cynops ensicauda, is represented in the lower Sashiki Fissure by two right and one leftmaxillae, one parietal-prootic-exoccipital, one left frontal, two right squamosals, six right and three left dentaries, five atlantes, 108 postatlantal precaudal vertebrae, 14 right and 11 left ribs, 41 right and 44 left humeri and 51 right and 41 left femora.
Fossils referred to Cynops ensicauda. (1–4) Postatlantal precaudal vertebra in anterior (1) left lateral (2) dorsal (3) and ventral (4) views; (5 and 6) atlas in anterior (5) and left lateral (6) views; (7) parietal-prootic-exoccipital in dorsal view; (8) right maxilla in lateral view; (9) right dentary in medial view; (10) right rib in posterior view; (11) right humerus in lateral view; and (12) right femur in posterior view. Abbreviations: con, condyle; diap, diapophyses; epi.pr, epipleural processes; neu.sp, neural spine; n.prep, notch for prearticular; parap, parapophyses; pos.pr, posterior process; subd.d, subdental ditch; zygap, zygapophyses. The arrow in 12 indicates a concavity. Scale bars equal 1 mm. Nakamura & Ota (2015).
Anderson’s Crocodile Newt, Echinotriton andersoni, is represented in the lower Sashiki Fissure unit by 11 right and 19 leftmaxillae, seven right and six left frontals, one parietal-prootic-exoccipital, nine right and nine left squamosals, three right and two left quadrates, 28 right and 42 left dentaries, 11 atlantes, 290 postatlantal precaudal vertebrae, 89 right and 92 left ribs, 68 right and 76 left humeri and (71 right and 92 left femora.
In the upper Sashiki Fissure unit, which is Middle Holocene in age, none of the species currently confined to northern Okinawa is present.
One species found in southern Okinawa today, and absent from the Pleistocene deposits is the Okinawa Narrow-mouthed Toad, Microhylao kinavensis, which is represented in the upper Sashiki Fissure deposits by two right ilia.
Right ilium of Microhylao kinavensisin lateral view, tub.sup, tuber superior. Scale bar equals 1mm. Nakamura & Ota (2015).
Also present in the upper unit at the Sashiki Fissure are one left ilium of the Ryukyu Kajika Frog, Buergeria japonica, one male left humerus of the Okinawa Green Tree Frog, Rhacophorus viridis viridis, and one right maxilla of the Sword-tailed Newt, Cynops ensicauda.
This study presents a very different view of the Pleistocene Amphibian fauna of southern Okinawa, with no species not found on the island today found in either the Late Pleistocene or Middle Holocene deposits, and the introduced Polypedates leucomystax absent from even the Holocene deposits. This leads Nakamura and Ota to conclude that the previous identifications of a range of exotic Frogs in Late Pleistocene deposits from the Minatogawa Fissure as erroneous.
The situation with Frogs found today on the north of the island is more complex, with Nakamura and Ota able to confirm the presence and even abundance of several north Okinawan species in Late Pleistocene deposits from the south of the island. This is highly problematic, as all previous evidence has suggested that the Late Pleistocene climate of the island was drier than today, with the south of the island less able to support such moisture loving species. Neither should pools of standing water, necessary for the survival of many of these species, form on the karst terrain that produced the fossils in anything short of a major flood event.
Maps showing current distributions (shaded areas) of five forest Frogs (Limnonectes namiyeia, Babina holstia, Odorrana ishikawaea, Odorrana narinaa, and Rana ulmaa, and the Newt Echinotriton andersonia on Okinawajima Island and adjacent islets. ‘?’ Denotes a population lacking a record or observation in the past 30 years. Nakamura & Ota (2015).
Nakamura and Ota observe that while the limestone areas of southern Okinawa are more-or-less free from streams or bodies of standing water, due to the drainage provided by the highly porous limestone, they are home to several natural springs, fed by waters flowing along the top of the water impermeable Shimajiri Group, which underlies the limestone units in places where these units are relatively thin or the limestone becomes slightly more dense (where water flowing through a rock body encounters a less porous area it must go somewhere, and tends to rise to the surface). They therefore suggest that in the Late Pleistocene the Minatogawa and Sashiki Fissures might have been home to such springs, providing standing bodies of water, which could have supported the moisture-loving Amphibians found preserved in the deposits there.
None of these species are present in the Middle Holocene deposits, suggesting that any such springs did not last beyond the last Pleistocene glaciation, and that the Amphibians of Okinawa Island had reached roughly their modern distributions by this time.
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