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Stegodon aurorae; Temporal range: Plio-Pleistocene
	Replica of the well-preserved Akebono elephant skeleton from Taga, Shiga Prefecture (Mie Prefectural Museum)
	Model of Stegodon aurorae
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Proboscidea
Family:	†Stegodontidae
Genus:	†Stegodon
Species:	†S. aurorae
Binomial name
	†Stegodon aurorae; (Matsumoto, 1918)
Synonyms
	Elephas aurorae (protonym); Parastegodon aurorae; Parastegodon akashiensis; Parastegodon infrequens; Parastegodon? kwantoensis; Parastegodon sugiyamai; Stegodon (Parastegodon) aurorae; Stegodon akashiensis; Stegodon infrequens; Stegodon kwantoensis; Stegodon sugiyamai; Stegodon shodoensis (partim);

Stegodon aurorae is an extinct species of Stegodon known from Early Pleistocene (2.0 Ma – 1.0 Ma) of Japan and possibly Taiwan.^[7]^[8]^: 487 It appears to be an insular dwarf species, exhibiting smaller body size than its mainland ancestors.

Description

The best-preserved Stegodon aurorae skeleton, that from Taga in Shiga Prefecture, stands to a shoulder height of 1.93 m (6 ft 4 in), with a body length of 4.58 m (15.0 ft).^[9]^: 43 It is relatively short-legged, the ratio between vertebral column length and shoulder height being 0.88.^[9]^: 43 The body mass of S. aurorae has been calculated at one quarter (around 2,122 kilograms (4,678 lb)) of that of its mainland ancestor S. zdanskyi, which had a shoulder height of around 3.6 m (12 ft);^[10]^: 1658 as such, it is an example of insular dwarfism.^[10]^: 1657

Distribution

Remains of Stegodon aurorae have been found in over forty-five localities in the Japanese archipelago, mainly in central Honshū.^[9]^: 43 The type specimen was from Ishikawa Prefecture,^[1] while among the eight skeletons, one was found in Saitama Prefecture, two in Nagano Prefecture, one in Shiga Prefecture, one in Mie Prefecture, and three in Hyōgo Prefecture.^[11]^: 202 A fragmentary lower jaw with part of a third molar attributed has also been recovered from "Cho-chen", Taiwan.^[12]^: 31However, other authors have argued that Stegodon remains from Taiwan should be included in S. orientalis instead.^[13]

Biozone

The species lends its name to the biostratigraphic assemblage zone referred to as the "S. aurorae Zone".^[8]^: 487 This biozone, from two to one million years ago,^[8]^: 487 includes deer (Elaphurus spp., Cervus sp.) and rhino (Rhinoceros sp.) as species that competed for resources, as well as the predatory canine Canis falconeri.^[10]^: 1662

Evolution

Cladistic analysis of cranial characters suggests a close relationship between Stegodon aurorae and Stegodon zdanskyi from northern China.^[9]^: 41 S. zdanskyi is thought to have reached Japan over a land bridge in the Early Pliocene, around 5-4 million years ago giving rise there to the smaller S. miensis (c. 4–2.9 Ma) the earliest of the four stegodontids found in the Japanese archipelago, due to isolation from the mainland.^[9]^: 41^[8]^: 482^[14] S. miensis was then succeed by S. protoaurorae (c. 2.9–2 Ma), and then S. aurorae (2-1 Ma)^[8]^: 487 The progressive size reduction from S. miensis to S. aurorae is suggested to have been driven marine transgression reducing the land area of the Japanese archipelago.^[8]^{: 484–488} The absence of S. aurorae from the fossil record of China and the Korean Peninsula supports the theory of the species arising within the archipelago.^[15]^: 261

Following the extinction of Stegodon aurorae around 1 million years ago, it was succeeded in Japan by Stegodon orientalis, which is also known from mainland East Asia,^[7] which arrived in the archipelago as a result of a separate migration during the Middle Pleistocene.^[9]

Taxonomy

Elephas aurorae was described by Matsumoto Hikoshichirō in 1918, based on an upper molar from Mount Tomuro in the old Province of Kaga (present-day Ishikawa Prefecture).^[1] In 1924, Matsumoto erected the genus Parastegodon and transferred to it the Akebono elephant, the new combination being Parastegodon aurorae.^[2] In 1938, Makiyama Jirō synonymized Parastegodon with Stegodon.^[16]^: 15 In a 1991 review of the former genus Parastegodon, Taruno Hiroyuki confirmed the synonymization of Stegodon kwantoensis (Tokunaga, 1934), Stegodon sugiyamai (Tokunaga, 1935), Stegodon akashiensis (Takai, 1936), and Stegodon infrequens (Shikama, 1937), as well as some of the material referred to Stegodon shodoensis Matsumoto, 1924, with Stegodon aurorae.^[17]

Stegodon orientalis shodoensis was described by Matsumoto in 1924, from material from the village of Yoshima [ja] (now Sakaide), Kagawa Prefecture.^[18]^: 333 The subspecies was elevated to species rank by Makiyama in 1938.^[16]^: 17 Some of the material referred to S. shodoensis (ショウドゾウ also ミツゴゾウ) belongs to S. aurorae.^[17]^: 5 Parastegodon? kwantoensis, the protonym of Stegodon kwantoensis (カントウゾウ), was described by Tokunaga Shigeyasu in 1934 from dental material excavated in the village of Kakio [ja] (now Kawasaki), Kanagawa Prefecture.^[5]^: 17 Parastegodon sugiyamai, the protonym of Stegodon sugiyamai (スギヤマゾウ), was described by Tokunaga Shigeyasu in 1935, from a molar unearthed during the building of a road in the village of "Saida", Kagawa Prefecture.^[6] Parastegodon akashiensis, the protonym of Stegodon akashiensis (アカシゾウ), was described by Takai Fuyuji in 1936, from an upper molar (a cheek tooth) from the cliffs along the shore of the village of Ōkubo (now Akashi), Hyōgo Prefecture.^[3] Additional material from the coast of and sea bed off Akashi, including two skulls and a lower jaw, was published at the same time and referred to P. akashiensis by Shikama Tokio.^[19] Parastegodon infrequens, the protonym of Stegodon infrequens (インフリークエンスゾウ also タキガワゾウ), was described by Shikama Tokio in 1937, from material from the seabed off Akashi, Hyōgo Prefecture.^[4]^[17]^: 12 P. kwantoensis, P. sugiyamai, and some of the material referred to P. akashiensis were synonymized with P. aurorae, and P. infrequens with P. akashiensis, by Takai in 1938.^[20]^: 753–4 Taruno synonymized P. akashiensis with S. aurorae in 1991.^[17]^: 5

References

^ ^a ^b ^c ^d Matsumoto Hikoshichirō (1918). "On a New Archetypal Fossil Elephant from Mt. Tomuro, Kaga". Science Reports of the Tohoku University. Second Series, Geology. 3 (2): 51–56. hdl:10097/30161. ISSN 0082-464X.
^ ^a ^b Matsumoto Hikoshichirō (1924). 日本産化石象の種類(略報) [Types of Japanese Fossil Elephant]. Journal of the Geological Society of Tokyo (in Japanese). 31 (371): 255–272. doi:10.5575/geosoc.31.371_255.
^ ^a ^b ^c Takai Fuyuji 高井冬二 (1936). "On a New Fossil Elephant from Okubo-mura, Akashi-gun, Hyogo Prefecture, Japan". Proceedings of the Imperial Academy. 12 (1): 19–21. doi:10.2183/pjab1912.12.19.
^ ^a ^b Shikama Tokio (1937). "Parastegodon infrequens sp. nov. from the Akashi district". Japanese Journal of Geology and Geography. 14 (3–4): 127–131.
^ ^a ^b Tokunaga Shigeyasu (1934). 横濱市及び神奈川縣柿生村發見の象齒化石に就て [On Fossil Elephant Teeth Found in Yokohama City and Kakio Village, Kanagawa Prefecture]. Journal of Geography (Chigaku Zasshi) (in Japanese). 46 (8): 363–371. doi:10.5026/jgeography.46.363.
^ ^a ^b Tokunaga Shigeyasu (1935). "A New Fossil Elephant Found in Shikoku, Japan". Proceedings of the Imperial Academy. 11 (10): 432–434. doi:10.2183/pjab1912.11.432. S2CID 128053908.
^ ^a ^b Takahashi, K.; Chang, C.H.; Cheng, Y.N. (2001). "Proboscidean fossils from the Japanese Archipelago and Taiwan Islands and their relationship with the Chinese mainland". In Cavarretta, G.; Gioia, P.; Mussi, M.; Palombo, M.R. (eds.). The World of Elephants (PDF). Rome: Consiglio Nazionale delle Ricerche. pp. 148–151. ISBN 88-8080-025-6.
^ ^a ^b ^c ^d ^e ^f Aiba, H.; Baba, K.; Matsukawa, M. (2010). "A new species of Stegodon (Mammalia, Proboscidea) from the Kazusa Group (lower Pleistocene), Hachioji City, Tokyo, Japan and its evolutionary morphodynamics". Palaeontology. 53 (3): 471–490. Bibcode:2010Palgy..53..471A. doi:10.1111/j.1475-4983.2010.00953.x. S2CID 128161878.
^ ^a ^b ^c ^d ^e ^f Saegusa Haruo; Thasod, Yupa; Ratanasthien, Benjavun (2005). "Notes on Asian stegodontids". Quaternary International. 126–128: 31–48. Bibcode:2005QuInt.126...31S. doi:10.1016/j.quaint.2004.04.013.
^ ^a ^b ^c Geer, A.A.E. van der; et al. (2016). "The effect of area and isolation on insular dwarf proboscideans". Journal of Biogeography. 43 (8): 1656–1666. Bibcode:2016JBiog..43.1656V. doi:10.1111/jbi.12743. S2CID 87958022.
^ 北御牧村アケボノゾウ発掘調査団 (2002). Kitamimakimura Akebonozou Excavation Research Group (ed.). "Uncommon occurrence of Stegodon aurorae (Matsumoto) (Mammalia; Proboscidea) from Kitamimaki-mura, Nagano Prefecture, Japan". Earth Science (Chikyū Kagaku). 56 (3): 197–202. doi:10.15080/agcjchikyukagaku.56.3_197. ISSN 0366-6611.
^ Shikama Tokio; Ōtsuka Hiroyuki 大塚裕之; Tomida Yukimitsu (1975). "Fossil Proboscidea from Taiwan (I)". Science Reports of the Yokohama National University. Section II, Biological and Geological Sciences. 22: 13–62. doi:10.15080/agcjchikyukagaku.54.4_257.
^ Kawamura, Ai; Chang, Chun-Hsiang; Kawamura, Yoshinari (March 2016). "Middle Pleistocene to Holocene mammal faunas of the Ryukyu Islands and Taiwan: An updated review incorporating results of recent research". Quaternary International. 397: 117–135. Bibcode:2016QuInt.397..117K. doi:10.1016/j.quaint.2015.06.044.
^ Taruno Hiroyuki (2010). "The stages of land bridge formation between the Japanese Islands and the continent on the basis of faunal succession" 哺乳類化石の変遷から見た日本列島と大陸間の陸橋の形成時期 [The stages of land bridge formation between the Japanese Islands and the continent on the basis of faunal succession]. The Quaternary Research (Daiyonki Kenkyū) (in Japanese and English). 49 (5): 309–314. doi:10.4116/jaqua.49.309.
^ Takahashi Keiichi; Namatsu Keiko (2000). "Origin of the Japanese Proboscidea in the Plio-Pleistocene". Earth Science (Chikyū Kagaku). 54 (4): 257–267. doi:10.15080/agcjchikyukagaku.54.4_257.
^ ^a ^b Makiyama Jirō [in Japanese] (1938). Japonic Proboscidea. Memoirs of the College of Science, Kyoto Imperial University. Series B. 14 (1): 1–59. hdl:2433/257870. ISSN 0368-8895.
^ ^a ^b ^c ^d Taruno Hiroyuki 樽野博幸 (1991). 日本列島産"Parastegodon"属の分類学的再検討 [Systematic revision of the genus "Parastegodon" from the Japanese Islands (Mammalia: Proboscidea)]. Bulletin of the Osaka Museum of Natural History (Thesis) (in English and Japanese). Vol. 45. pp. 5–16.
^ Matsumoto Hikoshichirō (1924). 日本産ステゴドンの種類(略報) [Species of Stegodon From Japan]. Journal of the Geological Society of Japan (in Japanese). 31 (373): 323–340. doi:10.5575/geosoc.31.373_323.
^ Shikama Tokio (1936). "Note on Parastegodon akashiensis Takai from the Akasi District". Proceedings of the Imperial Academy. 12 (1): 22–24. doi:10.2183/pjab1912.12.22.
^ Takai Fuyuji 高井冬二 (1938). 本邦に於ける新生代哺乳動物(豫報) [Cenozoic Mammalian Fauna of the Japanese Empire (A Preliminary Note)]. Journal of the Geological Society of Japan (in Japanese). 45 (541): 745–763. doi:10.5575/geosoc.45.745.