''Stegodon aurorae'', also known as the , is a species of fossil

elephantoid Elephantoidea is a taxonomic group that contains the elephants as well as their closest extinct relatives. The following cladogram shows the relationships among elephantoids, based on hyoid The hyoid bone (lingual bone or tongue-bone) () is a ...

known from

Early Pleistocene The Early Pleistocene is an unofficial sub-epoch in the international geologic timescale in chronostratigraphy, being the earliest division of the Pleistocene Epoch within the ongoing Quaternary Period. It is currently estimated to span the time ...

(2.0 Ma – 1.0 Ma) Japan and Taiwan.

Description

The best-preserved ''Stegodon aurorae'' skeleton, that from Taga in

Shiga Prefecture is a prefecture of Japan located in the Kansai region of Honshu. Shiga Prefecture has a population of 1,412,916 (1 October 2015) and has a geographic area of . Shiga Prefecture borders Fukui Prefecture to the north, Gifu Prefecture to the nort ...

, stands to a shoulder height of , with a body length of . It is relatively short-legged, the ratio between

vertebral column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordata, ...

length and shoulder height being 0.88. The body mass of ''S. aurorae'' has been calculated at one quarter of that of its mainland ancestor ''S. zdanskyi'', which had a shoulder height of around ; as such, it is an example of

insular dwarfism Insular dwarfism, a form of phyletic dwarfism, is the process and condition of large animals evolving or having a reduced body size when their population's range is limited to a small environment, primarily islands. This natural process is disti ...

Distribution

Remains of ''Stegodon aurorae'' have been found in over forty-five localities in the

Japanese archipelago The Japanese archipelago (Japanese: 日本列島, ''Nihon rettō'') is a archipelago, group of 6,852 islands that form the country of Japan, as well as the Russian island of Sakhalin. It extends over from the Sea of Okhotsk in the northeast to t ...

, mainly in central

Honshū , historically called , is the largest and most populous island of Japan. It is located south of Hokkaidō across the Tsugaru Strait, north of Shikoku across the Inland Sea, and northeast of Kyūshū across the Kanmon Straits. The island separa ...

. The

type specimen In biology, a type is a particular wiktionary:en:specimen, specimen (or in some cases a group of specimens) of an organism to which the scientific name of that organism is formally attached. In other words, a type is an example that serves to a ...

was from

Ishikawa Prefecture is a prefecture of Japan located in the Chūbu region of Honshu island. Ishikawa Prefecture has a population of 1,140,573 (31 October 2019) and has a geographic area of 4,186 km2 (1,616 sq mi). Ishikawa Prefecture borders Toyama Prefecture to ...

, while among the eight skeletons, one was found in

Saitama Prefecture is a landlocked prefecture of Japan located in the Kantō region of Honshu. Saitama Prefecture has a population of 7,338,536 (1 January 2020) and has a geographic area of 3,797 km2 (1,466 sq mi). Saitama Prefecture borders Tochigi Prefecture ...

, two in

Nagano Prefecture is a landlocked prefecture of Japan located in the Chūbu region of Honshū. Nagano Prefecture has a population of 2,052,493 () and has a geographic area of . Nagano Prefecture borders Niigata Prefecture to the north, Gunma Prefecture to the ...

, one in

Mie Prefecture is a prefecture of Japan located in the Kansai region of Honshu. Mie Prefecture has a population of 1,781,948 () and has a geographic area of . Mie Prefecture is bordered by Gifu Prefecture to the north, Shiga Prefecture and Kyoto Prefecture to ...

, and three in

Hyōgo Prefecture is a prefecture of Japan located in the Kansai region of Honshu. Hyōgo Prefecture has a population of 5,469,762 () and has a geographic area of . Hyōgo Prefecture borders Kyoto Prefecture to the east, Osaka Prefecture to the southeast, an ...

. A fragmentary lower jaw with part of a third molar has also been recovered from " Cho-chen",

Taiwan Taiwan, officially the Republic of China (ROC), is a country in East Asia, at the junction of the East and South China Seas in the northwestern Pacific Ocean, with the People's Republic of China (PRC) to the northwest, Japan to the nort ...

Biozone

The species lends its name to the

biostratigraphic Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them.Hine, Robert. “Biostratigraphy.” ''Oxford Reference: Dictionary of Bio ...

assemblage zone referred to as the "''S. aurorae'' Zone". This

biozone In biostratigraphy, biostratigraphic units or biozones are intervals of geological strata that are defined on the basis of their characteristic fossil taxa, as opposed to a lithostratigraphic unit which is defined by the lithological properties of ...

, from two to one million years ago, includes

deer Deer or true deer are hoofed ruminant mammals forming the family Cervidae. The two main groups of deer are the Cervinae, including the muntjac, the elk (wapiti), the red deer, and the fallow deer; and the Capreolinae, including the reindeer ...

(''

Elaphurus ''Elaphurus'' is a genus of deer Deer or true deer are hoofed ruminant mammals forming the family Cervidae. The two main groups of deer are the Cervinae, including the muntjac, the elk (wapiti), the red deer, and the fallow deer; and ...

spp. In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate se ...

, ''

Cervus ''Cervus'' is a genus of deer that primarily are native to Eurasia, although one species occurs in northern Africa and another in North America. In addition to the species presently placed in this genus, it has included a whole range of othe ...

'' sp.) and

rhino A rhinoceros (; ; ), commonly abbreviated to rhino, is a member of any of the five extant species (or numerous extinct species) of odd-toed ungulates in the family Rhinocerotidae. (It can also refer to a member of any of the extinct species o ...

(''

Rhinoceros A rhinoceros (; ; ), commonly abbreviated to rhino, is a member of any of the five extant species (or numerous extinct species) of odd-toed ungulates in the family Rhinocerotidae. (It can also refer to a member of any of the extinct species o ...

'' sp.) as species that competed for resources, as well as the predator Falconer's wolf (''Canis falconeri'').

Faunal succession

Proboscidea The Proboscidea (; , ) are a taxonomic order of afrotherian mammals containing one living family (Elephantidae) and several extinct families. First described by J. Illiger in 1811, it encompasses the elephants and their close relatives. From ...

n fossils have been recovered from over three hundred and fifty sites in Japan. In accordance with the

principle of faunal succession The principle of faunal succession, also known as the law of faunal succession, is based on the observation that sedimentary rock strata contain fossilized flora and fauna, and that these fossils succeed each other vertically in a specific, rel ...

, the following sequence has been established: ''Gomphotherium annectens'', ''Stegolophodon pseudolatidens'', ''Sinomastodon sendaicus'', ''Stegodon miensis'', ''S. protoaurorae'', ''S. aurorae'', ''

Mammuthus trogontherii The steppe mammoth (''Mammuthus trogontherii'', sometimes ''Mammuthus armeniacus'') is an extinct species of Elephantidae that ranged over most of northern Eurasia during the late Early and Middle Pleistocene, approximately 1.8 million-200,000 y ...

'', ''S. orientalis'', ''

Palaeoloxodon naumanni ''Palaeoloxodon naumanni'', occasionally called Naumann's elephant, is an extinct species belonging to the genus ''Palaeoloxodon'' found in the Japanese archipelago during the Middle to Late Pleistocene around 430,000 to 24,000 years ago. It is na ...

'', and '' M. primigenius''.

Speciation

Cladistic analysis Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived chara ...

cranial Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...

characters Character or Characters may refer to: Arts, entertainment, and media Literature * ''Character'' (novel), a 1936 Dutch novel by Ferdinand Bordewijk * ''Characters'' (Theophrastus), a classical Greek set of character sketches attributed to The ...

suggests a close relationship between ''Stegodon aurorae'' and ''Stegodon zdanskyi'' from northern China. ''S. zdanskyi'' are thought to have reached Japan over a

land bridge In biogeography, a land bridge is an isthmus or wider land connection between otherwise separate areas, over which animals and plants are able to cross and Colonisation (biology), colonize new lands. A land bridge can be created by marine regre ...

in the

Early Pliocene Early may refer to: History * The beginning or oldest part of a defined historical period, as opposed to middle or late periods, e.g.: ** Early Christianity ** Early modern Europe Places in the United States * Early, Iowa * Early, Texas * Early ...

, giving rise there to the smaller '' S. miensis'', the earliest of the four stegodontids found in the

, after isolation from the mainland due to the receding ice. The "biostratigraphical gap" between ''S. miensis'' (c. 4–2.9 Ma) and ''S. aurorae'' (c. 2–1 Ma) is filled by ''S. protoaurorae'' (c. 2.9–2 Ma), its

speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...

perhaps "triggered" by

marine transgression A marine transgression is a geologic event during which sea level rises relative to the land and the shoreline moves toward higher ground, which results in flooding. Transgressions can be caused by the land sinking or by the ocean basins filling ...

, with successive decreases in size an "evolutionary trend". The absence of ''S. aurorae'' from the

fossil record A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in ...

of China and the Korean Peninsula supports the theory of the species arising within the archipelago.

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

). In 1924, Matsumoto erected the genus ''Parastegodon'' and transferred to it the Akebono elephant, the

new combination ''Combinatio nova'', abbreviated ''comb. nov.'' (sometimes ''n. comb.''), is Latin for "new combination". It is used in taxonomic biology literature when a new name is introduced based on a pre-existing name. The term should not to be confused wi ...

being ''Parastegodon aurorae''. In 1938, Makiyama Jirō synonymized ''Parastegodon'' with ''

Stegodon ''Stegodon'' ("roofed tooth" from the Ancient Greek words , , 'to cover', + , , 'tooth' because of the distinctive ridges on the animal's molars) is an extinct genus of proboscidean, related to elephants. It was originally assigned to the famil ...

''. 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''. ''Stegodon orientalis shodoensis'' was described by Matsumoto in 1924, from material from the village of Yoshima a(now Sakaide),

Kagawa Prefecture is a prefecture of Japan located on the island of Shikoku. Kagawa Prefecture has a population of 949,358 (as of 2020) and is the smallest prefecture by geographic area at . Kagawa Prefecture borders Ehime Prefecture to the southwest and Tok ...

. The

subspecies In biological classification, subspecies is a rank below species, used for populations that live in different areas and vary in size, shape, or other physical characteristics (morphology), but that can successfully interbreed. Not all species ...

was elevated to species rank by Makiyama in 1938. Some of the material referred to belongs to ''S. aurorae''. ''Parastegodon? kwantoensis'', the

protonym In the scientific name of organisms, basionym or basyonym means the original name on which a new name is based; the author citation of the new name should include the authors of the basionym in parentheses. The term "basionym" is used in both botan ...

of , was described by Tokunaga Shigeyasu in 1934 from dental material excavated in the village of Kakio a(

now Now most commonly refers to the present time. Now, NOW, or The Now may also refer to: Organizations * Natal Organisation of Women, a South African women's organization * National Organization for Women, an American feminist organization * Now ...

Kawasaki),

Kanagawa Prefecture is a prefecture of Japan located in the Kantō region of Honshu. Kanagawa Prefecture is the second-most populous prefecture of Japan at 9,221,129 (1 April 2022) and third-densest at . Its geographic area of makes it fifth-smallest. Kana ...

. ''Parastegodon sugiyamai'', the

of , was described by Tokunaga Shigeyasu in 1935, from a molar unearthed during the building of a road in the village of "

Saida Saida may refer to: Places * Saïda, Algeria, a city in Algeria * Saïda Province, a province of Algeria * Saida, Lebanon, the Arabic name for Sidon, a city in Lebanon * Saida, a village in Helan, Mandi Bahauddin, Punjab province, Pakistan * Sai ...

. ''Parastegodon akashiensis'', the protonym of , was described by Takai Fuyuji in 1936, from an upper molar (a

cheek tooth Cheek teeth or post-canines comprise the molar and premolar teeth in mammals. Cheek teeth are multicuspidate (having many folds or tubercles). Mammals have multicuspidate molars (three in placentals, four in marsupials, in each jaw quadrant) and ...

) from the cliffs along the shore of the village of Ōkubo (now

Akashi Akashi may refer to: People *Akashi (surname) Places *Akashi, Hyōgo *Akashi Station, a Japanese railroad station on the Sanyō Main Line *Akashi Strait *Akashi Kaikyō Bridge, crossing the former *Akashi Castle *Akashi Domain * Akashi, the name ...

. 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 (1912–1978) was a Japanese vertebrate palaeontologist. Considered the leading Japanese figure in the field in the immediate pre- and post-war years, species he described include Yabe's giant deer ('' Sinomegaceros yabei''). References Ext ...

. ''Parastegodon infrequens'', the protonym of , was described by Shikama Tokio in 1937, from material from the

seabed The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...

off

. ''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. Taruno synonymized ''P. akashiensis'' with ''S. aurorae'' in 1991.

References

{{Taxonbar, from=Q11283098 Stegodontidae Pleistocene proboscideans Pleistocene mammals of Asia Fossils of Japan Fossil taxa described in 1918