The Tsubusagawa Formation is distributed in a small area of the northern part of Kyushu Island, and is composed of Pliocene fluvio-lacustrine deposits. Many important fossils for understanding the fauna of this age have been found from this formation, and are called the “Ajimu Fauna” (Takahashi, 2001). The age of the lower part of this formation, which is 3.5 Ma, was established by tephra correlations between this formation and the Miyazaki Group (Satoguchi, 2016). The location where fossils are excavated from, in the Hirayama area, is a middle part of this formation, and is composed mainly of organic mud with thin sand beds. The depositional facies is estimated to have been around a swamp and/or pond. Two new described tephra beds, which are named the Hirayama 1 and 2 volcanic ash beds, are intercalated in this formation at this location. These beds are not correlated with any volcanic ash beds in this area.

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Late Early Pliocene palaeo-vegetation based on the plant macrofossil assemblages of fossil elephant skull Stegodon miensis containing layer founded from the middle part of the Tsubusagawa Formation distributed in Hirayama area, Ajimu Town, Usa City, Oita Prefecture reconstructed. There was a swamp zone where aquatic plants Potamogeton and Trapa are growing, and an open wetland spreading over the genera Carex, Cyperus, and Schoenoplectiella was grown. There also exist sunny and dry land area, where the genera Persicaria, Boehmeria, Lamium, and Angelica grow. Evergreen coniferous Cryptomeria, evergreen broad-leaved tree Cinnamomum, deciduous broad-leaved tree Litsea, Cornus, and Betulaceae have been grown around sedimentary areas.

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One of the authors, E. Kitabayashi, has excavated more than 16,000 cyprinid and cobitid fossils of pharyngeal bones and teeth. Among them, we have identified 198 fossils. They consist of cobitids and six subfamilies, Danioninae, Cultrinae, Acheilognathinae, Leuciscinae, Gobioninae, Cyprininaeo of cyprinids. Danionine fossils contained two genera Nipponocypris and Zacco, cultrine ones were identified as genus Culter, acheilognathine ones contained two genera Acheilognathus and Tanakia, leuciscine ones contained two genera Phoxinus and Tribolodon, gobionine ones contained five genera Coreius, Squalidus, Gnathopogon, Hemibarbus, and Pseudorasbora, and cyprinine ones were genus Carassius. This fossil assemblage detected with water sieve method was more diverse than previous ones (Nakajima and Kitabayashi, 2001).

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Several fossil fragments of anurans and snakes were collected from the Pliocene Tsubusagawa Formation of Ajimu-machi, Oita Prefecture, Northern Kyushu, Japan. Among anuran fossils, four ilia were identified as Ranidea gen. et sp. indet, and two vertebrae as Rhacophoridae gen. et sp. indet, Two snake vertebrae were identified as Colubridae gen. et sp. indet. The identification were based on morphological characters of extant anurans and snakes from Japan and previous report from other countries Specific identifications of the genera or species for these fossils have yet to be performed due to their poor preservation and insufficient comparisons between fossils and extant species occurring in East and South Eastern Asia. However, the results of this study indicates that the faunas of the Pliocene Tsubusagawa Formation includes two anuran families and one snake family, each of which includes two morphotypes.

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The crocodilian caudal vertebra is obtained from the Tsubusagawa Formation. Its size is same and its gross shape is resemblance to the 20th caudal vertebra of Crocodylus porosus, NTM R16036, total length for 513 cm. However, in comparison with NTM R16036, the direction of neural spine is more dorsocaudal, and the M. longissimus caudae and M. semispinalis caudae might be more developed in the Tsubusagawa specimen. With respects to the hitherto regarding crocodilian fauna, the Tsubusagawa specimen is identified to the crocodyline crocodylid, Toyotamaphimeia machikanensis. The caudal vertebrae are preserved only three, 1st to 3rd , in the type specimen of T. machikanense, and thus direct comparison with the Tsubusagawa specimen is impossible. According to Chinese classic dictionary by Lu (1174, the 12th Century), the tail of crocodile which may be identified as T. machikanensis is prehensile as if elephant trunk. The posterior condyle of this vertebra is well developed, and thus there is some possibility to support the evidence of prehensile tail. At the early Miocene, T. machikanensis might be derived from crocodyline crocodylid Maomingosuchus petrolicus. T. machikanensis is differ from Maomingosuchus petrolicus in having the largest 7th maxillary teeth and much larger body. The large canini-form development of 7th maxillary teeth is an adaptation for the relatively larger prey. As a larger prey, the presence of oxygasterine cyprinid Hypophthalmichthys (Silver Asian Carp), total length for 1 metre or more, is seemed to be involved with the secondary caniniform development of 7th maxillary teeth and gigantism of Toyotamaphimeia. The oldest fossil record of Toyotamaphimeia is the early Miocene and that of Hypophthalmichthys is too. Hypophthalmichthys is famous for its frequently jumping when disturbed. Although this jumping is hitherto claimed as enigmatic, it could be interpreted as anti-predatory behavior against Toyotamaphimeia.

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Fossil rodents, represented by nine isolated teeth, were obtained by wet sieving sediments from the Upper Pliocene Tsubusagawa Formation in Ajimu, Oita Prefecture. The rodent remains include Micromys sp., Arvicolinae gen. et sp. indet., and unidentified incisors. This is the first report of the Pliocene rodent fauna in Japan. Micromys sp. fossils differ from modern species in occlusal outline, number of roots, crown size, position of t4 and t6, size of t9, and presence or absence of t7 in the upper M1. Arvicorinae gen. et sp. indet. have such features as a hypsodont, very high sinuous line, thick cementum, and relatively wide crown in comparison to other arvicolids. Historically, Micromys and Arvicolinae have had a Palearctic distribution. This is consistent with previous plant fossil research that indicates a temperate-zone climate in the lower to middle part of the Tsubusagawa Formation. However, it is not consistent with the fossil tortoise and large mammals included in the Ajimu Fauna, which represent a subtropical or tropical This contradiction may suggest that the fossil assemblage was formed in an environment under the transition from tropical to temperate zone in the gradually cooling climate of the Late Pliocene.

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Large-sized stegodontid left and right fossil molars were discovered during dam construction for a storage reservoir in Hirayama, Ajimu-machi, Usa District, Oita Prefecture in February 1987. Subsequently, an excavation of the locality was undertaken in November 2004 by the Lake Biwa Museum, Ajimu-machi Board of Education, and local volunteers. The skull of the molars was discovered in the middle part of the Tsubusagawa Formation, but unfortunately, due to strong compression in the bed, its form could not be observed in detail. The presence of nine ridges and a talon in the fossil molars indicates that they are from Stegodon miensis.

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The results of investigations of 20 footprint fossil sites in Oita Prefecture, which were performed before December 2015, are described. Fourteen (70%) sites are from riverbeds and riverbanks, three (15%) are from cliffs of construction sites and quarries, two (10%) are from coastal cliffs, and one (5%) is from an excavation site. The type of animal footprints at eight sites of the 20 have not been identified, but at the other sites footprints of deer or possibly deer were identified at 16 sites, of elephants or possibly elephants at 13 sites, of rhinoceroses at three sites, of birds at two sites, and of crocodiles at one site. These trends are similar to the national situation. The tendency that elephant and deer footprints are often found together is also similar to other domestic cases.

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The Pliocene Tsubusagawa Formation is distributed in Ajimu Basin, Oita Prefecture, Japan. To collect small vertebrate fossils of the Tsubusagawa Formation, sedimentary rocks in mesh sacks were first left unaltered on the riverbank. Over the winter, the rocks weathered by freeze-thawing. The weathered rocks were then washed through gardening sieves and window screen nets. This resulted in a large collection of small vertebrate fossils, such as cyprinid pharyngeal teeth.

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