[in Japanese]

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A well-preserved intact skeleton of paleoparadoxiid was excavated from the river bed of the Toki River in Shimogiri, Kamado-cho, Mizunami City, Gifu, central Japan in June 10th, 2022. We call this specimen “Paleoparadoxiid Mizunami-Kamado specimen” (MFM18130). It is from the Lower Miocene Shukunohora Formation of Mizunami Group (ca. 16.5 Ma). The skeleton is nearly complete, including the skull, mandible, vertebrae, ribs, hindlimbs, and all sternal bones. The paper records and figures the excavation and preparation process (June, 2022 to August, 2023) of “Paleoparadoxiid Mizunami-Kamado specimen”.

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Paleoparadoxiid fossils were discovered in the Shukunohora Formation of the Mizunami Group exposed in the Toki River bed in Shimogiri-ku, Kamado-cho, Mizunami City, in 2022. In order to clarify the age of the Shukunohora Formation in this locality, we determined strontium isotopic ages using shell fossil samples that were cooccurred with paleoparadoxiid fossils. The strontium isotopic ages suggest that the depositional age of the paleoparadoxiid-bearing horizon is ca. 16.5 Ma.

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This paper presents the results of magnetic measurements on the sandstone block containing the “Paleoparadoxiid Mizunami-Kamado specimen” from the Shukunohora Formation of the Miocene Mizunami Group in central Japan. Stepwise demagnetization suggests the presence of both magnetite and greigite, with the latter being probably authigenic minerals formed during early diagenesis. The anisotropy of magnetic susceptibility (AMS) analysis shows a highly variable magnetic fabric with no preferred orientation, suggesting the absence of a consistent grain alignment in the sandstones. This likely reflects disturbances by storms or bioturbation. The sandstone block-mean remanent magnetization direction may be attributed to chemical remanent magnetization (CRM) carried by greigite and, therefore, may not represent the paleofield direction at the time of deposition. However, assuming CRM acquisition during early diagenesis shortly after deposition, the deposition was during a reverse polarity period around 16.5 Ma, potentially either Chron C5Cr (17.154–16.637 Ma), C5Cn.2r (16.532–16.434 Ma), or C5Cn.1r (16.351–16.261 Ma). The deviation of the block-mean remanent magnetization direction from the geocentric axial dipole field suggests a clockwise tectonic rotation (28 ± 20°) for the crust beneath the study area relative to the Asian continent. This rotation coincides with the clockwise rotation of Southwest Japan during the major opening of the Japan Sea, suggesting the “Paleoparadoxiid Mizunami-Kamado specimen” lived amidst this tectonic event.

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Paleoparadoxiid Mizunami-Kamado specimen is a well-preserved whole skeleton of paleoparadoxiid that was excavated from Kamado-cho, Mizunami City Gifu prefecture in June 2022. Each bone is preserved very well, and the bone and other fossils have plenty of information about the situation before they became fossilized. Therefore, this study records the positions of each body part of this specimen and other fossils preserved together with the specimen to discuss the taphonomy and the growth of bones of the specimen. A total of 119 bones were collected, including the skull, mandible, vertebrae, ribs, hindlimbs, and sternal bones of the specimen, but it lacks the forelimbs. The growth of these bones was almost stopped, which suggests the specimen is a mature individual. After comparison with other specimens of paleoparadoxiid, the specimen in this study turned out to be one of the most mature specimens of Paleoparadoxiidae. Fossil remains of molluscs, barnacles, decapods, sharks and plants were collected from around the specimen. Some barnacles and molluscs attached to some bones. The information of this specimen and other fossils suggest that the carcass drifted on the sea before going down to the sea bottom and took much time to deposit.

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A nearly complete paleoparadoxiid skeleton, i.e., MFM 18130, from the Lower to Middle Miocene Shukunohora Formation, Kamado Town, Mizunami City, Gifu Prefecture, Japan, is identified based on the cranial, mandibular and postcranial characters. MFM 18130 has such unique characters as presence of the postzygomatic foramen on the skull, collumnar cusps on the molars, paired sterna, strongly twisted tibia and astragalus, and therefore, belongs in the family Paleoparadoxiidae of the Desmostylia. In addition, MFM 18130 has some derived characters that are possessed in the genera Archaeoparadoxia and Neoparadoxia such as dorsally high suprraoccipital process and shortened zygomatic process. On the contrally, the conditions of these characters are opposite in the monospecific Paleoparadoxia. In addition, the presence of the hypoconulid on M/3 is different from that of the monospecific Archaeoparadoxia, and the body size is much smaller than any species of Neoparadoxia. Accordingly, MFM 18130 is provisionally identified as Paleoparadoxiidae genus and specis undetermined.

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A paleoparadoxiid specimen from Kamado, Mizunami exhibits numerous bone fragment-like fossils ranging in diameter from a few millimeters to about 3 centimeters. These fossils are associated with various morphological features, such as rod-like arrangements depending on the preservation state, surface features rich in irregularities and nutrient foramina, and histological characteristics including a peripheral compact layer composed of calcified cartilage and a spongy interior composed of bone tissue. Based on these features, we preliminarily identify the segmented structure as mineralized tissue developed within the costal cartilage. Most of the isolated bone fragment-like fossils probably have belonged to mineralized costal cartilage judging from the morphological and histological similarity to the elements of the rod-like aggregate. The structure and mineralization process of rib cartilage in desmostylians, including the family Paleoparadoxiidae, remain unclear, and this collection of rib cartilage specimens is expected to serve as a novel source of information for clarifying the physiology, ecology, and phylogeny of desmostylians. Future research should focus on further elucidating the details of rib cartilage fossils in the Mizunami-Kamado specimen and exploring the relationships between the pattern of rib cartilage mineralization and ecology or physiology by comprehensively investigating the morphological features of rib cartilage in various mammalian lineages. This approach is anticipated to provide new biological insights into desmostylians.

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Formation processes of calcite concretions co-occurred with the “Paleoparadoxiid Mizunami-Kamado specimen” and calcite fillings formed in bones were studied and discussed based on geological observations and geochemical analyses. The un-deformed fecal pellet-like structures in concretion show that concretions formed before the compaction of sediments. The low δ¹³C values (approximately −10‰) of concretions and calcite fillings in bones and similar δ¹⁸O values to each other suggest that they were formed from organic carbon at almost the same time during early diagenesis. The calcite fillings in bones protect the skeleton from deformation due to the compaction of sediments.

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Molluscan fossils including Crenomytilus grayanus (Dunker) and Chlamys itoigawae Masuda were collected from the locality of the “Paleoparadoxiid Mizunami-Kamado specimen” in Kamado-cho, Mizunami City, Gifu Prefecture, central Japan. The specimen of Mytilus coruscus Gould reported by Itoigawa et al. (1974, 1981, 1982) and Taguchi (2002) are reexamined, and these specimens are referable to C. grayanus by having fine radial striae on the shell exterior, fine crenulations on the interior shell margin and a non-vacuolated psedonymph.

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Many specimens of a small echinoid (sand dollar) species were collected from the excavation site of paleoparadoxiid in Kamado, Mizunami, central Japan. These are safely assigned to Kewia minoensis because of their morphology and their smallness. Occurrence of these echinoid enables to reconstruct the paleoenvironment as a purely marine environment without influence of brackish water from nearby.

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Total 124 elasmobranch teeth were co-occurred with “Paleoparadoxiid Mizunami-Kamado specimen”. Ninety-seven specimens were identified with Galeocerdo aduncus, and other teeth were also identified with genera Carcharhinus and Squatina, except for one batoid tooth. Many bite marks were recognized on the left femur, middle phalanx of left hindlimb and left ¹⁴th to ¹⁵th ribs. The present paleoparadoxiid seemed to be preyed upon those sharks.

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Twenty-seven species of fossil Ostracoda, 17 species of planktonic Foraminifera, and 18 species of benthic Foraminifera were found in sandstones around the paleoparadoxiid-bearing horizon of the Miocene Shukunohora Formation, Mizunami Group, central Japan. Based on the results of the microfossil analysis, the depositional age was assigned to the lower part of planktonic foraminiferal zone N8 (~17.0–16.0 Ma) during the Mid-Miocene Climatic Optimum. The paleoenvironment is thought to have been sandy sea bottoms in upper sublittoral zone (~20–50 m) near seaweed or seagrass beds under the influence of strong warm currents.

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The pollen assemblage from the sediment sample containing the whole skeleton of a paleoparadoxiid (“Mizunami-Kamado specimen”), is mainly composed of Pinus, Dacrydium, Quercus (evergreen type) and others. This assemblage includes warm elements such as Keteleeria, Pseudolarix, Quercus (evergreen type), Dacrydium, Liquidambar, and Reevesia, many of them are now extinct in Japan. The assemblage is comparable with the pollen assemblages previously documented from the Shukunohora Formation, Mizunami Group, Gifu Prefecture, Japan. These assemblages correspond to the NP-2B pollen zone, indicative of the Middle Miocene climatic optimum. The paleovegetation is inferred to be evergreen conifer forests characterized by Pinus and Dacrydium with evergreen broad-leaved trees such as evergreen Quercus and deciduous broad-leaved trees. The paleoclimatic conditions are estimated to be about 21°C for the annual mean temperature with an annual precipitation of 1,500–1,800 mm. Similar pollen assemblages found in sediments containing Paleoparadoxia in Niigata and Okayama prefectures also represent the NP-2B pollen zone and include mangrove plant pollen fossils.

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Laser scanning and X-ray CT scanning were performed on “Paleoparadoxiid Mizunami-Kamado specimen” to determine the condition of the vertebrae, right hindlimb, and skull buried in rock, which are extremely difficult to extract due to their state of preservation, to observe the interior of the skull, and to create 3D digital reconstructions of the skeleton. In addition, not only bone elements but also changes in the specimen due to the preparation process were also digitized. The digitization of the bone elements of the specimen made it possible to easily create whole-body skeletal reconstruction models, which will be a precious resource for research and educational purposes.

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A paleontological restoration of the “Paleoparadoxiid Mizunami-Kamado specimen” which was excavated from the Shukunohora Formation of the Mizunami Group in Kamado-cho, Mizunami City, Gifu, Japan, was created as a 2D illustration. The form and ecology of when the specimen was preyed by sharks, were reconstructed based on skeleton of MFM18130, its burial position. The illustration was revised by the Kitagawa and Kohno’s scientific comments before its completion.

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