The ostracod muscle scars have been used as important characters to define the evolutionary lineages of ostracods from the Palaeozoic to the Recent, though the formation and calcification of their muscle attachment have never been investigated. The present paper describes, for the first time, the cuticle formation of muscle attachment in a podocopid ostracod, as an example of a calcified arthropod. The formation of the muscle attachment structure in a podocopid ostracod progresses through the nearly same pathway as in other arthropods. The muscle connects to the cuticle by means of a specialised epidermal cell, the tendinal cell. Strong desmosomes adhere the muscle cell to the tendinal cell, while the tendinal cell is connected to the cuticle by hemidesmosomes with intracuticular fibres (tonofibrillae). The deposition of the new pre-exuvial cuticle takes place around the extended intracuticular fibres and these fibres maintain the connection between the tendinal cell and the old cuticle. At postecdysis the cuticle of the carapace begins its calcification, and immediately and rapidly increases its thickness, but the calcification of the muscle scars evidently progresses later than other parts of the procuticle, due to the fact that part of the tendinal cell is fully occupied with force-transmitting structures, like microtubules. Only the parts not engaged in force-transmitting action have enough space in the cell to store the granules to start the calcification. Furthermore, the less organic matrix in the procuticle of the muscle attachment area also contributes to the delay of calcification. The results of this paper provide information on cuticle formation in the calcified arthropods, which can be applied to fossil taxa, and show the relationships between the calcified and organic matrix during calcification. In addition, this study suggests that arthropod muscle scars, including in the fossil state, are useful characters when discussing the construction of muscular systems at the cellular level, and available as stable criteria through ontogeny for the comparative morphology of the exoskeleton.

Lower Permian (Asselian to Sakmarian) radiolarians and conodonts are identified from the bedded chert of the Chanthaburi area along the Sra Kaeo Suture Zone in eastern Thailand. The radiolarian and conodont-bearing bedded chert including radiolarian species such as Parafollicucullus bulbosus, P. lomentarius, P. u-formus, Pseudoalbailella scalprata, and others is exposed as large-size exotic blocks within the Thung Kabin mélange. This radiolarian fauna further includes Triplanospongos musashiensis, Triaenosphaera minuta, Latentifistula texana, Latentibifistula asperspongiosa and others, which have previously been reported from the Middle to Upper Permian. Except for these Middle to Upper Permian species, this radiolarian fauna is quite similar to those from Japan, Oregon, South China, West Texas, Cis-Urals and Central and Northeastern Thailand. Asselian to Sakmarian conodont faunas represented by Streptognathodus constrictus which have been reported from the Southern Urals and South China, and several sponge spicules are also contained in this bedded chert. Occurrence of these Lower Permian conodonts is the first record from the Sra Kaeo Suture Zone. This radiolarian- and conodont-bearing bedded chert was probably deposited in the Palaeotethys /or Palaeotethyan back-arc basin which is thought to have been a pelagic environment at low latitudes of the Southern Hemisphere.

Nonmarine stromatolites sporadically occur in the Lower Cretaceous Kanmon Group in northern Kyushu, Japan. We examined the mode of occurrence and morphological variations of the stromatolites that were found in the Wakino area, the stratotype of the Wakino Subgroup of the Kanmon Group, and analyzed their mineralogical compositions. These stromatolites occur at 7 horizons all from mudstone-dominant facies of the Barremian Lower Wakamiya Formation. The associated tepee structure, mud cracks, and gypsum pseudomorphs suggest that the stromatolites of the Lower Wakamiya Formation were deposited in extremely shallow-water environments with intermittent subaerial exposure. The studied stromatolites comprise 3 distinct morphotypes: flat, columnar, and nodular. XRD analysis confirms that all 3 types have the same mineralogical composition, i.e., calcite, quartz, and clay minerals. They are composed of alternating clastic layers and calcareous layers, or alternating siliciclastic carbonate layers and organic carbon-rich carbonate layers on a submillimeter scale. Based on their morphological characteristics, a sequential transition is suggested from the flat type to columnar type, then to nodular type. The columnar type has the greatest morphological variety, and shows a characteristic pattern of branching in which the bifurcation always initiates from an episodically intervened thick layer of siliciclastics. This suggests that the branching of stromatolite columns is likely triggered by blocking the upward growth of bacterial mats with intermittent supply of a thick sediment cover.

Variation in the stable isotopes of land-snail shells potentially provides useful information for reconstructing the terrestrial paleoenvironment. In this study, we investigated the relationship between vegetation and variation in the shell carbon (¹³C/¹²C) isotope ratios in the land snail Mandarina chichijimana on Chichijima of the Ogasawara Islands. In modern samples, the mean δ¹³C value of the adult shell (range -13.8∼-9.6) was higher at sites that contain a greater proportion of C₄ plants, especially near the coast (regression equation, mean δ¹³C=6.594×[C₄ plant proportion]-12.43). The shell δ¹³C of the living snails was also significantly positively correlated with the δ¹³C of the body tissues. As no differences were found in the δ¹³C values of shells collected in carbonate-rich areas and volcanic rock settings, we conclude that δ¹³C in M. chichijimana is almost exclusively influenced by the plants that they consume. Also, in fossil shells from limestone outcrops, the mean δ¹³C value (-11.6∼-11.4) was significantly lower than in modern shells from the same limestone outcrops (-11.0∼-9.6). This is therefore preliminary evidence that C₄ vegetation declined in line with a decrease in sea level around the time of the Last Glacial Maxima. Together, the findings may form a basis for the future use of land snail shells to estimate the paleoenvironment of the Pacific Islands in this region.

Patchy organic sheets often exist in the inner shell layers of endolithic mytilids. They are commonly distributed beneath damaged portions of the shell both in mechanical and chemical borers (Adula falcatoides, Botula fusca and many species of Leiosolenus). The shell damage results from the abrasion and/or corrosion during boring activity. Shell dissolution by calcium-binding mucus or by cold seawater undersaturated with calcium ions occurs only in regions where the periostracum has worn out. The shell microstructure of the repaired portions of 14 species of boring mytilids suggests that internal organic sheets protect the shell from dissolution and three shell repair patterns using the organic sheets evolved independently in at least two clades of boring mytilids.

A platanistoid fossil was recovered from the upper Lower Miocene Akeyo Formation, Mizunami Group, Gifu Prefecture, Japan. It consists of a right scapula, two cervical and three thoracic vertebrae, and an isolated tooth. The following key apomorphies of Platanistoidea are found in the scapula of the specimen: loss of or greatly reduced coracoids process, acromion located on the anterior edge of the scapula, and disappearance of the supraspinatus fossa on the lateral side of the scapula. This is the fourth platanistoid to be reported from Japan.

A “Miocene” molluscan fauna of the Yunokogawa Formation is reexamined taxonomically and its paleobiogeographic implications are discussed. The fauna is composed of three species of Gastropoda and 18 species of Bivalvia. The new data indicate that the Yunokogawa molluscan fauna is not of Miocene age, but is referable to the Omma-Manganji Fauna of Pliocene—Early Pleistocene age. No warm-water species are included in the Yunokogawa molluscan fauna. This is probably because the fauna is older than ca. 3.5 Ma or because the influence of warm-water (the Paleo-Tsushima Current) in the Paleo-Tsugaru Strait was much weaker than today. The following taxa are described taxonomically: Fusitriton aff. oregonensis (Redfield), Anadara (Anadara) amicula amicula (Yokoyama), Chlamys (Leochlamys) foeda (Yokoyama), Thracia (Homoeodesma) kakumana (Yokoyama), Protocallithaca Nomura, 1937, and Protothaca (Protocallithaca) adamsii (Reeve).

Cheilostomes, the dominant group among living bryozoans, appeared in the Late Jurassic and began to extensively diversify late in the Early Cretaceous. Knowledge of the early diversification phase comes mostly from fossils from Europe and the USA, but recently Early to Late Cretaceous cheilostomes have proven to be much more common in Japan than previously suspected. Modes of preservation in Japanese material include 1) the exposed colony surface, which is rare, 2) zooidal vertical walls embedded in matrix, and 3) colony molds, which are common. Here we illustrate and describe in detail a method adapted from dental technology to produce from colony molds fine-quality vinyl polysiloxane (VPS) silicone casts suitable for scanning electron microscopy (SEM). Improvements over previous methods include the use of paraffin dams, direct use of low-viscosity VPS medium for casts, and pressure curing. Our method will be useful for any preservational mode leaving molds or imprints organisms, including bioimmuration.

The inoceramid bivalve Mytiloides ipuanus (Wellman, 1959) from the Lower Albian is reported for the first time in the Northwest Pacific and described in detail. This is the oldest record of a Cretaceous inoceramid in this region. It is possible that the present study documents the origin of the extreme diversification and abundance of inoceramid bivalves in the Late Cretaceous of the Northwest Pacific. The beginning of inoceramid diversification in the Late Cretaceous is seen during the latest Early Cretaceous, which suggests that M. ipuanus is an important species for inferring the causes of the Late Cretaceous flourishing of inoceramids.