Gengo Tanaka

Chelicerate neural ground pattern in a Cambrian great appendage arthropod

Preservation of neural tissue in early Cambrian arthropods has recently been demonstrated, to a degree that segmental structures of the head can be associated with individual brain neuromeres. This association provides novel data for addressing long-standing controversies about the segmental identities of specialized head appendages in fossil taxa. Here we document neuroanatomy in the head and trunk of a ‘great appendage’ arthropod, Alalcomenaeus sp., from the Chengjiang biota, southwest China, providing the most complete neuroanatomical profile known from a Cambrian animal. Micro-computed tomography reveals a configuration of one optic neuropil separate from a protocerebrum contiguous with four head ganglia, succeeded by eight contiguous ganglia in an eleven-segment trunk. Arrangements of optic neuropils, the brain and ganglia correspond most closely to the nervous system of Chelicerata of all extant arthropods, supporting the assignment of ‘great appendage’ arthropods to the chelicerate total group. The position of the deutocerebral neuromere aligns with the insertion of the great appendage, indicating its deutocerebral innervation and corroborating a homology between the ‘great appendage’ and chelicera indicated by morphological similarities. Alalcomenaeus and Fuxianhuia protensa demonstrate that the two main configurations of the brain observed in modern arthropods, those of Chelicerata and Mandibulata, respectively, had evolved by the early Cambrian.

Exceptionally preserved 450-million-year-old ordovician ostracods with brood care.

Ostracod crustaceans are the most abundant fossil arthropods and are characterized by a long stratigraphic range. However, their soft parts are very rarely preserved, and the presence of ostracods in rocks older than the Silurian period [1-5] was hitherto based on the occurrence of their supposed shells. Pyritized ostracods that preserve limbs and in situ embryos, including an egg within an ovary and possible hatched individuals, are here described from rocks of the Upper Ordovician Katian Stage Lorraine Group of New York State, including examples from the famous Beechers Trilobite Bed [6, 7]. This discovery extends our knowledge of the paleobiology of ostracods by some 25 million years and provides the first unequivocal demonstration of ostracods in the Ordovician period, including the oldest known myodocope, Luprisca incuba gen. et sp. nov. It also provides conclusive evidence of a developmental brood-care strategy conserved within Ostracoda for at least 450 million years.

Effects of mass sedimentation events after the 2011 off the Pacific coast of Tohoku Earthquake on benthic prokaryotes and meiofauna inhabiting the upper bathyal sediments

We examined the effects of mass sedimentation events caused by the 2011xa0off the Pacific coast of Tohoku Earthquake on abundances and vertical distributions of prokaryotes and metazoan meiofauna in sediments, using sediment cores collected from eight bathyal stations off Tohoku 1xa0year after the M9.0 earthquake. Event deposits 1–7xa0cm thick were observed at the topmost part of the sediment cores at all sampling stations. At some stations, prokaryotic cell abundances were lower in the surface event-deposit layers compared to those in deeper sediments. These variations were explained by environmental parameters such as a dimensionless sorting factor and mean grain size, suggesting that turbidite sedimentation affected prokaryotic cell abundances. Nematodes had anomalously higher subsurface abundances at the stations where subsurface peak prokaryotic cell numbers were observed, whereas copepods always showed peak densities in the sediment surface layer. Although there are no available data for prokaryotic cell abundances and meiofaunal densities before the earthquake from the same sites, it is likely that the subsurface peaks in prokaryotic cell numbers and nematode densities resulted from the sedimentation events. The effects of sedimentation events on the organisms were observed 1xa0year after the earthquake, indicating that episodic sedimentation events on scales of several centimeters have a large effect on small organisms inhabiting sediments.

Mineralized rods and cones suggest colour vision in a 300 Myr-old fossil fish

Vision, which consists of an optical system, receptors and image-processing capacity, has existed for at least 520 Myr. Except for the optical system, as in the calcified lenses of trilobite and ostracod arthropods, other parts of the visual system are not usually preserved in the fossil record, because the soft tissue of the eye and the brain decay rapidly after death, such as within 64 days and 11 days, respectively. The Upper Carboniferous Hamilton Formation (300 Myr) in Kansas, USA, yields exceptionally well-preserved animal fossils in an estuarine depositional setting. Here we show that the original colour, shape and putative presence of eumelanin have been preserved in the acanthodii fish Acanthodes bridgei. We also report on the tissues of its eye, which provides the first record of mineralized rods and cones in a fossil and indicates that this 300 Myr-old fish likely possessed colour vision.

Crittendenia (Bivalvia) from the Lower Triassic (Olenekian) Bac Thuy Formation, an Chau Basin, Northern Vietnam

Abstract. n Well preserved molluscan fossils of Olenekian age (Early Triassic) were obtained from the upper part of the Bac Thuy Formation in Lang Son City, northern Vietnam. We report here an ammonoid, Xenoceltites variocostatus Brayard and Bucher, and describe two bivalve species, Crittendenia australasiatica (Krumbeck) and Crittendenia langsonensis sp. nov. Xenoceltites variocostatus is a characteristic species of the uppermost Smithian Anasibirites ammonoid zone and the earliest Spathian Tirolites ammonoid zone. Crittendenia australasiatica and C. langsonensis may thus be significant diagnostic species suggesting a middle Olenekian age. In addition, these occurrences demonstrate a probable faunal exchange between the eastern Tethys and eastern Panthalassa during the Olenekian, because many species of Crittendenia have been reported from Asia and from Panthalassic basins in the United States.

Redescription of Two Krithid Species (Crustacea, Ostracoda) from the Sea of Japan, with a Comment on the Taxonomic Characters of Krithidae

Abstract. n The marine podocopid ostracod Krithe antisawanensis is redescribed on the basis of its appendages and the carapaces of adult male and female specimens. The krithid species Parakrithella pseudadonta, which has only been previously described on the basis of the carapace morphology and male appendages excluding the copulatory organ, is redescribed here with its female appendages and male copulatory organ. Both krithid genera (Krithe and Parakrithella) have three articulated podomeres in the fifth and sixth limbs and an axe-head anterior vestibulum. Such characters are useful as diagnostic features for the family.

Miocene Ostracods from the Itahana Formation in the Tomioka District, Gunma Prefecture, Central Japan: Paleoenvironmental Significance and Systematics

Abstract. n Sixty-two podocopid ostracod species belonging to 37 genera were identified during the analysis of 57 samples from the Miocene Itahana Formation (ca. 11.0 to 10.5 Ma) in the Tomioka district, Gunma Prefecture, central Japan. Two assemblages were recognized in the lower member of the Itahana Formation: Assemblage I is characterized by Aurila joushuensis, Loxoconcha nozokiensis, Paracytheridea neolongicaudata, Callistocythere spp., and Cythere omotenipponica; Assemblage II is characterized by Krithe sp., Acanthocythereis sp., Ambtonia takayasui, Ambtonia tamayuensis, and Argilloecia sp. Assemblage II indicates a depositional environment of muddy bottoms in the upper bathyal open sea, while for the species of Assemblage I a habitat of sandy bottoms in warm shallow open seas is indicated, suggesting that the depositional environment of the lowermost Itahana Formation was bathyal and that the ostracods of Assemblage I were transported to that environment by turbidity currents. Fifteen new species are described in this study.

Middle Permian Ostracods from the Akasaka Limestone, Gifu Prefecture, Central Japan

Abstract. n Samples of Middle Permian organic-rich black unconsolidated mud were collected from a fissure of black limestone in Kinshozan Mountain, Akasaka City, Gifu Prefecture, central Japan. Nine new species, one new genus and one new family are described herein: Ikeyaparchitidae Tanaka fam. nov., Gifuaparchites Tanaka and Maeda gen. nov., Aurikirkbya kinshozanensis Tanaka sp. nov., Gifuaparchites takagii Tanaka and Maeda sp. nov., Cavellina hashintotoi Tanaka and Maeda sp. nov., Bairdia nishiwakii Tanaka and Nishimura sp. nov., Bairdia akasakaensis Tanaka sp. nov., Bairdia oogakiensis Tanaka sp. nov., Bairdiacypris? hayasakai Tanaka sp. nov., Acratia? okumurai Tanaka sp. nov., and Acratia? hamadai Tanaka, Ono and Maeda sp. nov. This is the first report from Japan of a typical Panthalassa ostracod assemblage during the Middle Permian, and of which is characterized by typically endemic species.

A new bivalved arthropod from the Devonian of Japan

Abstract. n Other than trilobites and ostracods only a few species of arthropods are known from the Palaeozoic of Japan. Here we describe a new bivalved arthropod from the Lower Devonian of central Japan, Norikeya onoi gen. et sp. nov. It shows morphological similarities to both palaeocopine ostracods and phyllocarid malacostracans, but its systematic position remains uncertain. N. onoi lived in warm temperate shallow marine waters. Leperditicopid arthropods that occur in the same formation in central Japan are associated with the Early Devonian leperditicopid biogeographic province of South China, which concurs with the probable palaeogeographic position of central Japan based on other types of evidence.

Evolution of antennules of cytheroidean ostracods (Crustacea).

Phylogenetic analysis and fossil records indicate that antennules with whip-like setae are the most plesiomorphic state in cytheroidean ostracods and that antennulae with claw-like setae are derived from antennulae with whip-like setae. Character distributions on the 18S rDNA molecular phylogenetic tree suggest that two phenotypic features of cytheroidean antennules (W/L ratio and claw-like/whip-like setae) have morphological plasticity. These features have evolved as an ethological adaptation rather than due to phylogenetic constraints such as the evolution of shell outline in cytheroideans. However, the species of the Leptocytheridae-Trachyleberididae clade generally have stout, robust antennules, indicating a phylogenetic constraint. The character state of setae (claw/whip) in cytheroidean ostracods is reflected more through their ethology than the W/L ratio of the antennules. On the basis of the present analysis and the fossil record, diversification in the morphology of the antennules seems to have occurred during the early Mesozoic.