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Article type: Cover
1994 Volume 56 Pages
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Article type: Appendix
1994 Volume 56 Pages
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Article type: Appendix
1994 Volume 56 Pages
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Article type: Index
1994 Volume 56 Pages
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Article type: Appendix
1994 Volume 56 Pages
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Article type: Appendix
1994 Volume 56 Pages
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Atsushi Matsuoka
Article type: Article
1994 Volume 56 Pages
1-8
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Discoidal spumellarian species belonging to the Family Spongodiscidae and Porodiscidae float in seawater extending an axoflagellum and numerous axopodia. Observation of their skeletons by scanning electron microscopy (SEM) reveals that they possess a pore larger than openings of the spongiose meshwork in the skeleton surface. The pore is the site through which the axoflagellum projects outward and is named axoflagellum pore. This paper describes some features of the axoflagellum pore of Dictyocoryne truncatum (Ehrenberg), Spongaster tetras tetras Ehrenberg, Spongodiscus biconcavus Haeckel, and Euchitonia elegans (Ehrenberg), showing SEM photos which were taken from different view points on a single specimen. These discoidal spumellarian species float in seawater generally keeping their skeletons horizontal. The position of axoflagellum pore gives useful information on the orientation in floating. This paper draws the orientation of floating D. truncatum and E. elegans. The position and shape of axoflagellum pore differs among species, suggesting that the features of axoflagellum pore can be useful in distinguishing species and in discussing their phylogenetic relationship.
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Masakazu Nara
Article type: Article
1994 Volume 56 Pages
9-20
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Macaronichnus segregatis, which is recognizable as white colored spots having 2 to 5mm in diameter, occurs in the Middle to Late Pleistocene foreshore deposits distributed in the Kanto plain, central Japan. It is a long, sinuous, unbranched burrow without apparent wall or burrow lining. This fossil has been regarded in Japan as trace of locomotion of a small isopod, Excirolana chiltoni. The microscopic observation reveals that the particles in the burrow filling are composed of sand grade colorless minerals such as quartz and feldspar, rich in poorly rounded particles as compared to the surrounding sediments. A deformed lamination, which may have been synchronously formed with the burrow, is discernible in the laminated sand of the burrow-bearing horizon. The tracemaker seems to penetrate the sediments with peristaltic movement, called intrusion. According to my field observation and experimental study E. chiltoni never make a trace like M. segregatis. The isopod moves through sediments by backfilling but not by intrusion. Taking all things into consideration, the tracemaker is not E. chiltoni, but a probable vermiform animal, and the present trace fossil is probably of fecal origin. The tracemaker may have selectively ingested particles in relation to their shape and specific gravity, though the exact mechanism of particle selection is yet unknown.
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Koichiro Masuda, Takashi Sasaki
Article type: Article
1994 Volume 56 Pages
21-25
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Fossil boring shell burrows collected from the Late Pliocene Dainenji Formation at Iwanuma City, Miyagi Prefecture are described and their geological significance is discussed.
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Hitoshi Furusawa, Naoki Kohno
Article type: Article
1994 Volume 56 Pages
26-32
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An isolated rib bone belonging to the Steller's sea-cow, Hydrodamalis gigas (Zimmermann, 1780), is described from the Middle Pleistocene Mandano Formation of the Boso Peninsula, central Japan. The specimen has a typical sirenian dense bone, and is designated as H. gigas on the basis of its morphological characters, size, geological age, and cool-condition of environment which is suggested by molluscan assemblage. The shape of cross section and curvature of the specimen indicate that this rib is of a fore-part (〜5th). This is the first record of the Order Sirenia from the Mandano Formation (including westward equivalent Nagahama Formation) of the upper Kazusa Group, and it represents the southernmost occurrence of the Steller's sea-cow in the world. The specimen also provides evidence that the Steller's sea-cow had distributed at least by the Middle Pleistocene in the adjacent sea around the Japanese Islands.
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Toshiyuki Yamaguchi, Kazushige Tanabe, Tomoki Kase
Article type: Article
1994 Volume 56 Pages
33-
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Takeshi Setoguchi
Article type: Article
1994 Volume 56 Pages
34-36
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Charles Dawin advocated the concept of 'living fossils' in his book, "The Origin of Species". According to his original idea, living fossils are to be found in isolated, and specialized environments where ordinary or normalized organisms are hardly to live. Recently, this concept has been strongly seconded in a popular book entitled "On Methuselah's Trail" written by P. D. Ward. Among the higher primates, Anthropoidea, owl monkeys, the genus Aotus living in South America, can be solely regarded as living fossils, because their Middle Miocene (14 Ma BP) relative, Aotus dindensis found in La Venta, Colombia is hardly distinguishable morphologically from the living form A. trivirgatus. They have been living in dense tropical rain forests along Amazon River. South America has long been an isolated island continent, but the habitat itself cannot be regarded as 'specialized'. Aotus became a living fossil under isolated but "normalized" environment in constrast to other living fossils as Darwin imagined.
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Toshiyuki Yamaguchi
Article type: Article
1994 Volume 56 Pages
37-41
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The most primitive living members of all the suborders in the Cirripedia; Scalpellomorpha, Verrucomorpha, Brachylepadomorpha and Balanomorpha, have been discovered at the deep-sea hydrothermal vents of the Western Pacific. The notable feature here, in contrast to their conservative but distinctly different shell morphologies, is the uniquely convergent adaptation of their setose feeding mechanism to vent-related food sources. Some relatives of them have fossil record in the middle Mesozoic age. It may be puzzling why four such antiquated morphologies have persisted in association with abyssal hydrothermal springs while their antecedents became extinct elsewhere. However, barnacles are noted for their adaptability to a wide range of habitats including rigorous environments such as estuaries and the highest intertidal zone.
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Kazushige Tanabe
Article type: Article
1994 Volume 56 Pages
42-46
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As compared with extant coleoids, species of Nautilus retain plesiomorphic conditions in various anatomical characters, such as the pin-hole type eyes without lenses, numerous tentacles without suckers, and two pairs of gills. Their reproductive biology is fairly different from that of extant coleoids by having much longer life span and embryonic period, and a smaller number of offspring. The embryo of early organogenetic stage possesses a cup-shaped, Monoplacophora-like primary shell (cicatrix), and its overall morphology is similar to the cephalopod prototype supposed by previous authors. Relatively large embryonic shells starting with the cicatrix have been widely recognized in fossil nautiloids of various geologic ages and are distinguished from the small embryonic shells of coeloids and ammonoids starting with a spherical initial chamber ("protoconch"). Oxygen isotopic analysis of aquarium-reared specimens strongly suggests that in the wild, Nautilus migrates into a deeper water soon after hatching in relatively shallower and warmer environments. This phenomenon can be interpreted as one of the adaptive stratetgies to escape increasing predatory pressure in shallower environments since the Cenozoic.
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Koichi Sekiguchi
Article type: Article
1994 Volume 56 Pages
47-50
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1) Adaptation : The horseshoe crabs adapt highly to the environment during their life ecologically and physiologically as well as morphologically. For example, the spawning behavior of horseshoe crabs shows a semidiurnal rhythm. As they lay eggs in the sand near the high tide level at high tide twice a day, egg clusters are submerged for a relatively short time and exposed to the air most of the day. This environment is favorable to protect the eggs from enemies, but it is not always adequate for the eggs because of dryness and low or high salinity. As a countermeasure, the horseshoe crab embryo forms a thick and strong membrane, inner egg membrane, under the chorion. The embryo holds seawater in the perivitelline space surrounded by the inner egg membrane and the egg with this membrane is gradually swollen by influx of seawater during embryonic development. The developing embryo is protected in this expanding aquatic space for about 30 days until hatching. 2) Speciation : The American horseshoe crab shows a distant relation to the Asian species at the higher level than at the genus level, judging from the external and internal morphology, artificial crossing experiments among four extant species, biochemical studies of high molecular protein and so on. From the morphological point view, it is reasonable to divide the Asian species into two genera, Tachypleus and Carcinoscorpius. However, from our results of artificial crossing and analyses of proteins, T. tridentatus has rather closer relation to C. rotundicauda than to T. gigas. Thus, we consider that the three Asian species of horseshoe crabs belong to the same genus, Tachypleus.
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Tatsuaki Kimura
Article type: Article
1994 Volume 56 Pages
51-55
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This brief note reviews the plants barely survived from the thorough extinction within each higher plant taxon during the geological time, on the basis of our recent knowledge of the transition of terrestrial vascular plants. Many evolutionally interesting higher plant taxa, such as pteridosperms, Bennettitales, Pentoxylales, Czekanowskiales, Cordaitales and Cheirolepidiaceae became extinct by the end of the Mesozoic without leaving any descendants. Some of the survivors can be regarded as 'living plant fossils', such as living matoniaceous, dipteridaceous and dicksoniaceous ferns, Ginkgo biloba, and monotypic taxodiaceous conifers with some hesitation and doubt.
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Keiichi Takahashi
Article type: Article
1994 Volume 56 Pages
56-
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Kiyotaka Chinzei
Article type: Article
1994 Volume 56 Pages
57-
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Kiyotaka Chinzei
Article type: Article
1994 Volume 56 Pages
58-
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Teiichi Kobayashi
Article type: Article
1994 Volume 56 Pages
59-60
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Michiko Yajima
Article type: Article
1994 Volume 56 Pages
60-62
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Michiko Yajima
Article type: Article
1994 Volume 56 Pages
62-63
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Yokichi Takayanagi
Article type: Article
1994 Volume 56 Pages
63-65
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Article type: Appendix
1994 Volume 56 Pages
66-72
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Article type: Bibliography
1994 Volume 56 Pages
73-75
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Article type: Appendix
1994 Volume 56 Pages
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Article type: Appendix
1994 Volume 56 Pages
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Article type: Appendix
1994 Volume 56 Pages
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Article type: Appendix
1994 Volume 56 Pages
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Article type: Cover
1994 Volume 56 Pages
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1994 Volume 56 Pages
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