Fossils

Cover

2023 Volume 114 Pages Cover-1-
Published: September 30, 2023
Released on J-STAGE: October 09, 2023

DOIhttps://doi.org/10.14825/kaseki.114.0_Cover-1

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Adaptive significance of sexual dimorphism in an opportunistic ammonite species Yezoites puerculus (Jimbo)

Ai Suhama, Takashi Okamoto

2023 Volume 114 Pages 3-18
Published: September 30, 2023
Released on J-STAGE: October 09, 2023

DOIhttps://doi.org/10.14825/kaseki.114.0_3

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The ecological adaptive significance of sexual dimorphism in an opportunistic ammonite species, Yezoites puerculus (Jimbo), was investigated by means of computer simulations of population dynamics under unsteady food supply and low predatory pressure conditions. The results of our simulations showed that: (1) the peak in the adaptive landscape diagram is generally located where adult males are smaller than females. A similar result was obtained through additional simulations of anagenetic evolution based on a simple hereditary model (e.g. phyletic transition, thereafter). (2) The sexual dimorphism in this specie is controlled by the difference in size between males and females: a smaller adult size is suitable for males and females to avoid starvation, whereas a larger adult size increases breeding opportunities for females. (3) High starvation risk, resulting from high population density in a domestic environment, particularly facilitates sexual dimorphism in size. Sexual dimorphism was however not observed when the starvation risk was set to be low enough. These results suggest that other ammonite species which commonly co-exist with Yezoites but do not show explicit sexual dimorphism, likely have a pelagic mode of life during the latter half of their life.

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At the beginning of deposition of Kazusa Group in the western part of Kanto Plain (Kazusa Group in the western part of Kanto Plain deposited in western Tokyo)

Hajime Taru, Toshiyuki Kimura

2023 Volume 114 Pages 19-22
Published: September 30, 2023
Released on J-STAGE: October 09, 2023

DOIhttps://doi.org/10.14825/kaseki.114.0_19

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Stratigraphic distribution of the fossil diatom Lancineis rectilatus in the Lower Pleistocene Kazusa Group in the Tama Hills, central Japan, and its geological and palaeontological significance

Tomonori Naya, Takehiko Suzuki

2023 Volume 114 Pages 23-35
Published: September 30, 2023
Released on J-STAGE: October 09, 2023

DOIhttps://doi.org/10.14825/kaseki.114.0_23

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Lancineis rectilatus is a fossil diatom that appeared in the middle Early Pleistocene and disappeared in the early Middle Pleistocene in the shallow marine sediments of the central Kanto Plain, central Japan. In this study, diatom assemblages of the Oyamada to Iimuro Formations of the Lower Pleistocene Kazusa Group exposed along the Tama River and in Honmoku, Yokohama City, are investigated to clarify the stratigraphic relationship between the tephra beds and the occurrence of L. rectilatus in the Tama Hills. The Oyamada to Inagi Formation section is dominated by coastal shallow-marine diatoms. The Ozenji to Iimuro Formation of the Yokohama-Honmoku section is dominated by pelagic diatoms. L. rectilatus is identified from above the NK tephra layer of the Iimuro Formation. The NK tephra in the Tama Hills and the Kd12 Type tephra in the central Kanto Plain are possibly correlative.

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A new Pleistocene physeterid specimen from the Kazusa Group in the western part of Kanto Plain and a review of the fossil cetaceans from the Pleistocene of Japan

Toshiyuki Kimura, Hajime Taru, Cheng-Hsiu Tsai

2023 Volume 114 Pages 37-45
Published: September 30, 2023
Released on J-STAGE: October 09, 2023

DOIhttps://doi.org/10.14825/kaseki.114.0_37

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This study describes a previously undescribed fossil physeterid from the Pleistocene of Tokyo. This fossil physeterid was recovered from the Early Pleistocene, Fukushima Formation (ca. 1.60‒1.77 Ma), Kazusa Group in 1971 and has long been known as “Hino Kujira” without detailed descriptions. The specimen consists of fragments of the left maxilla, and we estimate the original size of the maxilla can reach at least 3 m long, suggesting an individual at least 11.5 m long. The maxilla is dorsoventrally thick (eroded, but at least 188 mm), and such a thick maxilla clearly differs from that of mysticetes but is similar to that of the modern sperm whale, Physeter macrocephalus. In addition, the average size of each pore on the spongy bone under the compact bone layer is prominent (10 to 15 mm in diameter), consistent with the macroporous structure of the maxilla of modern Physeter macrocephalus. We then provisionally make the taxonomic assignment of this Pleistocene specimen as Physeter sp. Besides, we review seventy-three documented fossil cetaceans from the Pleistocene of Japan, suggesting that the known paleodiversity of the Japanese Pleistocene cetaceans (12 genera) is much lower than that of modern composition (26 genera). This evident discrepancy likely results from the research effort that currently only produces an inadequately known fossil record of cetaceans.

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Proceedings

2023 Volume 114 Pages 52-89
Published: September 30, 2023
Released on J-STAGE: October 09, 2023

DOIhttps://doi.org/10.14825/kaseki.114.0_52

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Back Cover

2023 Volume 114 Pages Cover-2-
Published: September 30, 2023
Released on J-STAGE: October 09, 2023

DOIhttps://doi.org/10.14825/kaseki.114.0_Cover-2

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