ヒドロダマリス亜科Hydrodamalinae(海牛目:ジュゴン科)における下顎と脊柱の運動機能に関する進化的変化  [in Japanese] Evolutional change of the masticating and spinal movement in Hydrodamalinae  [in Japanese]

Access this Article

Search this Article

Author(s)

Abstract

系統的にDusisiren jordaniとHydrodamalis gigasの間にあるヤマガタダイカイギュウD. dewana,アイヅタカサトカイギュウD. takasatensis,タキカワカイギュウH. spissa等をふくむヒドロダマリス亜科の下顎(咀嚼),頭の背屈・側屈,脊柱運動機能の変化について,骨格の観察と現生の海牛(ジュゴンとマナティー)の筋学的な知見をもとに考察を試み,次の結果を得た.1)後期中新世のD. takasatensisとD. dewanaでは歯が小さくなった後,翼状突起腹側部が広がり,下顎骨の吻部咀嚼面が狭い長方形となることから下顎骨を動かし吻部で餌をはさむ力が増したと考えられる.歯を欠如した鮮新世のH. spissaではそれがさらに強くなったと推測される.また側頭稜の発達から鮮新世のH. cuestae, H. spissaでは側頭筋の力が現世のH. gigas以上に強いと考えられる.2)鱗状骨外側部,上後頭骨・外後頭骨背外側縁の形状比較から,後期中新世のD. jordaniのように板状筋が発達する傾向はD. takasatensis, D. dewanaにも認められる.しかし,H. gigasのように板状筋より頭半棘筋の発達が目立つのはすでにH. spissaから認められる.3) D. dewanaとD. takasatensisはほぼ同じ大きさの頭蓋をもつが,鱗状骨外側部,上後頭骨・外後頭骨背外側縁の形状の差異から,D. takasatensisが頭の背屈・側屈がより強く,頭の可動範囲も広いと考えられる.4)ヒドロダマリス亜科は中位胸椎の棘突起が低いことから,最長筋などの軸上筋の発達が弱く,脊柱を背屈させる力もジュゴンの場合より弱い,特にH. spissaで顕著であると考えられる.ヒドロダマリス亜科は水流エネルギーのあまり強くない環境に穏やかな脊柱の動きで生息していたと推測できる.

Evolutional change in function of the jaw, mobility of the neck and the vertebral column in Hydrodamalinae is examined on the basis of the fossil skeleton, the myological and osteological knowledge of living sirenians. In Dusisiren dewana, Dusisiren takasatensis and Hydrodamalis spissa, the holding function of their rostra probably had been developed like Hydrodamalis gigas, because they had a mandibular masticating surface which is narrow and rectangular in outline, ventrally broadened pterygoid process that attached the pterygoid muscles. In D. dewana and D. takasatensis, the splenius muscles may have been developed like Dusisiren jordani, but in H. spissa, the semispinalis capitis muscle may have been more developed than the splenius muscle like H. gigas. D. takasatensis has some other features, longer sigmoid ridge of squamosal, larger occipital condyle and occipitosphenoidal eminence, and thicker lateral flange of exoccipital than those of D. dewana. Therefore, there is a possibility that in D. takasatensis the neck force for raising and depressing the head had been strengthened by the developed splenius, semispinalis capitis and longus capitis muscles. The low spinous process of the middle part of the thoracic vertebrae in Hydrodamalinae indicates that the epaxial muscles (longissimus dorsi and transversospinalis muscles) are less developed, and therefore that the mobilities of the vertebral column are gentle. They might have been swimming in the calm environment.

Journal

  • Earth Science (Chikyu Kagaku)

    Earth Science (Chikyu Kagaku) 60(1), 49-62, 2006

    The Association for the Geological Collaboration in Japan

References:  17

Codes

  • NII Article ID (NAID)
    110004750871
  • NII NACSIS-CAT ID (NCID)
    AN00141269
  • Text Lang
    JPN
  • Article Type
    ART
  • ISSN
    0366-6611
  • NDL Article ID
    7822823
  • NDL Source Classification
    ZM41(科学技術--地球科学)
  • NDL Call No.
    Z15-171
  • Data Source
    CJP  NDL  NII-ELS  J-STAGE 
Page Top