Why did rudist bivalves flourish in the reefal environments in the Cretaceous Greenhouse Earth? -Early evolution of the Hippuritidae

2023 Fiscal Year Final Research Report

• Project/Area Number: 20K04145
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Multi-year Fund
• Section: 一般
• Review Section: Basic Section 17050:Biogeosciences-related
• Research Institution: University of Toyama
• Principal Investigator: SANO Shin-ichi 富山大学, 学術研究部都市デザイン学系, 教授 (50823259)
• Co-Investigator(Kenkyū-buntansha): 伊庭 靖弘 北海道大学, 理学研究院, 准教授 (80610451)
• Project Period (FY): 2020-04-01 – 2024-03-31
• Keywords: 白亜紀 / 赤道太平洋域 / フィリピン / ヒップリテス科 / ポリコニテス科 / 厚歯二枚貝 / 系統分類 / 古生物地理

Outline of Final Research Achievements

The Hippuritidae is one of the representative rudist families in the Late Cretaceous, but its origin and early evolutionary history remain uncertain. A new hippuritid from the Caramoan Peninsula, the Philippines, has a distinct multiple-fold system having seven or eight radial infoldings, which are inserted into fixed places by fixed order, in the outer shell layer along the whole commissural margin of the right valve. It uniquely combines primitive and “derived” morphological features from different lineages of this family. Cladistic analysis of the almost all of the hippuritid genera and some advanced forms of the polyconitids, the candidate ancestors for the Hippuritidae, is conducted. The Caramoan hippuritid comprises the clade with Torreites and Polytorreites, which represents the systematic position derived from the polyconitids and sister to other hippuritids. It is supposed that the Hippuritidae most likely originated in the Central Pacific area in the Cenomanian.

Free Research Field

古生物学

Academic Significance and Societal Importance of the Research Achievements

後期白亜紀は，地球史上で唯一，二枚貝が礁性環境の主役になった時代である．ヒップリテス科は厚歯二枚貝を代表する存在で，蓋付き湯飲み型の形態を持ち，後期白亜紀の炭酸塩プラットフォームに繁栄し，この成功の理由を探る上で重要な存在である．本科は，後期白亜紀チューロニアンに，科の様々な標徴を既に獲得した状態で，地中海地域とカリブ海地域にほぼ同時に出現したとされ，その起源は謎とされてきた．本研究によって，本科がセノマニアンの赤道太平洋域に起源し，本科の標徴も，祖先形となるポリコニテス科から，従来想定されていない形態を原始的段階として，段階的に獲得されたという，全く新しい初期進化史を提案することができた．