体サイズから見る三葉虫モルフォタイプの多様性 : 収斂形質群の機能形態学の応用(<特集>生物多様性と古生物学)

抄録

As to document a long-term biodiversity pattern of an extinct organism, I focused on size changes in homeomorphic olenimorph and illaenimorph trilobite species based on physiological background. The former type is characterized to have two dimensionally widened pleural area which represents respiratory region. The latter type is characterized to exhibit large and complicated cephalic muscle scars which could control movements of cephalic appendages more sophisticatedly than other trilobites. The olenimorph species were established in the Early Cambrian, and diversified their maximum body sizes from the Middle Cambrian to the Early Ordovician involving the occurrence of the largest species. From the Middle Cambrian to the Middle Devonian, some of the olenimorph species retained small body sizes that allowed them an efficient respiratory system as a strategy against predators. On the other hand, the illaenimorph trilobites, which were established in the Late Cambrian, diversified their maximum sizes gradually toward the Middle Silurian, but reduced their sizes abruptly after the Late Devonian. The difference in the tempo of diversification of the body sizes between olenimorph and illaenimorph species implies that it took a longer time for illaenimorph species to establish developmental integration of functionally related traits unlike olenimorph species. The study on body size in each homeomorphic morphotype from the physiological viewpoint appears to be applicable to extinct organisms. Detail morphological information, such as the change of functionally significant characters with the increase of body size, is required for further progress of the functional morphology, and for understanding the biodiversity pattern of extinct organisms from the functional-morphological point of view.