隠蔽環境の化石動物種群 : 4.5億年前の石灰泥丘を例に(2009年度日本動物分類学会シンポジウム)

抄録

Upper Ordovician cave-dwelling trilobite species, Isocolus sjoegreni, Ityophorus undulatus and Prionocheirus obtusus, found in originally subsurface open-spaces of the Boda Limestone, Sweden, show two distinct occurrence patterns. Blind and dwarfed I. sjoegreni and I. undulatus co-occur with non-trilobite animal taxa, the former with ostracods and the latter microgastropods, which indicates ecological similarity with the taxa such as in preferable food resources. The two species show morphological simplification in their cephalic suture pattern, which is alike to the general morphological characteristics recognised in the modern cryptobionts. P. obtusus is generally found in a fossil assemblage that consists exclusively the specimens of the species. No other than its dwarfed body size, P. obtusus shows no corresponding morphological characteristics that are exhibited in the other two cave dwelling trilobite species. Its ancestral habitual characteristic, living in a deeper-water and stagnant muddy level bottom environment, is an indication that the species preferred on physical aspects of cryptic environments such as darkness and lack of predators. There appeared at least two general directions for the biodiversity of fossil cryptobionts.