|Budget Amount *help
¥3,400,000 (Direct Cost : ¥3,400,000)
Fiscal Year 2002 : ¥1,100,000 (Direct Cost : ¥1,100,000)
Fiscal Year 2001 : ¥1,100,000 (Direct Cost : ¥1,100,000)
Fiscal Year 2000 : ¥1,200,000 (Direct Cost : ¥1,200,000)
On the basis of this study, it is found that bivalve habitat expanded rapidly during the late Cretaceous to Paleogene, from physically stable shelf and bay environments to physically more unstable environments such as beaches, rocky shores, gravelly substrates, and channel and related current-swept environments. Reconstruction of colonization history of rocky shore by bivalves is difficult owing to the low fossilization potential, but boring bivalves of the family Pholadidae left reasonably good fossil record. Analysis of the functional shell morphology, Pholadid bivalves are interpreted to have colonized firstly plant remains in the Late Jurassic and they could colonize hard rock in the Late Cretaceous. Physically more unstable semi-consolidated sedimentary rock became colonized in the Miocene. Shallow subtidal gravelly substrates became colonized by epibyssate pectinids, oysters and isognomonids in the Paleogene. Similar environments have been inhabited by free-living pectinids after the Miocene.
Members of the Glycymerididae appeared in the shelf facies in the Aptian (Early Cretaceous). It is interpreted that they shifted their habitat in a stepwise manner to lower shoreface in the Late Cretaceous. In the Eocene, their habitat reached gravelly substrates in upper shoreface or related current-swept environments. In accordance with this habitat change, their shell size increased remarkably. It is also noted that the habitat of Glycymeridids dispersed after the Miocene not only nearshore, but also offshore shelf and upper bathyal muddy bottoms.
To summarize the result, the distribution pattern of bivalves are understood as dispersed one, which was much more restricted to physically more stable shelf or bay environments up to the early Cretaceous : Late Cretaceous to Paleogene was a time of dramatic change in the pattern of bivalve distribution.