CLARIFICATION OF HABITAT EXPANSION DURING THE EVOLUTIONARY HISTORY OF BIVALVES

在此基础上，研究发现，在晚白垩世至古近纪，双壳类动物的栖息地迅速扩展，从物理上稳定的陆架和海湾环境扩展到物理上更不稳定的环境，如海滩、岩石海岸、砾石基质、河道和相关的水流冲刷环境。由于化石化潜力低，双壳类在岩岸的定殖历史重建较为困难，但无趣双壳类的Pholadidae留下了相当好的化石记录。功能壳形态分析表明，双壳类生物最早在晚侏罗世的植物遗存中定居，并可在晚白垩世的硬岩中定居。物理上更不稳定的半固结沉积岩在中新世成为殖民地。浅层潮下砾石底物在古近纪开始被附生果胶、牡蛎和等食石类生物定植。中新世之后，自由生活的果胶虫也在类似的环境中生活。Glycymerididae的成员出现在早白垩世Aptian陆棚相中。据解释，它们是在晚白垩世逐步将栖息地转移到下海岸的。在始新世，它们的栖息地达到了上滨面或相关的水流环境中的砾石底物。随着生境的变化，它们的壳尺寸显著增大。中新世以后，Glycymeridids的栖息地不仅在近岸，而且在近海陆架和上部深海泥质底也有分布。综上所述，早白垩世以前，双壳类的分布格局是分散的，更多地局限于物理上更稳定的陆架或海湾环境；晚白垩世至古近纪是双壳类分布格局发生剧烈变化的时期。

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.