Inoceramus - Inoceramus

A genus of marine bivalve mollusks that flourished in the Mesozoic era (251 to 65.5 million years ago). It is included in the order Pterygota, and together with other genera, it forms an independent family (Inoceramidae). However, because the structure of the hinge is different from that of modern-day Inoceramids (including pearl oysters and scallops), there is an alternative theory that it belongs to a different lineage (Incus subclass).

The soft body of this species remained a mystery for a long time, but the discovery of well-developed gill fossils replaced by phosphate minerals revealed that it had excellent seawater filtering capabilities. The shell is medium to large in size, with equal or unequal shells, and its shape is highly variable. It is almost toothless, and muscle scars are underdeveloped. The calcareous shell consists of a nacreous layer (inner layer) and a developed prismatic layer (outer layer), and the outer surface is covered by a thin organic shell skin. The shell has well-developed concentric ribs (linear elevations), and rarely radial ribs. It lived by attaching itself to features on the seafloor, or to driftwood or seaweed in the sea by extending byssus threads from the front edge of the shell.

This species is thought to have appeared in the Permian Period (299-251 million years ago), but due to the paucity of fossil records from the Triassic Period, the relationship between the Permian species and those from the Jurassic Period and later has been called into question. They became extinct in the late Cretaceous Period (145-65.5 million years ago). In particular, species that lived in the Mesozoic Era are found in abundance all over the world, including Japan, and are important in dividing geological strata into time periods.

[Tanabe Kazunari]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

中生代（2億5100万～6550万年前）に栄えた海生二枚貝の一属。翼形亜綱ウグイスガイ目に含められ、他の属とともに独立した科（Inoceramidae）をつくる。ただし、現生ウグイスガイ類（アコヤガイ、ホタテガイなどを含む）と蝶番(ちょうつがい)の構造が異なるため、別の系統（キヌタレガイ亜綱）だとする異説もある。

　軟体部については長らく不明であったが、よく発達したえらの化石がリン酸塩鉱物に置換した状態で発見され、優れた海水の濾過(ろか)能力をもっていたことがわかった。殻(から)は中形ないし大形で、左右等殻ないし不等殻で形態は変異に富む。ほとんど無歯で筋痕(きんこん)も未発達である。石灰質の殻は真珠層（内層）と発達した稜柱(りょうちゅう)層（外層）からなり、薄い有機質の殻皮が外表面を覆う。殻には同心円状の肋(ろく)（線状の高まり）が発達し、まれに放射肋もみられる。殻の前縁から足糸を出して海底の地物または海中の流木、海藻に付着して生活した。

　この仲間はペルム紀（二畳紀。2億9900万～2億5100万年前）に出現したと考えられるが、三畳紀の化石記録が乏しいため、ペルム紀の類とジュラ紀以降の類の間の類縁関係が疑問視されている。白亜紀（1億4500万～6550万年前）後期に絶滅した。とくに中生代に生息した類は日本を含む世界各地から多産し、地層の時代区分に重要である。

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