小嘴贝纲纲

概述

小嘴贝纲物种在旧的分类系统中，被归类为有铰纲的小嘴贝形亚纲。小嘴贝纲物种肉茎发达，外壳有阔有窄，大多数皆有两面凸出的外壳。

分类

• 以下为小嘴贝纲包含的目：
• †原正形贝目（Protorthida，早寒武纪 - 晚泥盆纪）
• †正形贝目（Orthida，早寒武纪 - 晚二叠纪）
• †五房贝目（Pentamerida，中寒武纪 - 晚泥盆纪）
• †无洞贝目（Atrypida，早奥陶纪 - 晚泥盆纪）
• 小嘴贝目（Rhynchonellida，早奥陶纪 - 现代）
• †石燕贝目（Spiriferida，晚奥陶纪 - 晚二叠纪）
• †无窗贝目（Athyridida，晚奥陶纪 - 早侏罗纪）
• †准石燕贝目（Spiriferinida，早泥盆纪 - 早侏罗纪）
• 穿孔贝目（Terebratulida，早泥盆纪 - 现代）
• 鞘壳贝目（Thecideida，晚三叠纪 - 现代）

在1952年及1965年的分类中，无洞贝目、无窗贝目、石燕贝目、准石燕贝目这四个目本来被归类成为较大的石燕贝目以下的四个亚目，因为这些物种皆having coiled spiralia for lophophore support，而其他目的腕足成圈。

The Rhynchonellata is a class of Lower Cambrian to Recent articulate brachiopods that combines orders from within the Rhynchonelliformea (Articulata revised) with well developed pedicle attachment. Shell forms vary from those with wide hinge lines to beaked forms with virtually no hinge line and from generally smooth to strongly plicate. Most all are biconvex. Lophophores vary and include both looped and spiraled forms. Although morphologically distinct, included orders follow a consistent phylogenetic sequence.

Orders

• Orders assigned to the Rhynchonellata, in temporal sequence, include:
• †Protorthida (Lower Cambrian – Upper Devonian)
• †Orthida (Lower Cambrian – Upper Permian)
• †Pentamerida (Middle Cambrian – Upper Devonian)
• †Atrypida (Lower Ordovician – Upper Devonian)
• Rhynchonellida (Lower Ordovician – Recent)
• †Spiriferida (Upper Ordovician – Lower Jurassic)
• †Athyridida (Upper Ordovician – Lower Jurassic)
• †Spiriferinida (Lower Devonian – Lower Jurassic)
• Terebratulida (Lower Devonian – Recent)
• Thecideida (Upper Triassic – Recent)

The Atrypida, Athyridida, Spiriferida, and Spiriferinida were previously considered as suborders in the Spiriferida and in the Treatise on Invertebrate Paleontology part H, all having coiled spiralia for lophophore support. In the other orders the supporting brachidia are commonly looped.

The Orthida and Spiriferida have wide hinge lines where the two valves (or shells) articulate. The Pentamerida, Terebratulida, Atrypida and Athyridida have narrow hinge lines. Some, like the Rhynchonellida and Spiriferida, may be strongly plicate, with a median fold and sulcus. Others, like the Spiriferinida and Terebratulida are basically smooth. Most have shells without perforations known as punctae and are impunctate but some e.g. Orthida and Rhynchonellida have punctate offshoots.