Rhynchonelliformea

Rhynchonelliformea is a major subphylum and clade of brachiopods. It is roughly equivalent to the former class Articulata, which was used previously in brachiopod taxonomy up until the 1990s. These so-called articulated brachiopods have many anatomical differences relative to "inarticulate" brachiopods of the subphyla Linguliformea and Craniformea. Articulates have hard calcium carbonate shells with tongue-and-groove hinge articulations (hence the name) and separate sets of simple opening and closing muscles.

The Rhynchonelliformea (as described in the Treatise Part H, revised 1997–2007) is divided into five classes: Obolellata, Kutorginata, Chileata, Strophomenata, and Rhynchonellata. The Rhynchonellata are found living today, as the major constituent of modern brachiopod faunas. The other classes are all extinct: the Obolellata and Kutorginata are restricted to the Cambrian, while the Chileata and Strophomenata range through most of the Paleozoic.

Like all brachiopods, rhynchonelliforms have a shell with two stacked components (valves): a ventral (pedicle) valve which hosts the fleshy pedicle, and a dorsal (brachial) valve which acts as a platform for the lophophore and other important organs.

Rhynchonelliform shells are composed of low-magnesium calcium carbonate fragments stabilized by proteins and other organic molecules. The shell has several major layers. The thicker but less dense inner (secondary) mineralized layer is secreted from below by the mantle. In living rhynchonelliforms, the secondary layer is a mesh of calcite fibers, but extinct groups were more variable in structure. The thinner but denser outer (primary) mineralized layer is composed of calcite grains. It was emplaced from above by the periostracum, a protein-rich outer organic sheath.

The two valves meet at a mobile hinge at the back of the body. In nearly all rhynchonelliforms, this hinge is strengthened by a pair of teeth on the ventral valve, fitting into a pair of sockets on the dorsal valve. This tooth-and-socket articulation is responsible for rhynchonelliforms' alternative name as articulated brachiopods. Tooth shapes typically range from deltidiodont (simple and knob-like) to cyrtomatodont (hooked) or transverse (wider than long).

The teeth may be supported by a variety of specialized structures. The most common of these are dental plates, thin crests extending forwards along the inner surface of the ventral valve. The dental plates also act as attachment points for the brachiopod's diductor muscles. In some rhynchonelliforms, the dental plates are enlarged to the point that they converge into a bowl-shaped structure known as a spondylium.

Internally, the rear part of the dorsal valve is complex, bearing a variety of calcified structures originating near the hinge line. These structures, termed cardinalia, come in many varieties with crest-like, plate-like, or prong-like shapes. Cardinal processes host muscle attachments, clustering near the hinge line or projecting along the midline of the valve. In early articulates, brachiophores (also called crural bases) develop near the front rim of each tooth socket. In more recent species, the brachiophores extend into paired crura (singular: crus), which diverge away from the midline and reinforce the base of the lophophore. In some rhynchonelliform groups, the crura develop further into brachidia (singular: brachidium), looping or spiraling strands which support the entire length of the lophophore.

In general, rhynchonelliforms have mixoperipheral growth: through the shell's development, the commissure (rim) of the shell expands forwards and outwards, away from the hinge. In addition, the valves converge towards each other, creating a curved valve profile tapering to a pointed umbo ("beak") overlooking the hinge. When seen from behind, each valve may bear a triangular depression (interarea) in the space between the umbo and the hinge line.