brachclass

BRACHIOPODA

CLASSIFICATION

CLASSIFICATION & GEOLOGIC RANGES

Phylum Brachiopoda (Cambrian-Recent)

Class Inarticulata (Cambrian-Recent)

Class Articulata (Cambrian-Recent)

Order Orthida (Cambrian-Permian)

Order Strophomenida (Ordovician-Jurassic)

Order Pentamerida (Cambrian-Devonian)

Order Rhynchonellida (Ordovician-Recent)

Order Spiriferida (Ordovician-Jurassic)

Order Terebratulida (Devonian-Recent)

Class INARTICULATA

Inarticulate brachiopods do not posses teeth and sockets, nor do they have clearly defined diductor muscles. Instead, the valves are held together by a complex of adductor muscles. Although some inarticulates construct their valves of calcite, most have shells of a mineral composition of chitin and calcium phosphate which can be recognized by its shiny, enamel-like luster. Inarticulate brachiopods usually lack surficial ornamentation except growth lines. The classic example of the inarticulates belong to the order Lingulida. The linguloids are small, biconvex, astrophic, with usually oval or circular outlines. This order has quite a long geologic history with some genera (and possible species) remaining relatively unchanged since the Cambrian. See these specimens for examples. -

Class ARTICULATA

Articulate brachiopods differ from inarticulates in that the articulates posses teeth and sockets and mineralized lophophore supports. The classification of articulate orders and suborders depends primarily upon characters of the hinge and beak areas (including hinge length, teeth and sockets, pedicle opening, interarea, etc....) and perhaps more importantly, although more difficult to asses, the nature of the lophophore support (i.e., spiralium, brachiophores, loops, and hooks). Other features (such as the shell microstructure, surface ornamentation) sometimes are quite diagnostic of several orders and suborders of brachiopods. If you are unfamiliar with these morphologic terms your should review the last lab before continuing. A summary of the various brachiopod groups is found at the end of this page.

Order ORTHIDA

Shells of orthids are typically strophic (having an elongated hinge line) about equal to or slightly less than the maximum shell width dimension. The shape is generally semi- or sub-circular in outline. Valve convexity is usually unequally biconvex with a slightly inflated pedicle valve. The pedicle opening is usually triangular (although very rarely absent). Typically an interarea is developed on both pedicle and brachial valves. Orthids are typically covered with fine diverging radial costae. Examples of orthids can be found in these specimens.

Order STROPHOMENIDA

Shells of strophomenids are typically strophic, whose valves are either planoconvex or concavoconvex, or less commonly biconvex. The shell structure of strophomenidids is typified by small calcite rods perpendicular through shell surfaces (in the literature this shell structure is called pseudopunctate). The interarea is usually lacking in strophomenidids. Strophomenidids typically do not posses a pedicle opening, and are therefore not believed to have attached themselves by a pedicle; instead, they were mostly free-living (reclining) epi- or slightly semi-infaunal critters, floating on the substrata by means of their large surface area and/or spines. Diagnostics for three common Suborders are given below.

(1) Suborder Strophomenidina. Most are semicircular or subquadrate in outline with plano-convex or concavo-convex inflation. When taken together, the two valves of strophomenidinids are not as thick as those belonging to the productids (see below). See these specimens for examples. -

(2) Suborder Chonetidina. These resemble semicircular plano-convex or concavo-convex strophomenidids (as above), but differ in that they typically have well developed tubular spines along posterior margin of the pedicle interarea (i.e., along the hinge) such as is seen in this specimen. Furthermore, several groups of chonetids have elongated hinge margins which appear as spines. Chonetids are usually smaller than other strophomenidids. This group is quite common in many Devonian strata from central New York.

(3) Suborder Productidina. Easy to recognize, productids are often strongly concavo-convex with a greatly inflated pedicle valve. Many productids had during life numerous spines on the pedicle valve enabling them to float in soft muds. These spines are typically broken off by taphonomical processes, but their original presence can usually be deduced by knobs or slight thickenings where they once were attached such as in these specimens.

Order PENTAMERIDA

Shells of pentamerids are generally biconvex; they can be either strophic or more commonly astrophic. Pentamerids are typically ovoid, circular, triangular, or more commonly pentameral in outline. The interior of the shell is typified by a prominent medial ridge or septa in the brachial and/or pedicle valve. Also diagnostic of pentamerids is a spoon shaped structure modified from plates in the pedicle valve called the spondylium which supported muscle tissues.

Order RHYNCHONELLIDA

Shells of rhynchonellids are unequally biconvex and astrophic. Rhynchonellids typically have radial costae and a strong fold and sulcus which can clearly be seen in the commissure along the anterior margin. The juncture of radial ornamentation with the commissure produces a zig-zag pattern which is very characteristic of this group. The pedicle opening in rhynchonellids is usually not observed, but when present it typically slit or triangular shaped. See this example making sure you observe the crenulated commisural plane.

Order SPIRIFERIDA

Spiriferid brachiopods are a diverse group whose biconvex shells can be astrophic to highly strophic and circular to alate in outline. The unifying feature among spiriferid brachiopods is the spiral lophophore support. Detailed diagnostics for three common suborders are given below.

(1) Suborder Atrypidina. These are astrophic, and have spiralia that are oriented parallel to commissural plane. Shells commonly have radial ornamentation such as these examples.

(2) Suborder Spiriferidina. Theses are strophic spiriferids, whose pedicle valve interarea is well developed. The orientation of the spiralia are perpendicular to the commissural plane and parallel to the plane of symmetry in brachiopods. Spirifirids typically have a well defined fold and sulcus and are frequently ornamented with radial plicae and/or costae. Commonly (but not exclusively) spiriferids are alate shaped such as Mucrospirifer from the Middle Devonian of New York.

(3) Suborder Athyrididina. These are astrophic and generally smooth shelled. They typically have a rounded pedicle opening and very much resemble the terebratulids (see below), but differ in having spiralia lophophore supports and less pronounced pedicle opening. The spiralia are oriented perpendicular to the commissural plane.

Order TEREBRATULIDA

Terebratulids are astrophic brachiopods which typically have biconvex shells that are usually ovoid to circular in outline. They can be either smooth or have radial ribbing. The lophophore support is loop shaped in contrast to the spiralia of similar looking spiriferids.

TABLE 1 - Some characteristic morphologic features of articulate brachiopods

Note: H=hinge, L=length, PV = pedicle valve

Order-Suborder	Convexity	Pedicle opening	Hinge	H:L	Other
Orthida	unequally biconvex	triangular	strophic	< 1	diverging costae
Strophomenidina	plano- concavo-convex	absent	strophic	1	thin-shelled
Chonetidina	plano- concavo-convex	absent	strophic	1	spines on hinge
Productidina	plano- concavo-convex	absent	strophic	1	spines on PV
Pentamerida	biconvex	absent	astrophic	--	medial septum
Rhynchonellida	biconvex	absent	astrophic	--	strongly plicate
Atrypidina	biconvex	small	astrophic	--	often costate
Spiriferidina	biconvex	triangular	strophic	1	wing-shaped
Athyrididina	biconvex	small, circular	astrophic	--	often smooth
Terebratulida	biconvex	circular	astrophic	--	smooth

From McRoberts (1998)

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