Orthida

Orthids typically possess biconvex, radially ornamented valves that join into a strophic(straight) hinge. The ventral(pedicle) valve is much taller than the dorsal(brachial) valve, which is relatively flat. Because of that, the interarea is also taller on the ventral valve. Radial ornamentation typically occur as costae, which often appear as grooves on the interior surface as well as the exterior. A single median sulcus on the dorsal valve is often present.

Orthids are articulate brachiopods, meaning they have hinges with teeth and sockets. Their teeth are mostly deltidiodont, meaning the teeth and hinges do not completely interlock. A pair of rod or blade-like brachiophores are present on the dorsal interior, which may have aided in supporting lophophores. Also on the dorsal interior is a cardinal process, a projection located in the median posterior end. Members of Orthoidea tend to have a simple cardinal process shaped like a single rod, while in other taxa more derived types are seen, including bilobed and trilobed forms^[2]. Diductor muscle scars are often found on the cardinal process, which implies that it aided in the opening of the valves. The diductor muscles, projecting through the posterior opening known as the delthyrium, would have used the cardinal process as a sort of lever when lifting the dorsal valve^[3].

The microstructure of the Orthid shell can be divided into two types: punctate and impunctate. Punctate microstructure refer to a state that contain punctae, which are perforations within the shell layers. On the other hand, impuncate microstructure refers to the absence of such punctae. The existence of punctae is a key feature in dividing the two suborders of Orthida - the impunctate Orthids are classified into the suborder Dalmanellidina, and the punctate Orthids are classified into the suborder Orthidina^[2].

There is some debate over the forms that first appeared of this order as to how they should be classified. However, they began to differentiate themselves by the late Early Cambrian period, and by the late Cambrian period had diversified into numerous varieties and reach 2 to 5 cm in width. Specimens from the late Cambrian to the earliest Ordovician exhibit shells with rounded and pointed pedicle valves, with sharp to obtuse extremities and ridges that are fine to course. Punctate shells appear during the mid-Ordovician, which establish the suborder Dalmanellidina. The Ordovician is a productive period which gives rise to numerous genera in this order. However, they started to become greatly reduced by the end of the Ordovician extinction event. Both the Orthidines and Dalmanellidines survived through to the early Devonian Eventually, though, only the Dalmanellidines lived on, and would play a minor role in benthic ecosystems until the late Permian, when they became extinct.^[4]

The taxonomy taken from the 2000 treatise^[2] is as follows: