Paterinata


Paterinata is an extinct class of linguliform brachiopods which lived from the lower Cambrian to the Upper Ordovician. It contains the single order Paterinida and the superfamily Paterinoidea. Despite being some of the earliest brachiopods to appear in the fossil record, paterinides stayed as a relatively subdued and low-diversity group even as other brachiopods diversified later in the Cambrian and Ordovician. Paterinides are notable for their high degree of convergent evolution with rhynchonelliform brachiopods, which have a similar set of muscles and hinge-adjacent structures.

Anatomy

Paterinides had organo-phosphatic shells which were ventribiconvex and strophic. Shell ornamentation usually consists of concentric fila and tiny pits. The tiny larval shell has a smooth outer halo and strongly ornamented inner portion. There are no canals in the shell microstructure.

Hinge structures

When seen from behind, each valve appears triangular, with a system of superimposed plates and furrows along the hinge. Each valve has a broad triangular depression edging the hinge, known as a pseudointerarea. The middle of each pseudointerarea hosts a narrower excavation or furrow, known as a notothyrium or a delthyrium. Finally, these furrows may be partially covered by a convex plate-like overgrowth, known as a homeochilidium or a homeodeltidium. The furrow-and-plate pairing creates closed pockets near the hinge, where the pedicle presumably emerged.

Soft tissue

The musculature of paterinides seemingly relied on a small number of broad muscles within the shell. There are a pair of large triangular muscle scars near the hinge of the ventral valve, conjoining under the homeodeltidium. There are two pairs of scars in the dorsal valve, one pair set medially and the other set posterolaterally. These major scars likely correspond to adductor muscles, which close the shell. A subtle pit was present at the tip of the homeochilidium, possibly hosting diductor muscles, which open the shell. Diductor muscles are otherwise only found in articulate brachiopods. The network of mantle canals usually had a saccate form, emphasizing a pair of midline canals which arc outwards once they reach the shell margin.

Subgroups