Strophomenida

Ordovician to Carboniferous

Introduction

The order Strophomenida originally referred to a diverse Paleozoic assemblage of Brachiopods, but in the most recent classification it is used in a much more restricted sense, with the more specialised forms being placed in their own distinct orders.

Strophomenids have one valve is convex while the other is generally concave, so the living space for the animal inside is very reduced. It has a wide hinge line, and one concave valve, which may be either brachial or pedicle. The surface is ornamented by fine radiating lines (costae) and a wide hinge line is present in this group. The early strophomenids had concave-convex pseudopunctate shells, and were attached by a pedicle only in early growth stages, becoming free-lying later in ontogeny.

These are the brachiopods often referred to as "petrified butterflies."   One major characteristic peculiar to this group is the presence of pseudo-punctate structure.

Appearing in the mid-Ordovician, strophomenids were both varied and abundant but declined after the Devonian. They have a well defined interarea (cardinal area) on one or both valves and lack spines.

The strophomenid body was very thin, it generally built a thick shell with muscle scars. Such shells are abundant Paleozoic fossils, and pedicle valves are exceedingly attractive. Many species have prominent growth lines that show how shapes changed with increasing size.

Systematics

Strophomenida
Strophomenoidea (Early Ordovician-Early Carboniferous)
Plectambonitoidea (Early Ordovician-Late Devonian)