EVOLUTIONARY AND FUNCTIONAL SIGNIFICANCE OF THE FOLD AND SULCUS IN SUBORDER PRODUCTIDINA (BRACHIOPODA)

The fold and sulcus is a widespread, distinct, morphological feature of many brachiopods throughout the Phanerozoic, and has been studied extensively in biconvex brachiopods. It is, however, convergently present in many concavo-convex brachiopods, but the evolutionary and functional significance of the fold and sulcus of concavo-convex taxa has not been previously examined. The feeding apparatus (lophophore) of concavo-convex brachiopods was probably very different from that of biconvex brachiopods. It is not evident that the fold and sulcus in concavo-convex brachiopods would have a function similar to that of biconvex taxa. This study examined the evolutionary pattern and function of the fold and sulcus in a major group of concavo-convex brachiopods, the suborder Productidina (Pragian-Changhsingian), using diversity data, and fluid-dynamic biomechanic analysis.

The earliest productidines were non-sulcate. During the Devonian, non-sulcate forms had greater origination and lower extinction rates than the sulcate productidines. By the Visean, both groups had similar origination and extinction rates, whereas by the Permian, the sulcate genera had a much lower extinction rate. The proportion of sulcate taxa increased consistently throughout the group’s stratigraphic range (r=0.909, p<<0.01), and by the end-Permian, almost half of all productidines were sulcate. The strong increase in the proportion of sulcate taxa suggests selective pressure for a fold and sulcus across the Productidina.

Examination of fluid flow within a model of a non-sulcate productidine in a recirculating flume reveals a whorl of water inside the shell cavity, spinning on an axis parallel to the hingeline, with the downward flow of the whorl closest to the hingeline and the height of the whorl dependent on shell gape. The presence of the sulcus forces a split in this whorl into two separate streams. This could aid in suspension feeding, as each separate whorl would be located over a separate branch of the lophophore, potentially improving feeding efficiency. If having a sulcate shell increases feeding efficiency, there would be significant selective pressure for that shape, which would drive the increase in the proportion of sulcate productidines through the late Paleozoic.