MIND THE GAPE: THE SOLUTION TO THE LIFE HABITS OF RAFINESQUINA ALTERNATA MAY TURN ON A 3D-PRINTED HINGE

Rafinesquina alternata is at the center of a debate over whether free-living concavo-convex brachiopods lived concave side up or down. Whereas epibionts and bio-sedimentary structures indicate a concave-down life position, the counter-argument is that the downturned narrow gape would have been buried in the muddy substrate preventing water circulation to the lophophore.

This biomechanical constraint hinges on the assumption that Rafinesquina gaped at <5° as observed in most modern brachiopods. However, modern thecideide brachiopods open their valves to ~90°, and Rafinesquina lever systems allow a gape of ~50°. Nevertheless, the structure of the hinge could limit gape, and should not be neglected.

The challenge of studying a precision mechanism like the Rafinesquina hinge is that separated valves show internal detail but are unlikely to have been life pairs and will not fit precisely. The hinge cannot be observed directly on still-articulated valves because these are generally cemented together with calcite.

Using serial sagittal sections of hinges on articulated valves, it is possible to estimate muscle leverage but one cannot determine how features in different sagittal planes interact, including hinge function. For example, the hinge, more than muscle leverage, is sensitive to the precise position of the fulcrum, but the exact fulcrum line cannot be determined from sagittal slices or examining isolated valves.

We addressed this knowledge gap by serial grinding of typically preserved specimens and microCT scanning silicified specimens to generate, enlarge and print 3D models of articulated valves. These models were articulated and manipulated from the outside to approximate the movement that would have resulted from muscle contraction.

The fulcrum articulation is clear when manipulated. Shell structures previously attributed to the fulcrum actually stabilize closed valves. Two radiating rows of teeth on each valve mesh progressively like shears as the valves open or close. At the full gape the valves fit together on a few radial crenulations in the interarea. Interarea alignment allows gape angle of any articulated valves to be measured directly at~50°.

This hinge analysis confirms a wide gape, and indicates that concave-down life position was possible.