BRACHING THE MOLD: BIOMECHANIC TESTS OF THE LIFE ORIENTATION OF A DORSIBICONVEX BRACHIOPOD

The abundance of brachiopods in Palaeozoic outcrops makes them ideal palaeoenvironmental indicators, yet there are few independent, biomechanical studies on fossil brachiopod life orientations. Brachiopod orientations can be used to determine if animals were alive at the time of burial, the rate of sediment deposition, and if transport or time averaging occurred. We assess potential orientations of a common dorsibiconvex brachiopod, Desquamatia, from the mid-Late Devonian of North America, using a realistic model in a flume.

A well preserved Desquamatia from the Waterways Fm. (Early Frasnian, Alberta, Canada) was modeled using materials with the specific gravity of calcium carbonate (the original shell material), and by measuring thickness of disarticulated Desquamatia. A recirculating flume was used to test the hydrodynamic stability of the model in three initial orientations: 1) anterior commissure upstream, 2) umbo upstream, and 3) lateral (specimen was perpendicular to flow); all orientations were performed with the dorsal valve topmost, and repeated with the ventral valve on top. The six scenarios were conducted on plexiglass, and on well-sorted, medium-grained sand. All scenarios were repeated 5 times for a total of 60 trials.

Flume trials indicate that the model brachiopod has no true hydrodynamically stable orientation. The most common reorientations were 1) dorsal valve on top, umbo upstream (41/60 trials), and 2) ventral valve on top, umbo upstream (17/60 trials). However, these reorientations occurred at low velocities (~ 0.2 m/s), and entrainment occurred soon after (~ 0.3 m/s).

Assuming that a juvenile, pedunculate, dorsibiconvex brachiopod would have initially been oriented during life with its ventral valve on top, these results suggest two important outcomes: 1) the brachiopod either was attached by a pedicle throughout its life, or lived in quiet, undisturbed waters; and 2) if the brachiopod was attached by a pedicle during life, fossil Desquamatia found oriented with the dorsal valve on top likely indicate that the specimens were dead prior to burial.

Ongoing investigation includes flume trials of Desquamatia models with frills, and trials with a gaping model. Biomechanical tests using more realistic models are invaluable tools easily applied to a wide range of palaeoecological problems.