PRECOCIOUS SCAPHITES: HYDRODYNAMIC ADVANTAGES OF SHAPE AND ORNAMENTATION IN JUVENILE AMMONITES

Basic conch geometry roughly constrains how different ammonites could balance and swim. But it remains unclear what costs - or advantages - are added by complex ribbing and other conch ornamentation. We present a case study with juvenile scaphite ammonites from the Western Interior Seaway of the Late Cretaceous.

Scaphitid conchs typically change shape considerably through ontogeny. Juvenile planispiral conchs offer varied streamlining that reduces drag resistance when swimming. Adult conchs produce a hook-shaped body chamber that forfeits streamlining for hydrostatic stability. Ribbing, however, persists throughout an individual’s life with little change in amplitude and wavelength. We weigh the hydrodynamic cost of ribbing in these juvenile conchs using computational fluid dynamic simulations (CFD).

We create synthetic scaphitid 3D models of two representative shapes from specimens collected in the Upper Cretaceous Pierre Shale: an inflated discocone and a moderately compressed oxycone. We add simple ribs to each, for a total of four test cases. Flow simulations target a series of velocities from 1-20 cm/s on specimens 4-5 cm in diameter. Generally, an inflated conch shape generates more overall drag force than a compressed shape. Ribs, meanwhile, cause an inflated specimens yield drag force to be closer to those of a compressed specimen. We interpret this as a consequence of premature shear experienced at the ribbing interface along the conch flank, and at the umbilical shoulder.

For juvenile scaphites, inflation and ornamentation cause opposing trade-offs in drag force with increasing turbulence. If our ammonites were traveling slower than one body-length per second (or were smaller in size), compression and/or ribbing exacerbate drag force. If our ammonites were traveling faster than one body-length per second (or were larger in size), compression and/or ribbing reduce drag force. Between two individuals, the inflated form should favor maneuverability and acceleration, while the compressed form should travel greater distance for the same effort. Ribbed ornament appears to help the inflated conch behave more like a compressed conch. Collectively, juvenile scaphites appear to occupy every position within a highly constrained maneuverability-efficiency-speed landscape.