Keeping A Flow Profile: The Fluid Dynamics and Biomechanics of Trilobite Genal Spines

Abstract

Arthropods are well known for being extraordinarily diverse. Variation of form and niche-occupation in arthropods relies on specialized appendages and regionalization of a common bauplan. In the marine realm, ancient trilobites exhibit extreme exoskeleton disparity. It is commonly accepted that much of the variety of trilobite evolution, particularly in the head, reflects adaptation to assorted environments. Here, I demonstrate the relationship between cephalon morphology and fluid flow through turbulence and drag force. I examine the effect of genal spine length on (1) degree and position of turbulence relative to the trilobite body along with (2) amount of drag felt on the organism. General morphology of Devonian Eldredgeops (Trilobita: Phacopida) was used as a template. I do not suggest that Eldredgeops may have possessed cephalic spines, but rather, models were constructed with varying lengths of genal spines as a proof of concept. Trilobite models were constructed with five different genal spine lengths, from absent genal spines to spines extending past the rear of the body. In a flow tank, the forms were analyzed with dye stream tests to identify minimum turbulence and with a force transducer to measure drag forces. Laser techniques were used to visualize flow. Results address biomechanics of trilobite cephalon morphology as they affect the fluid dynamics around the entire body. I discuss ecological implications of these findings.