Trilobite tridents: hydrodynamic lift and stability mechanisms for queue formation

TL;DR

This paper proposes that Walliserops trifurcatus's trident-like structures evolved primarily for hydrodynamic lift and stability in group movement, enhancing collective efficiency rather than for combat or sexual selection.

Contribution

It introduces a biomechanical and collective behavior hypothesis for trident evolution, emphasizing cooperative locomotion advantages over individual combat functions.

Findings

Tridents improve hydrodynamic lift and stability in group formations.

Linked formations reduce energy expenditure and collision risk.

Hydrodynamic drafting benefits weaker individuals within groups.

Abstract

The bizarre trident-like cephalic projections of Walliserops trifurcatus have previously been interpreted as sexually selected weapons for intraspecific combat. We propose an alternative hypothesis grounded in biomechanics and collective behavior, that tridents evolved as adaptations for hydrodynamic lift and queue stability, conferring energetic advantages during group locomotion. Under this hypothesis, lift could offset gravitational forces, enabling greater locomotor efficiency, while mechanically linked formations, where tridents rested on the pygidia of leading individuals, enhanced pitch and roll stability and minimized costly accelerations and collisions. These formations also facilitated hydrodynamic drafting, allowing weaker individuals to conserve energy and remain integrated within the group. The trident's structure, though inefficient for solitary lift or combat, functioned effectively in cooperative formations, suggesting that its original selective advantage lay not in individual performance but in enhancing group cohesion and efficiency. Rather than emerging from competitive pressures alone, the trident may have arisen through selection for coordinated, cooperative movement -- potentially representing a precursor stage to traits later exapted for sexual selection.