Table 1 Ecological niches of trilobites based on morphological or sedimentary evidence

Reduced thorax and pygidium; smoothed cephalon; downward projecting spines; facies independence

Light, streamlined body allows fast swimming. Spines prohibit effective movement on the sediment surface. Distribution controlled by water column characteristics rather than sediment characteristics.

Pelagic lifestyle/swimming

Smooth exterior, broad & flat axial lobe

Reduce friction

Burrowing

Larger muscle attachments

Stronger limb motions to move sediments

Eyes reduced or absent

Darker conditions, less need/no need for keen eyesight

Living in deep water

Wide bodies, genal spines

Added support for soupy substrate

Well-developed limbs, flexible hypostome, Cruziana feeding traces

Food passed anteriorly towards the mouth during the course of movement, flexible hypostome used as a scoop.

Particle feeding

Unusual occipital angle, pitted fringe

Pits allow water to flow through cephalon from leg-generated currents

Filter feeding

Rigid, strongly braced hypostome; forked hypostome projections

Ability to process relatively large food particles

Predatory, feeding on soft bodied worms