The strong, fusiform (spindle-shaped) build
of Palaeoniscus indicates that it was a
powerful swimmer rather than a
bottom-dweller. Its large, forward-
placed eyes also suggest that vision
was its most important sense. It had
proportionately very large jaws,
lined with rows of short, sharp teeth
that continuously replaced
themselves (as in sharks),
Palaeoniscus is an early or basal
actinopterygian (ray-fin). Its deeply
forked, strongly hererocercal tail (the
two lobes unequal in size) is entirely
fleshy to the tip of its upper lobe -
while, in more derived ray-fins, only the
bony rod-like rays support the tail, which is
often homocercal (with equal lobes).
Additionally, Palaeoniscus regulated its
buoyancy using a pair of air sacs connected
to the throat, instead of the swim bladder as
in most later ray-finned fish, which develops
as an outgrowth of the digestive tract.