rapidly in the Silurian. Numerous species succumbed to extinction at the end
of the period.
The echinoderms with fivefold and bilateral symmetries and exoskele-
tons composed of numerous calcite plates were among the most prolific ani-
mals of the Silurian seas.The most successful of the Silurian echinoderms were
the crinoids, commonly called sea lilies because they resemble flowers atop
stalks anchored to the seabed. Some crinoids were also planktonic or free-
swimming types (Fig. 84). They became the dominant echinoderms of the
middle and upper Paleozoic, with many species still in existence.
The long stalks of the crinoids grew upward of 10 feet in length or longer
in good conditions.They were composed of perhaps 100 or more calcite disks
called columnals and anchored to the ocean floor by a rootlike appendage.A cup
called a calyx perched on top of the stalk and housed the digestive and repro-
ductive organs. The animal strained food particles from passing water currents
with five feathery arms that extended from the calyx, giving the crinoid a flow-
erlike appearance. The extinct Paleozoic crinoids and their blastoid relatives,
whose calyx resembled a rosebud, made excellent fossils, especially the stalks,
which on weathered limestone outcrops often looked like long strings of beads.
The echinoids are aclass of echinoderms that include sea urchins, heart
urchins, and sand dollars.They have exoskeletons composed of limy plates that
are characteristically spiny, spherical, or radially symmetrical. Some more
advanced forms were elongated and bilaterally symmetrical about a central
point. The sea urchins lived mostly among rocks encrusted with algae, upon
which they fed. Unfortunately, such an environment was not conducive to
fossilization. Likewise, the familiar sand dollars that occasionally washed up on
beaches are rare in the fossil record.
Figure 83The rudists
were major reef-building
mollusks.SILURIAN PLANTS