540 million years ago, as evidenced by the presence of Cambrian seashores in
such places as the interior of North America, which is remarkably similar on all
continents. In addition, a stable environment enabled marine life to flourish and
proliferate, becoming widely dispersed.The flora included cyanobacteria, red and
green algae, and acritarchs, a form of plankton that supported early Paleozoic
food chains. Some simple multicellular algae that built algal mats and stromato-
lites similar to those of today evolved more than a billion years ago.
Curiously, many trilobite fossils have bite scars predominantly on the
right side. Predators might have attacked from the right probably because
when the trilobite curled up to protect itself it exposed this side of its body.
(Trilobite fossils are often found with their bodies completely curled up.)
However, if the trilobite had a vital organ on its left side and an attack
occurred there, it stood a good chance of being eaten, thereby leaving no fos-
sil. Therefore, if attacked on the right side, the trilobite stood a better chance
of entering the fossil record, albeit with a chunk bitten out of its side.
Trilobites shed their exoskeletons as they grew. In this manner, an indi-
vidual could have left several fossils, which explains why whole fossils are rare
(Fig. 47). During molting, a suture opened across the head, and the trilobite
simply fell out of its exoskeleton. Sometimes, though, a clean suture failed to
open, and the animal had to wiggle its way out. Either way, it was still vul-
nerable to predators until its new skeleton hardened.
The population of trilobites peaked during the late Cambrian around
520 million years ago, when they accounted for about two-thirds of all marine
species. By about 475 million years ago, the number of trilobite species
abruptly declined to one-third following the rise of mollusks, corals, and other
stationary filter feeders. Later, the trilobites left the nearshore areas for the off-
shore probably in response to significant environmental changes in tempera-
ture and seawater chemistry.The demise of the trilobites also appears to follow
the arrival of the jawed fish.
CAMBRIAN PALEONTOLOGY
The sponges were primitive multicellular animals that thrived on the seafloor just
before the start of the Cambrian. The early sponges lived at a pivotal point in
Earth history, when ancestors of most modern animal groups suddenly appeared
in the seas. The ancient sponges lived much as their modern counterparts do.
They filtered nutrients from seawater, thus providing the earliest example of fil-
ter feeding, which is an extremely common mode of life in the modern seas.
The existence of sponges prior to the Cambrian suggests that more com-
plex animals must have evolved by the time the Ediacaran fauna appeared
around the world.The sponge skeletons consisted of tiny glassy spikes very sim-
CAMBRIAN INVERTEBRATES