which were among the first organisms to grow hard shells and the dominant
species for the next 100 million years.
Trilobites were primitive aquatic crustaceans and a favorite among fos-
sil collectors, second only to dinosaurs as fossils of public interest. They were
as common and diverse as their crustacean cousins are today. Because most of
the present world’s land areas were submerged during various parts of the
Paleozoic, trilobites are found in marine sedimentary rocks on all continents.
They lived mostly on the seafloor and were among the earliest known organ-
isms to grow hard shells.The remarkable structure and beauty of the trilobite
compound eye provides important information about visual capabilities early
in the history of invertebrates.
The trilobites were small, oval-shaped arthropods, ancestors to the
horseshoe crab, their only remaining direct descendent.Their body had three
lobes,hence the name. It consisted of a central axial lobe containing the crea-
ture’s essential organs and two side, or pleural, lobes. The giant paradoxides
were truly a paradox among trilobites, extending nearly 2 feet in length, while
most trilobites were generally less than 4 inches long. Since trilobites were so
widespread and lived throughout the Paleozoic, their fossils are important
markers for dating rocks of this era. They appeared at the very base of the
Cambrian and became the dominant invertebrates of the early Paleozoic.They
diversified into about 10,000 species before declining and becoming extinct
after some 300 million years of highly successful existence.
The trilobites occupied the shallow waters near the shores of ancient seas
that flooded inland areas, providing extensive continental margins from the coast-
line to the edge of the abyss. Seawater advanced to cover most of the continentsFigure 46Trilobites,
extinct ancestors of today’s
horseshoe crabs, first
appeared in abundance in
the Cambrian and became
extinct at the close of the
Paleozoic.
Historical Geology