example is Helicoplacus(Fig. 58), whose body parts were configured in a man-
ner not found in any living organism. It was about 2 inches long and shaped
like a spindle covered with a spiraling system of armor plates. It emerged dur-
ing the transition from the Precambrian to the Cambrian, when more types
of body plans arose than at any succeeding period.Helicoplacus,as with most
species of the early Cambrian, did not leave any living descendants and
became extinct about 510 million years ago, just a short 20 million years after
it first appeared.
During the Cambrian, continental motions assembled the present con-
tinents of South America, Africa, India, Australia, and Antarctica into Gond-
wana (Fig. 59), named for a geologic province in east-central India. Evidence
for Gondwana exists in geologic provinces with similar rock types from the
late Precambrian to the early Cambrian.These show matches between Brazil
and West Africa; eastern South America, South Africa,West Antarctica, and East
Australia; and East Africa, India, East Antarctica, and West Australia.The Pacific
margins of South America, Australia, and Antarctica had formed by the begin-
ning of the Cambrian.
East Antarctica was an old Precambrian shield lying to the south of Aus-
tralia, India, and Africa. A great mountain building episode deformed areas
between all pre-Gondwana continents, indicating their collision during this
interval.The Transantarctic Mountains arose during the collision of the larger
East Antarctica with the geologically younger West Antarctica, composed of
volcanic island arcs swept up from the ocean floor.
Much of Gondwana was in the southern polar region from the Cam-
brian to the Silurian. The present continent of Australia was at the northern
Figure 58Helicoplacus
was an experimental
species whose body parts
were arranged in a
manner not found in any
living creature and became
extinct about 510 million
years ago after surviving
for 20 million years.CAMBRIAN INVERTEBRATES