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Graptolites were colonies of cupped organisms that resembled stems and
leaves of plants but were actually animals, with individual organisms housed in
tiny cups. They clung to the seafloor like small shrubs, floated freely near the
surface, appearing much like tiny saw blades, or attached to seaweed. Large
numbers of graptolites buried in the bottom mud produced organic-rich
black shales that indicate poor oxygen conditions. They are important mark-
ers for correlating rock units of the lower Paleozoic.
THE BURGESS SHALE FAUNA
The Burgess Shale Formation in British Columbia, Canada, contains the
remains of bizarre, soft-bodied animals that first appeared in the lower Cam-
brian about 540 million years ago soon after the emergence of complex
organisms. The assemblage featured more diversity of basic anatomic designs
than all the world’s oceans today. It included some two dozen types of plants
and animals that have no modern counterparts. Many of these peculiar ani-
mals were possibly carried over from the upper Precambrian but never made
it beyond the middle Paleozoic. They were so strange that they defied efforts
to classify them into existing taxonomic groups.
The animals were surprisingly complex with specialized adaptations for
living in a variety of environments. Some species appear to be surviving Edi-
acaran fauna,most of which became extinct near the end of the Precambrian.
Indeed, the Cambrian explosion might have been triggered in part by the
availability of habitats vacated by departing Ediacaran species. The Burgess
Shale fauna comprised more than 20 distinct body plans. A leechlike animal
called Pikaia(Fig. 54) was the first known member of the chordate phylum
and one of the rarest fossils of the Burgess Shale Formation, which contains
the best-preserved Cambrian fauna.It had a stiff dorsal rod called a notochord
along the back made of cartilage that supported organs and muscles,the pre-
decessor of the spine in vertebrates.
The Burgess Shale fauna originated in shallow water on a gigantic
coral reef that dwarfed Australia’s Great Barrier Reef, the single most mas-
sive structure built by present-day living beings. The ancient reef sur-
rounded Laurentia, the ancestral North American continent, and was
covered with mud that readily trapped and fossilized organisms. Most
occurrences originated from the western Cordillera of North America, an
ancient mountain range that faced an open ocean in the middle Cambrian.
Similar faunas existed on other cratons, including the North and South
China blocks, Australia,and the East European platform.Their widespread
distribution around other continents suggests many members had a swim-
ming mode of life.
79CAMBRIAN INVERTEBRATES