British geologists Adam Sedgwick and Roderick Impey Murchison studied the complex geology of western Wales. In 1835 both Sedgwick and Murchison named the rocks they studied for ancient Welsh tribes: Sedgwick used “Cambrian” and Murchison used “Silurian.” Each worker attempted to recognize breaks in the stratigraphic record as boundaries for his subdivision. Murchison began with the top of the sequence in the southeast; Sedgwick began at the base in the northwest. Murchison carefully documented the abundant fossils of these Silurian strata. Sedgwick’s strata were poorly fossiliferous, and his breakdown of the Cambrian System was essentially lithologic. When it became clear that their systems overlapped, a quarrel ensued because systems must be contiguous; that is, they do not overlap in time. The controversy was not resolved until 1879 when Charles Lapworth proposed the name “Ordovician System,” taken from another Welsh tribe, to include the disputed interval between Cambrian and Silurian. The Ordovician System rounded out the threefold paleontological division of the Early Paleozoic. The boundaries of Lapworth’s Ordovician System were based solely on its distinctive fossil content (Eicher 1976).
Originally geologists defined the beginning of the Cambrian Period as the point where fossils appeared. Subsequently older fossils have been found, and this definition is no longer valid. Now the start of Cambrian time and end of the Precambrian is roughly determined to be the point at which numerous hard-shelled fossils first appeared.
Significant Cambrian events
In the latest Precambrian and Cambrian, the supercontinent Rodinia, which was centered about the South Pole, broke apart, and crustal blocks drifted northward. The largest fragment was Gondwana (a collection of today’s southern continents, including Antarctica, South America, Africa, Madagascar, Australia-New Guinea, and New Zealand, as well as Arabia and India, which are in the Northern Hemisphere today). The second-largest continent, Laurentia, included most of North America, though the southeastern United States was wedged between Africa and South America as part of Gondwana. Siberia (just south of the equator) and Baltica (Scandinavia, Eastern Europe, and Russia) were situated between Gondwana and Laurentia. The rest of Europe and much of what is present-day Asia was split into fragments along the north coast of Gondwana.
The Cambrian Period marks an important point in the history of life on Earth; it is the time when many kinds of invertebrates and the first vertebrates—fishes—appeared in the fossil record. The Burgess Shale contains the best record of Cambrian animal fossils including soft-bodied forms. This locality reveals the presence of creatures originating from the “Cambrian explosion”—an evolutionary burst of animal origins dating from 545 to 525 million years ago. The “explosion” describes the very rapid proliferation of a truly amazing diversity of living things on Earth. Most of these creatures are now extinct and are known only from their fossils.
During Cambrian time, life was only common in the watter. The land was barren and subject to erosion; these geologic conditions led to mudslides, where sediment periodically rolled into the seas and buried marine organisms. At the Burgess Shale locality in the Canadian Rocky Mountains, sediment was deposited in a deep-water basin adjacent to an enormous algal reef with a vertical escarpment several hundred feet high. Though not in a U.S. national park, the type locality of the Burgess Shale—Burgess Pass—is located in British Columbia’s Yoho National Park. In order to protect the site, UNESCO designated the Burgess Shale as a world heritage site in 1981.
Explosion of life
The discovery of fossils from the Precambrian revealed that life evolved long before the Cambrian Period began. Nevertheless, two things make the Cambrian Period notable. First, life exploded with almost all the major groups evolving in a relatively short time (about 40 million years). Second, the rise of animals with hard shells meant they had a much better chance of becoming fossilized. The explosion of life in the Cambrian Period is particularly apparent in oceanic fauna, which was without precedent in Earth’s history.
During the Cambrian, land plants had not yet evolved, so the terrestrial world was devoid of vegetation. However, in the oceans many marine invertebrates, including sponges and brachiopods (lamp shells), were present. Also, the first animals with backbones arose during the Cambrian: these were jawless fish called "agnathans". They were strongly armored creatures with bony skeletons. Most of their fossil remains are pieces of bony exterior plates. Because they lacked biting jaws, they were probably bottom dwellers that fed by filter feeding. A few descendants of jawless fish survive today, for example, the lamprey “eel” (Macdougall 1996).
Probably the best-known Cambrian animals were trilobites—a group of armored invertebrates that no longer exist. They were abundant in shallow Cambrian seas, which covered much of the world. Paleontologists have identified many species of Cambrian trilobites. Moreover, from the types of sediments in which the trilobites are fossilized, paleontologists have been able to determine something of their lifestyles. Trilobites included swimming forms; bottom dwellers; varieties that lived in warm, shallow waters; and those that lived in deeper, cooler regions. All species had hard, calcified external skeletons, which allowed them to be preserved in many Cambrian sedimentary rocks.
As the Cambrian progressed, there was significant turnover in the animals. Groups that had appeared early in the Cambrian, such as reef-building sponges, many-limbed Anomalocaris and its relatives, and many others that are hard to place, dwindled or went extinct altogether. Life at the end of the Cambrian was relatively less diverse than earlier in the period.
Every park contains some slice of geologic time. Below, we highlight selected parks associated with the Cambrian Period. This is not to say that a particular park has only rocks from the specified period. Rather, rocks in selected parks exemplify a certain event or preserve fossils or rocks from a certain geologic age.
- Chesapeake and Ohio Canal National Historical Park, West Virginia, Washington D.C., and Maryland—[Geodiversity Atlas] [Park Home]
- Death Valley National Park, California, Nevada—[Geodiversity Atlas] [Park Home]
- Great Smoky Mountains National Park, Tennessee and North Carolina—[Geodiversity Atlas] [Park Home]
- Harpers Ferry National Historical Park, Maryland, Virginia, and West Virginia—[Geodiversity Atlas] [Park Home]
- Yellowstone National Park, Idaho, Montana, Wyoming—[Geodiversity Atlas] [Park Home]