Cambrian Period free essay sample

The Cambrian Period marks the beginning of the Paleozoic Era. This period gets its name from a place in Wales where the first examples of this type of ancient life was found. The period lasted for nearly 53 million years, from about 543 million years ago until 490 million years ago. The Cambrian Period marks an important point in the history of life on earth; it is the time when most of the major groups of animals first appear in the fossil record. This event is sometimes called the Cambrian Explosion, because of the relatively short time over which this diversity of forms appears. It was once thought that the Cambrian rocks contained the first and oldest fossil animals, but these are now to be found in the earlier Vendian strata. Subdivisions The chart at right shows the major subdivisions of the Cambrian Period for North America (Laurentia during the Cambrian). International ages (subdivisions) have not been established. The size of the bars does not correlate with the length of time for each age. The oldest unnamed age is 543 to 520 million years ago, while the remaining six ages are from 520 to 490 million years ago, each approximately 5-6 million years long. Tectonics and Paleoclimate The Cambrian follows the Vendian period, during which time the continents had been joined in a single supercontinent called Rodinia (from the Russian word for homeland, rodina). As the Cambrian began, Rodinia began to fragment into smaller continents, which did not always correspond to the ones we see today. Green represents land above water at this time, red indicates mountains, light blue indicates shallow seas of the continental shelves, and dark blue denotes the deep ocean basins. The Cambrian world was bracketed between two ice ages, one during the late Late Proterozoic and the other during the Ordovician. During these ice ages, the decrease in global temperature led to mass extinctions. Cooler conditions eliminated many warm water species, and glaciation lowered global sea level. However, during the Cambrian there was no significant ice formation. None of the continents were located at the poles, and so land temperatures remained mild. In fact, global climate was probably warmer and more unifrom than it is today. With the beginning of the Cambrian at the retreat of Proterozoic ice, the sea level rose significantly. Lowland areas such as Baltica were flooded and much of the world was covered by epeiric seas. This event opened up new habitats where marine invertebrates, such as the trilobites, radiated and flourished. Plants had not yet evolved, and the terrestrial world was therefore devoid of vegetation and inhospitable to life as we know it. Photosynthesis and primary production were the monopoly of bacteria and algal protists that populated the worlds shallow seas. Also during the Cambrian, oxygen first mixed into the worlds oceans in significant quantity. Although there was plentiful atmospheric oxygen by the opening of the Cambrian, only in the Cambrian did the numbers of oxygen-depleting bacteria reduce in numbers sufficiently to permit the high levels we know today. This made dissolved oxygen available to the diversity of animals, and may have triggered the Cambrian Explosion. This was when most of the major groups of animals, especially those with hard shells, first appear in the fossil record. Localities Rocks of Cambrian age are distributed in the Great Basin of the western United States, parts of the northeastern United States, Wales, Scandinavia and the Baltic region, Siberia, and China, among other places. These localities were not where they are now: the position of the continents was very different. It may seem strange to imagine California on the equator, or Venezuela near the South Pole, but thats how things were! Cambrian localities on this server: * Aldan River Lower Cambrian fauna from this site in Yakutia, Siberia, trace the early evolution of animals with skeletons. Burgess Shale One of the greatest fossil finds ever made is the Burgess fauna of British Columbia. Thousands of soft-bodied animal fossils paint us a picture of early marine life. * House Range, Utah A varied array of Cambrian critters has been found in the Wheeler Shale and the Marjum Formation, both of which are exposed in the House Range. * Marble Mountains In the hottest part of the Mojave Desert of C alifornia is the rich Latham Shale where Olenellid trilobites are numerous. * White-Inyo Mountains You can visit ancient reefs in the mountains of eastern California. Cambrian life Explosion of Invertebrate Life During the Cambrian Period there was an explosion of life forms. Most of these were in the water. Many animals with no backbones lived in the shallow seas. These animals were invertebrates. Almost every metazoan phylum with hard parts, and many that lack hard parts, made its first appearance in the Cambrian. The only modern phylum with an adequate fossil record to appear after the Cambrian was the phylum Bryozoa, which is not known before the early Ordovician. A few mineralized animal fossils, including sponge spicules and probable worm tubes, are known from the Vendian period immediately preceding the Cambrian. However, the Cambrian was nonetheless a time of great evolutionary innovation, with many major groups of organisms appearing within a span of only forty million years. Trace fossils made by animals also show increased diversity in Cambrian rocks, showing that the animals of the Cambrian were developing new ecological niches and strategies such as active hunting, burrowing deeply into sediment, and making complex branching burrows. Finally, the Cambrian saw the appearance and/or diversification of mineralized algae of various types, such as the coralline red algae. This does not mean that life in the Cambrian seas would have been perfectly familiar to a modern-day scuba diver! Although almost all of the living marine phyla were present, most were represented by classes that have since gone extinct or faded in importance. Cambrian echinoderms were predominantly unfamiliar and strange-looking types such as early edrioasteroids, eocrinoids, and helicoplacoids. The more familiar starfish, brittle stars, and sea urchins had not yet evolved, and there is some controversy over whether crinoids (sea lilies) were present or not. Even if present, crinoids were rare in the Cambrian, although they became numerous and diverse through the later Paleozoic. And while jawless vertebrates were present in the Cambrian, it was not until the Ordovician that armored fish became common enough to leave a rich fossil record. Other dominant Cambrian invertebrates with hard parts were trilobites (like the one on the upper left, Nevadella from the Lower Cambrian of southwest Nevada); archaeocyathids (relatives of sponges that were restricted to the Lower Cambrian), and problematic conical fossils known as hyolithids (like the one on the upper right, also from the Lower Cambrian of Nevada). Many Early Cambrian invertebrates are known only from small shelly fossils tiny plates and scales and spines and tubes and so on. Many of these were probably pieces of the skeletons of larger animals. Arthropods Many of the Cambrian creatures appear to be arthropods, the large classification of animals that includes insects, spiders, crabs, and lobsters. An animal similar to the lobopod, Aysheaia might represent a step in this transformation: It appears to be a worm with ‘legs’, which are lobopods. A lobopod consists of muscles surrounding a blood-filled cavity. Lobopods are soft and pliable but they accomplish the purpose of moving the animal along. Each lobopod has a couple of claws at its end and the front two have spikes which apparently serve the purpose of grasping prey. So, what we find in the fossil record are animals with all stages of the transition between worms and arthropods: 1. worms; 2. lobopods; 3. Anomalocaris with both lobopods and arthropod appendages and possible incipient arthropod gills; and 4. arthropods. Trilobites The trilobite was an arthropod with a tough outer skin. It got its name from the three lobes in the hard skin. The trilobite was also one of the first animals to have eyesight. During the Cambrian there were more than 100 types of trilobites. Brachiopods Brachiopods are another distinct Cambrian animal form. The evolutionary lineage of the Brachiopods appears to begin with a creature called Halkieria Evangelista from the Sirius Passet formation of Greenland which is about 525 million years ago. This animal has three broad zones of scales. These scales are hollow and are inserted into the animals body in such a fashion as to make a chainmail-like armor covering the body of the metazoan. These small scales overlap and make a flexible armor with which to defend against attack. The back of the animal has palm-shaped scales, the sides have knife-shaped of scales and the edges of the belly have sickle-shaped scales. This will become important later when we talk about an animal transitional to the annelids. The bottom of Halkieria was soft, without scales and could best be described as being like a snails bottom. It enabled the animal to glide across the ocean floor. The shells at the ends of Halkieria are very similar to the shells of the brachiopods seen in the earliest Cambrian strata. What is believed to have taken place is that the two shells at each end of Halkieria were used by this animal as protection devices. The animal would curl up between the plates when threatened. There are other facts that support this interpretation. The edges of brachiopod shells have bristles, called setae, which extend away from the shell. In some of the brachiopods the setae are segmented as is the case with Halkierian scales. And finally, Neocrania, a modern brachiopod begins life as mobile animal that moves across the ocean floor, bearing the setae of a brachiopod but lacking the shell. Eventually it settles down, folds itself in the middle, secretes a shell and remains fixed like others of their kind. Other Invertebrates There were plenty of other species living during the Cambrian Period also. Mollusks, worms, sponges and echinoderms filled the Cambrian seas. No Backbones Yet, But There was even an early type of chordate living during the Cambrian Period. It was the Pikaia. Pikaia looked a bit like a worm with a long fin on each side of its body. The nerve cord was visible as a ridge starting behind the head area and extending almost to the tip of the body. The Top of The Food Chain One of the most fearsome hunters in the Cambrian seas was the Anomalocaris. This animal had an exoskeleton like an arthropod, but it did not have the jointed legs that would make it a true arthropod. This large animal fed on trilobites and other arthropods, worms and mollusks. Sponges Sponges grew in Cambrian seas, too. These animals belong to the phylum porifera because of all the tiny pores in their bodies. One species of sponge from this period had many branches that made it look like a tree. Another type of sponge looked like an ice cream cone-without the ice cream, of course! Many of the sponges became extinct when temperatures dropped at the end of the Cambrian period. Hard Parts Many of the creatures living in the Cambrian seas developed hard structures for defense, hard shells, scales, and spikes covering the outside of the body. The Wiwaxia lived on the bottom of the sea. The dorsal side of its body had scales and spikes for protection. The underside of Wiwaxia was soft and unprotected. Trilobites also living on the bottom could burrow under the Wiwaxia and attack the defenseless belly. Hallucigenia stood on seven pairs of tall legs. Its long, tube-shaped body had two rows of tall spikes along its back. This type of protection would have been very important for the animal because it had no eyesight to warn it of dangers. Plants The plants of the Cambrian were mostly simple, one-celled algae. The single cells often grew together to form large colonies. The colonies looked like one large plant. Mass Extinction The Cambrian Period began with an explosion of life forms. It ended in a mass extinction. Advancing glaciers would have lowered the temperature of the shallow seas where so many species lived. Changes in the temperature and the amount of oxygen in the water would have meant the end for any species that could not adapt. Why The most fundamental question about the Cambrian Explosion is why did it happen. There are many ideas, and of course it is possible that more than one of these effects worked together. 1. Climate Change   It appears that the period prior to the Cambrian era was marked by very cold temperatures. In fact, a somewhat controversial theory, termed Snowball Earth, claims that there was so much ice that much of the sun light was reflected and we came close to entering an irreversible downward spiral in temperature as less and less of the suns energy was absorbed by the surface of the planet. Thus, the Vendian/Ediacaran life forms would have been contending with very unfavorable conditions. Under this theory, the seeds of the Cambrian Explosion were planted during this earlier period, and when there were vast new possibilities for life in new and more favorable conditions, an explosion of new life forms occurred to fill these new niches. 2. Development of Predators PreCambrian fossils show little evidence for predators. Clearly, predators were prevalent and highly dangerous to Cambrian life. Evolution responds to challenge. Predators require many adaptations to find their prey ability to move rapidly toward a target without losing track of it, hunting strategies, grasping the prey all of which would also lead to development of brains and advances in nervous systems. The necessity to fend off predators to survive would have also triggered a great acceleration in the rate of evolutionary change in other animals, including the development of protective outer armor (shells, or the scale-like structures and spines of the Wiwaxia). 3. Evolutionary Breakthrough Perhaps some new feature developed by chance during evolution of the precambrian life forms that provided a huge advantage. An example would be the development of an exoskeleton that would provide rigidity against which muscles could work, making it suddenly advantageous for complex animals to develop. 4. Changes in Ocean Chemistry or in Geography The Cambrian occurred at the time plate tectonics were breaking up a super-continent, and also at a time when a large amount of phosphorus was being deposited on the shallow undersea shelves surrounding the continents.