Cambrian Invertebrates Reference Materials

CAMBRIAN INVERTEBRATES This chapter discusses invertebrate life forms and landforms from the lower Paleozoic era. The Cambrian period from 545 to 500 million years ago was named for a mountain range in central Wales, Great Britain that contained sediments with the earliest known fossils. Nineteenth-century geologists often puzzled why ancient rocks were practically without fossils until the Cambrian, when life suddenly sprung up in great abundance around the world. Therefore, the basis of the Cambrian (Figure 44) is considered the beginning of life, and throughout the times before it was simply called the Precambrian. This period is generally calm in terms of geological processes, with little mountain building, volcanic activity, glaciation, and climate extremes. The breakup of the late Precambrian supercontinent Rodinia and the flooding of the continent with inland seas created abundant, warm, and shallow shallow-water habitats, prompting an explosion of new species. For more than 80 percent of its history, life has remained almost exclusively unicellular. Then less than 600 million years ago, multicellular organisms suddenly appeared. Never before or since have there been so many new and unusual organisms; Surprisingly, no one has an equivalent in today's living world. CAMBRIAN EXPLOSION The Cambrian was the heyday of species evolution, featuring the rapid evolution of 10 million years from the first complex animals with exoskeletons. The Cambrian witnessed the first appearance of almost all animal phyla in the fossil record. After 3 billion years of unicellular life, a short period of intense creativity was followed by more than 500 million years of variations on anatomical themes set in the early Cambrian. So many new and diverse life forms emerged that this epoch has been described as a Cambrian explosion. The Cambrian explosion is the most remarkable and puzzling event in the history of life. The early Cambrian witnessed the loss of softbodied Ediacaran fauna and the rapid proliferation of clam fauna (Fig. 45). Biological proliferation peaked about 530 million years ago, filling the oceans with a wide array of life. It seems out of nowhere and in mind-boggling abundance, animals appear in very short spans of time shrouded in a bewildering array of exoskeletons. The introduction of hard skeleton parts has been called the largest discontinuity in Earth's history. It signifies a major evolutionary change by accelerating the rate of development of new organisms. Almost all large groups of modern animals appear in the fossil record. For the first time, animals put on shells, skeletons, legs and sensing antennae. In addition, a stable environment allows marine life to develop and spread to all parts of the world. This period followed the great Precambrian ice age, the worst the world had ever experienced, when ice sheets covered half the globe. It was also the time when the oxygen concentration rose to a significant level. After the ice receded and the sea began to warm, life took off in all directions. The Cambrian saw the highest percentage of experimental organisms than any other interval of geological history, with phyla perhaps three times more numerous than today. In the early Cambrian, the alternation of the ocean may have brought an unusual amount of nutrient-rich bottom water to the surface. Due to the increased calcium levels of seawater, early soft-bodied creatures developed skeletons as containers for the disposal of excess amounts of this toxic mineral from their tissues. As the concentration of calcium in the ocean increases, animal skeletons become more diverse and complicated. The oxygen level of the atmosphere seems to increase with the revolution of the skeleton. Higher oxygen levels increase metabolic energy, allowing for greater animal growth. This, in turn, requires stronger structural support. The skeleton also evolved in response to an incoming wave of ferocious predators. Paradoxically, most of these predators were soft-bodied and therefore poorly preserved as fossils. Soft-bodied organisms that lived before the arrival of shelled animals in the early Cambrian had great difficulty entering the fossil record. Animals with soft body parts rot quickly after death. Therefore, only traces of their existence remain, such as traces, tracks, and boring. These include fossil traces of softbodied animals found in the Ediacara Hills in southern Australia that date back to about 670 million years ago. The Ediacaran fauna arose from adaptation to extremely unstable conditions during the late Precambrian. Overspecialization under a narrow range of environmental conditions led to the great extinction of Ediacaran species about 540 million years ago at the beginning of the Cambrian explosion. Marine animals that survived death were very different from their Ediacaran ancestors and participated in the largest diversification of new species the world has ever known. In the early Cambrian, 125 million years after the emergence of the Ediacaran fauna, which was largely an evolutionary dead end, the seascape suddenly changed with the sudden arrival of animals with hard skeletal parts. Most phyla of living organisms appeared almost simultaneously, many of which originated in the late Precambrian. The body styles evolved in the Cambrian largely served as a blueprint for the modern species, with several radical body plans only emerging since then. As skeletons evolved, the number of organisms preserved in the fossil record skyrocketed. All known animal phyla that easily fossilized appeared during the Cambrian, after which the number of new classes sharply decreased. Fossils became abundant early in the period due to the development of fossilized hard body parts by replacement with calcium carbonate or silica, rapid excavation to prevent attack by scavengers and decay by oxidation, long periods of deposition with little erosion, and large populations of the species.