Cambrian Paleontology Tutorial Notes

CAMBRIAN PALEONTOLOGY Early sponges lived at a crucial point in Earth's history, when the ancestors of most modern animal groups suddenly appeared in the ocean. Ancient sponges lived like their modern counterparts. They filter nutrients from seawater, thus providing the earliest example of filter feeding, which is a very common way of life in modern seas. The existence of sponges before the Cambrian suggests that more complex animals must have evolved by the time the Ediacaran fauna appeared around the world. The sponge skeleton consisted of tiny glass spikes that closely resembled the spicules of some current sponges, suggesting little change had occurred during the intervention period unlike in most other groups. Glass spiked sponges, however, are very different from the types used in bathtubs. Coelenterates, from Greek meaning "intestine," is well represented in the Cambrian sea. They are the most primitive animals and include jellyfish, sea anemones, sea pens, and corals. Most coelenterates are radially symmetrical, with parts of the body radiating outward from a central point. They have a pouch-like body and a mouth surrounded by tentacles. Paleontologists have not found animal fossils in rocks that are more than 600 million years old. However, they have found the oldest preserved jellyfish animal embryos in small fossil eggs from early Cambrian discoveries. The discovery suggests that evidence for billion-year-old complex animals can be found by searching for embryos in ancient rocks. Primitive and radially symmetrical animals have only two types of cells, ectoderm and endoderm. In contrast, bilaterally symmetrical animals also have different mesoderms and intestines. During early cell division called division in bilateral animals, the fertilized egg forms two and then four cells, each of which produces several small cells. Many types have two stages of development, consisting of stationary polyps attached to the seafloor with a base with tentacles and a mouth directed upwards, and a mobile umbrella-shaped medusa or jellyfish-like phase with tentacles and mouth directed downward. Corals have a wide variety of skeletal shapes (Fig. 48), and subsequent generations build thick limestone reefs. Corals began to build reefs in the lower Paleozoic, forming island chains and barrier reefs along continental coastlines. Archaeocyathans resemble corals and sponges. However, they do not have a close relationship with living groups and thus belong to their own unique phylum. These cone-shaped creatures formed the earliest reefs, eventually extinct in the Cambrian. Many corals declined and were replaced by sponges and algae in the late Paleozoic due to the ocean recession in which they once thrived. Coral polyps are soft-bodied creatures that are essentially contract sacs crowned by rings of tentacles. The tentacles surround the hole like a mouth and are tipped with a poisonous sting. Polyps live in individual skeletal cups or tubes called thecae composed of calcium carbonate. They extend their tentacles to feed at night and withdraw to their thecae during the day or at low tide so as not to dry out in the Sun.Corals coexist symbiotically (in relationship) with zooxanthellae algae, which live inside polyp bodies. The algae consume coral waste products and produce organic matter that nourishes polyps. Some coral species receive 60 percent of their food from algae. Because algae need sunlight for photosynthesis, corals must live in warm, shallow water generally less than 100 feet and at temperatures between 25 and 30 degrees Celsius. Echinoderms, from Greek meaning "spiny skin," are perhaps the strangest animals ever preserved in the early Paleozoic fossil record. Their fivefold radial symmetry with arms radiating outward from the center of the body makes them unique among more complex animals. They are the only species to have a water vascular system consisting of internal canals that operate a series of tube legs, or podia, used for locomotion, feeding, and breathing. The great success of echinoderms is illustrated by the fact that more classes of these animals exist than any phylum both living and extinct. The main groups of echinoderms include starfish, brittle stars, sea urchins, sea cucumbers, and crinoids (Figure 49), known as sea lilies because of their plant-like appearance. Surprisingly, according to genetic studies, early echinoderms and chordates are more closely related to each other than other major phyla. Apparently, echinoderms and chordates branched off far from arthropods, annelids, and mollusks as early as 1.2 billion years ago. This happened long before the Cambrian explosion, when almost every large group of animals suddenly appeared in the fossil record. About 1 billion years ago, echinoderms and chordates separated. This suggests that slow-moving evolution was well underway half a billion years before the Cambrian explosion. Brachiopods (Fig. 50), also called lamp shells because of their resemblance to ancient oil lamps, were once the most abundant and diverse organisms. More than 30,000 species are catalogued in the fossil record. Productive brachiopods range from the Cambrian to the present but are most abundant in the Paleozoic and to a lesser extent in the Mesozoic.The appearance of fossil brachiopods in the strata suggests that moderate-to-shallow depths once covered the area. Brachiopods have two platelike shells called valves that are mounted face to face and opened and closed using simple muscles. Brachiopods filter food particles through a closed open shell to protect the animal from predators. More advanced species, including brachiopods called articulates, have ribbed shells with interlocking teeth that open and close along hinges. Many brachiopods are excellent index fossils for correlating rock formations around the world. They are important as guide fossils and are used to date many Paleozoic rocks. Mollusks are a very diverse group and leave behind the most impressive fossil record of all marine animals (Fig. 51). They form the second largest of the 21 basic animal groups, behind arthropods. This phylum is so diverse that paleontologists have difficulty finding common features among its members. The three main groups are snails, clams, and cephalopods. Snails and snails comprise the largest group and range from the Cambrian onwards. An ancient mollusk called kimberella (Figure 52) was a strange Precambrian creature shaped like a gherkin cucumber and lived just before the Cambrian explosion. Cephalopods, which include squid, cuttlefish, octopus, and nautilus, travel on propulsion jets. They suck water into the cylindrical cavity through openings on each side of the head and expel it under pressure through funnel-like appendages. Its straight, slender shell, up to 30 feet and longer, made nautiloids among the fastest animals in ancient seas. Ammonoids are the most spectacular marine predators, with a wide variety of circular shell shapes. Arthropods are the largest phylum of living organisms, comprising about 1 million species, or about 80 percent of all known animals. The 3-foot-long giant arthropod found in the Cambrian Central Cambrian Shale Formation of western Canada is one of the largest of all Cambrian invertebrates. Among the first and most famous ancient arthropods were trilobites. The body of arthropods is segmented, showing a relationship with annelid worms. Paired and jointed limbs are generally present on most segments modified for sensing, feeding, walking, and reproduction. Crustaceans appeared in the early Cambrian and soon became the dominant invertebrates. They occupied shallow waters near the coast of the ancient sea, which flooded the land to provide a wide continental margin. Crustaceans are primarily aquatic arthropods and include shrimp, lobsters, barnacles, and crabs (Fig. 53). Ostracods, or clam shrimp, are small crustaceans found in marine and freshwater environments. Their fossils are useful for correlating rocks from the early Paleozoic onwards, which makes them very important to geologists. Conodonts are fossil, small, jaw-like appendages that typically occur in marine rocks ranging from the Cambrian to the Triassic and are important for dating Paleozoic rocks. They are one of the most puzzling fossils and have baffled paleontologists since the 1800s. Paleontologists began to find these isolated tooth-like objects in rocks from the late Cambrian through the Triassic period. Conodonts are considered bone appendages of unusual soft-bodied animals resembling hagfish. However, the shape of the lost creature remained unclear until 1983, when paleontologists discovered a tooth-like cut at the front end of an eelshaped fossil from Scotland. In addition, the presence of distinctive eye muscles in conodonts unknown to invertebrates pushes the vertebrate fossil record as far away as the Cambrian. Conodonts showed their greatest diversity during the Devonian and were important for the correlation of long-distance rocks in that period. Graptolit are colonies of cupped organisms that resemble the stems and leaves of plants but are actually animals, with individual organisms placed in small cups. They stick to the seabed like small shrubs, float freely near the surface, look like small saw blades, or are attached to seaweed. Large amounts of graptolite buried in bottom mud produce organic-rich black shale indicating poor oxygen conditions. They are important markers for connecting lower Paleozoic rock units.