Ordovician Vertebrates

ORDOVICIAN VERTEBRATES This chapter examines early vertebrate life forms and the geology of the Ordovician period. The Ordovician, from 500 to 435 million years ago, was named for the ancient Ordovices tribe of Wales, Great Britain. Ordovician marine sediments are recognized on all continents of the Northern Hemisphere, in the Andes Mountains of South America, and in Australia. However, they are absent in Antarctica, Africa and India. Ordovician terrestrial deposits are not easy to recognize due to the lack of fossil terrestrial organisms. Many species that burst onto the scene in the early Cambrian advanced significantly in the warm Ordovician seas (Fig.60). Corals began to build extensive carbonate reefs in the Ordovician.The first fish appeared in the ocean. The presence of freshwater jawless fish suggests that unicellular plants, including red and green algae, inhabit lakes and rivers on land. JAWLESS FISH Beginning about 520 million years ago, the first vertebrates appeared on the scene. They have an internal skeleton made of bone or cartilage, one of the most significant life advances. The skeleton of vertebrates is light, strong, and flexible, with efficient attachment of muscles. The biggest advantage is that the skeleton grows together with the animal. Instead, invertebrates must shed their external skeletons to grow, putting them in grave danger to predators. This new internal skeleton allows a wide distribution of free-swimming species to various environments. The most primitive chordates, including vertebrates, are small, fish-like oddities called amphioxus. Although the animal has no actual backbone, it is nevertheless placed in a direct line to the vertebrate. The earliest vertebrates lacked jaws, paired fins on either side of the body, or true vertebrae and shared many of the characteristics of modern lampreys. The origin of vertebrates led to the evolution of one of the most important novelties—the head. It is packed with paired sensory organs, a complex three-part brain, and many other features missing in invertebrates. The traditional view holds that the eye has evolved independently several times in the past. However, the discovery of genes shared by fruit flies, squid, mice, and humans suggests that the eye probably evolved only once in the evolutionary history of life. The complex eyes of modern animals may have originally evolved from light-sensitive nerve cells that eventually evolved into specialized organs that could focus on specific objects such as prey. Only a small percentage of the major animal groups have true eyes, however, with 6 out of 30 phyla having complex eyes capable of providing images. However, having eyes is such an evolutionary advantage that species with complex eyes comprise 95 percent of the animals on Earth. Invertebrates, supported by an external skeleton, are at a disadvantage in terms of mobility and growth. Many animals such as crustaceans shed their shells as they grow, which often makes them vulnerable to predators. One such predator was an extinct giant sea scorpion called a eurypterid (Fig. 61), which ranged from the Ordovician to the Permian. It grew to be 6 feet long and a terror creature on the bottom of the sea with huge tongs. The earliest vertebrates were probably worm-like creatures with protruding rods called notochords on the back, the nervous system along the spine, and rows of muscles attached to the spine arranged in a banded pattern. A rigid structure made of bone or cuticle acts as a lever. Using flexible joints, they efficiently translate muscle contractions into organized movements such as rapid lateral flicks of the body to propel the animal through the water. Later, the tail and fins evolved for stabilization. The body became slimmer and torpedo-shaped for speed. With intense competition among stationary and slow-moving invertebrates, any advance in mobility is advantageous for vertebrates. The oldest known vertebrates are primitive, jawless fish called agnathans (Figure 62), which first appeared in the early Ordovician about 470 million years ago. The remains of this very well-preserved fish were found in the mountains of Bolivia, most of which were flooded by the sea in the Ordovician. Initially, fossil evidence was scant and fragmentary. Little is known about their appearance or about their evolutionary history. Previous descriptions dismissed agnathan as a mass of scales and plates without a head, tailless or confusing head with tail, giving it the dubious title "underdeveloped fossil." The wide distribution of primitive fish fossils around the world shows a long vertebrate record before the Ordovician.The first fish were small mud and sea spray that lacked jaws and teeth. The jawless fish, whose modern counterparts include lampreys and hagfish, had flexible trunks similar to cartilage, rather than the bony spine typical of most vertebrates. These ancient fish were probably poor swimmers and avoided deep water. They are generally small, about the size of small fish, and heavily armored with bony plates protecting the rounded head. The rest of the body is covered with thin scales that end near the narrow tail. Although well protected from invertebrate predators, the extra weight forces fish to live mostly at the bottom, where they sift sludge for food particles and excrete waste products through gaps on either side of the throat that then become gills. Gradually, the protofish acquires jaws and teeth, bone plates give way to scales, lateral fins develop on both sides of the lower body for stability, and the air bladder maintains buoyancy. Some primitive fish were surprisingly large, reaching 18 inches in length and 6 inches in width. So, for the first time, vertebrates skillfully propelled themselves through the sea. Fish soon became masters of the depths.