Paleontology

An invertebrate is essentially a multicellular animal that lacks a spinal column encased in vertebrae and a distinct skull. There are about 30 phyla, or groups, of invertebrates, and roughly 20 of these have been preserved as fossils. Still other phyla probably existed, but are not represented in the fossil record because the animals' soft bodies were not preserved. Only one invertebrate phylum is known to have become extinct—the Archaeocyathida. These organisms, which were superficially similar to sponges, did not survive past the Middle Cambrian period (530 million years ago).

The different body plans of invertebrates most likely evolved during the Precambrian, between 1,000 and 700 million years ago. There was an "explosion" of invertebrate evolution in the Lower Cambrian (beginning about 570 million years ago), which lasted perhaps only 10 million years. During this time the different phyla, including those existing today, developed. Meanwhile, the glaciers from a Proterozoic ice age were melting, raising sea levels above the continental shelves. This gave invertebrates more places to live. There are no fossils showing how the first invertebrates evolved; they just suddenly appear in the fossil record. This may be because they were evolving so quickly, and because they developed hard shells, allowing them to be preserved. Many organisms from this period were preserved in the Burgess Shale formation (530 million years ago) in British Columbia, Canada.

Figure 2. Eurypterid. Illustration by Hans & Cassidy. Courtesy of Gale Group.

Figure 3. Pikaia, the world's first known chordate; found at Burgess Shale. Illustration by Hans & Cassidy. Courtesy of Gale Group.

The sponges (phylum Porifera) appeared in the Middle Cambrian. The bodies of these "lower" invertebrates are neither symmetrical nor differentiated into tissues. Sponges are less evolutionarily advanced than members of the phylum Cnidaria, which includes jellyfish, corals, and sea anemones. These two phyla may have arisen directly and independently from the protists (simple onecelled organisms such as bacteria, algae, etc.).

The appearance of bilateral symmetry (two halves which are mirror images of each other) was an important evolutionary breakthrough. The most primitive bilaterally symmetrical animals are the flatworms (phylum Platyhelminthes). Platyhelminthes gave rise to the coelomates, which have a coelom, or internal body cavity. The coelomates split into two evolutionary lines, the protostomes (molluscs, annelids, and arthropods) and the deuterostomes (echinoderms and chordates). A few phyla, such as the phylum Bryozoa, are intermediate between the two lines. Of the nearly 20,000 species of bryozoans known, only 3,500 are still living.

The molluscs are a very diverse group of invertebrates. They include snails, chitons, and cephalopods (squids and octopuses). Recent studies of invertebrate genetic material has shown that molluscs and annelids (segmented worms) probably evolved from arthropods. Neopilina, which was discovered in 1957, is a modern, "primitive mollusc." It is similar to what the first molluscs are believed to have looked like. One of the main reasons molluscs have evolved so many different forms is that they have diverse methods of eating and of avoiding being eaten.

The arthropods (jointed foot) are the most successful group of organisms ever. They include centipedes, insects, crustaceans, horseshoe crabs, spiders, scorpions, and the extinct trilobites (Figure 1) and eurypterids (Figure 2). Arthropods evolved 630 million years ago. The trilobites lived for 350 million years, from the Lower Cambrian to the Late Permian, and developed into over 1500 genera. They had compound eyes with thin, biconvex lenses made of calcite. The last of the trilobites died out at the end of the Permian. The eurypterids were ancient water-scorpions. Most eurypterids were less than 7.9 in (20 cm) long, but some giant forms grew nearly 6.5 ft (2 m) long, making them the largest arthropods ever.

Insects colonized the land just after plants did, about 410 million years ago. Cockroaches and dragonflies appeared over 300 million years ago, and for the next 100 million years, insects were the only animals that could fly. The chelicerates (spiders, mites, scorpions, horseshoe crabs, and eurypterids) evolved in the Cambrian.

The echinoderms (phylum Echinodermata) include starfish, sea-urchins, sea cucumbers, and crinoids. A great many of these organisms were fossilized because they have skeletons made of calcite plates. The greatest number of different genera of echinoderms lived during the Carboniferous (360-286 million years ago). The embryology of modern echinoderms suggests that they are Figure 4. An ancient jawless fish, Hemicyclaspis. Illustration by Hans & Cassidy. Courtesy of Gale Group.
Figure 5. The coelacanth Latimeria. Illustration by Hans & Cassidy. Courtesy of Gale Group.
Figure 6. Ichthyostega. Illustration by Hans & Cassidy. Courtesy of Gale Group.
related to the chordates. Modern echinoderm larvae have a ciliated band that runs along both sides of their bodies. The "dipleurula theory" suggests that ancient adult echinoderms had this band also, and that in the ancestors of the chordates, it fused along the back to form the beginnings of the dorsal nerve. Pikaia (Figure 3), a worm-like creature found in the Burgess Shale, is the oldest known chordate. It lived in the Middle Cambrian (about 530 million years ago).

Vertebrates are a subphylum of the Chordata (chordates). Lower vertebrates have a notochord, which is a flexible cartilage rod that runs along their backs; this is replaced in higher vertebrates with a vertebral column. Vertebrates are also called craniate chordates because they are the only animals with a distinct cranium (skull). The oldest known vertebrate remains date from the Upper Cambrian and Lower Ordovician (around 505 million years ago).

The first vertebrates were fishes. They evolved primarily during the Devonian (408-360 million years ago). The earliest fishes did not have jaws (Figure 4). Unlike modern jawless fishes (hagfishes and lampreys), the extinct forms were heavily armored and had pairs of fins. Jaws may have evolved from gill arches near the head, but there are no transition fossils which show this. Rays and sharks, which have skeletons of cartilage instead of bone, are the most ancient living fishes with jaws. Bony fishes, such as coelacanths and the ancestors of most modern fresh and saltwater fishes, probably evolved early in the Devonian. Coelacanths (Figure 5) were believed to be extinct until a living one was discovered in 1938 in the Indian Ocean. Fishes thrived until the Late Devonian (360 million years ago), when a mass extinction wiped out 76% of fish families.

The amphibians were the first vertebrates to leave the seas for dry land. They evolved from fishes about 360 million years ago. One of the challenges to living on land is that animals must be able to support their own weight rather than simply allowing water to support them. The lobed fins of the bony fishes already contained the major bones that became the limbs of the early Figure 7. Two advanced pelycosaurs: Edaphosaurus (a) and Dimetrodon (b). Illustration by Hans & Cassidy. Courtesy of Gale Group.
tetrapods (four feet); many of these bones are still used in our own limbs today.

The earliest-known amphibian was the Ichthyostega (Figure 6). It and other early tetrapods probably ate invertebrates such as cockroaches and spiders, as well as little fishes. The diadectomorphs appear to have been somewhat transitional between amphibians and reptiles. They belong to a category of amphibians called reptiliomorphs (as opposed to the batrachomorphs, or "true" amphibians).

The first reptiles, which were about the size of small lizards, emerged about 300 million years ago. They had a crucial advantage over amphibians in that their eggs could hatch on land, freeing them from spending part of their lives in the water. Among these early reptiles were the ancestors of modern birds and mammals. The pelycosaurs (Figure 7) were the most varied of the Early Permian reptiles. Some had tall, skin-covered "sails" that may have helped regulate their body temperature. The main herbivores in the Late Permian were the dicynodonts, and the main carnivores were the gorgonopsians such as Arctognathus. Arctognathus had huge canines and could open its jaws 90°. The ancestors of the crocodilians arose in the late Triassic. Terrestrisuchus was small (1.64 ft [0.5 m] long), probably ate insects and small reptiles, and may have walked bipedally on its long hind legs. Unlike its descendants, it did not live in the water.

Triassic oceans were filled with placodonts, nothosaurs, and ichthyosaurs (Figure 8). Placodonts had heavy teeth, which were probably used to crush the hard shells of mollusks. Nothosaurs had pointed teeth in their small heads, and may have eaten fish. Ichthyosaurs (fish lizards) were shaped somewhat like giant porpoises with long, narrow jaws. They grew up to 49 ft (15 m) long in the Late Triassic, but were smaller during the Jurassic and Cretaceous. Plesiosaurs, along with ichthyosaurs, ruled the Jurassic and Cretaceous seas (Figure 9). Plesiosaurs were probably related to nothosaurs. Their paddles, however, were flat and like an airplane wing in cross-section, so that these animals may have moved through the water by "flying," the way penguins and sea turtles do.

The largest mass extinction of all time occurred at the end of the Permian (248-238 million years ago). All Figure 8. Ancient marine reptiles. Illustration by Hans & Cassidy. Courtesy of Gale Group.
but about ten tetrapod families died out, as well as 96% of marine species.

The dinosaurs arose in the Late Triassic (230 million years ago). The earliest dinosaurs walked upright on two legs and were carnivorous. Dinosaurs are divided into two groups, the Saurischia and the Ornithischia, based on their hips. The saurischians had "lizard hips" and the ornithischians had "bird hips." The lizard hips arose first; Triassic dinosaurs were saurischians. While saurischians included both carnivores (meat eaters) and herbivores (plant eaters), all of the ornithischians were herbivores. The carnivorous saurischians are known as the theropods. They included the Late Jurassic Allosaurus and Late Cretaceous Tyrannosaurus, which probably was the largest carnivore ever to walk the earth. Brachiosaurus was a herbivorous saurischian. This sauropod had a long neck and short, thick legs like an elephant's, which were designed for bearing its enormous weight. There were two kinds of bird-hipped dinosaurs, the Late Cretaceous Cerapoda and the Late Jurassic Thyreophora. The former included hadrosaurs (duck-billed dinosaurs) and ceratopsians ("horned faces"), and the latter included ankylosaurs and stegosaurs.

One of the major debates among paleontologists is whether or not dinosaurs were warm-blooded. Some scientists suggest that a four-chambered heart would have been necessary to pump blood up the long necks of the sauropods to their brains. Mammals and birds, which are warm-blooded, have four-chambered hearts, but so do the cold-blooded crocodilians. The debate has not been conclusively resolved.

There are two main models that attempt to explain the success of the dinosaurs. According to one, dinosaurs out-competed the mammal-like reptiles over a long period of time due to superior adaptations such as upright walking. The other model, which is supported by fossil evidence, says that the dinosaurs took advantage of openings created by two mass extinctions. By the end of the Triassic, dinosaurs had taken over the land. They were dominant for 165 million years, from the Late Triassic until their extinction at the Cretaceous-Tertiary (K-T) boundary some 65 million years ago. This massive extinction may have taken place in only a week or lasted for tens of thousands of years; this has not been determined yet, but further study may provide an answer. One prominent theory for the cause of this Figure 9. Late Jurassic plesiosaurs: Cryptoclidus (a & b) and Liopleurodon (c). Illustration by Hans & Cassidy. Courtesy of Gale Group.
Figure 10. The pterosaur Ramphorhynchus gemming. Illustration by Hans & Cassidy. Courtesy of Gale Group.
Figure 11. The first bird, Archaeopteryx. Illustration by Hans & Cassidy. Courtesy of Gale Group.
Researchers cleaning dinosaur fossils in a paleontology laboratory in Esperaza, France. The fossils arrive encased in a protective plaster cast and with some of the original surrounding rock still attached. They are cleaned thoroughly and treated with stabilizing chemicals before being studied or classified. Photograph by Philippe Plailly. Photo Research, Inc. Reproduced by permission.
event is that a meteorite hit the earth 65 million years ago, creating a cloud of dust which obscured the sun and prevented photosynthesis for several months. This set off a chain reaction which culminated in the death of many life forms on Earth.

The pterosaurs (winged reptiles) were closely related to the dinosaurs, and lived at the same time (Figure 10). They had short bodies with long necks, and pointed jaws. They ranged from the pigeon-sized Eudimorphodon to the largest of all flying creatures, Quetzalcoatlus, which is believed to have had a wing span of up to 49 ft (15 m). Thanks to some well-preserved pterosaurs, we know they had hair, and were therefore possibly warm-blooded.

Birds evolved from the theropod dinosaurs. The first bird, Archaeopteryx (Figure 11), lived in the Late Jurassic (150 million years ago). This small, magpie-sized bird had sharp teeth on both jaws, and feathers. The presence of teeth, claws on the fingers, and its bony tail are reptilian characteristics, whereas the feathers and "wishbone" (fused collarbones) are bird characteristics.

The ancestors of mammals were mammal-like reptiles called cynodonts. Cynodonts arose in the Late Permian (about 245 million years ago). Some were dogsized carnivores, and others were herbivores. One way the transition to mammals can be seen is in the manner in which the jaws are joined. Mammals have a new joint not present in reptile jaws, which allows for the side-to-side action of chewing. The earliest true mammals appeared in the Late Triassic (230 million years ago). One was the tiny, shrew-like Megazostrodon. Because of its size and the fact that it had pointed teeth, it was probably an insectivore. Mammals radiated widely in the Paleocene Epoch (66-58 million years ago), taking advantage of the openings left by the dinosaurs when they died out. The mammals were intelligent and took care of their young for an extended period of time, which probably gave them an edge over other animals. Mammals now appear in many very different forms, including the orders Insectivora (shrews), Carnivora (cats, dogs, seals), Chiroptera (bats), Cetacea (whales), Proboscidea (elephants), Tubulidentata (aardvarks), and Primates.

Figure 12. Illustration by Hans & Cassidy. Courtesy of Gale Group.