by John Pojeta, Jr. and Dale A. Springer 
Geologic time chart
Fossil Record
Change Through Time
Darwin's Theory
Mechanism for Change
Nature of Species
Nature of Theory
Paleontology, Geology & Evolution
Dating the Fossil Record
Examples of Evolution


References Cited

Suggested Readings
About the Authors

Paleontological Society


Evolution and the Fossil Record Examples of Evolution (Previous Page || Next Page)

The fossil record contains many well-documented examples of the transition from one species into another, as well as the origin of new physical features. Evidence from the fossil record is unique, because it provides a time perspective for understanding the evolution of life on Earth. This perspective is not available from other branches of science or in the other databases that support the study of evolution.

This section covers four examples of evolution from the incredibly rich and wonderful fossil record of life on Earth. We've chosen examples of vertebrates, animals with backbones, primarily because most of us identify more easily with this group rather than with sassafras or snails or starfish. However, we could have chosen any of many studies of evolutionary changes seen in fossil plants, invertebrates - animals without backbones such as the Chesapecten scallops (above), or single-celled organisms. We'll examine the evolution of legs in vertebrates as well as the evolution of birds, mammals, and whales.

Evolution of vertebrate legs

The possession of legs defines a group of vertebrate animals called tetrapods - as distinct from vertebrate animals whose appendages are fins, the fishes. In most fishes, the thin bony supports of the fins are arranged like the rays of a fan; hence these fishes are called 'ray-finned' fish. Trout, perch, and bass are examples of living ray-fins.

Certain fishes are called 'lobe-finned,' because of the stout, bony supports in their appendages. Lobe-finned fish first appear in the fossil record in early Late Devonian time, about 377 mya. The bony supports of some lobe-finned fishes are organized much like the bones in the forelimbs and hind limbs of tetrapods: a single upper bone, two lower bones, and many little bones that are the precursors of wrist and ankle bones, hand and foot bones, and bones of the fingers and toes that are first known in Late Devonian amphibian-like animals from about 364 mya. These animals were the first tetrapods. Many similarities also exist in the skull bones and other parts of the skeleton between Devonian lobe-finned fishes and amphibian-like tetrapods. In fact, in certain fossils the resemblances are so close that the definition of which are fish and which are tetrapods is hotly debated.

In 1998, a lobe-finned fish was described from Upper Devonian rocks from about 370 mya in central Pennsylvania (Daeschler and Shubin, 1998). This fish has bones in its forelimb arranged in a pattern nearly identical to that of some Late Devonian amphibian-like tetrapods. The pattern includes a single upper-arm bone (humerus), two forearm bones (radius and ulna), and many little bones connected by joints to the forearm bones in the positions of wrist and finger bones. However, the finger-like bones look like unjointed fin rays, rather than the truly jointed finger bones of tetrapods. Should the animal be called a fish or a tetrapod? It's hard to say. On the basis of the finger bones, it could be classified as a fish, whereas, on the basis of the large limb bones, the animal could be classified as a tetrapod.

Remember that we humans created the classification scheme for life on Earth, and we choose where to draw the boundaries. When dealing with transitional forms of life this is not an easy task!

Evolution of birds

Most paleontologists regard birds as the direct descendants of certain dinosaurs - as opposed to descendants of some other group of reptiles. Paleontologists and zoologists have long accepted that birds and reptiles are related. The two groups share many common traits including many skeletal features, the laying of shelled eggs, and the possession of scales, although in birds, scales are limited to the legs. Among modern birds, the embryos even have rudimentary fingers on their wings. In one modern bird, the South American hoatzin, Opisthocomus hoazin, the wings of the juvenile have large moveable claws on the first and second digits. The young bird uses these claws to grasp branches.

The descent of birds from dinosaurs was first proposed in the late 1860s by Thomas Henry Huxley, who was a famous supporter of Darwin and his ideas. Evidence from fossils for the reptile-bird link came in 1861 with the discovery of the first nearly complete skeleton of Archaeopteryx lithographica in Upper Jurassic limestones about 150 million years old near Solenhofen, Germany. The skeleton of Archaeopteryx is clearly dinosaurian. It has a long bony tail, three claws on each wing, and a mouth full of teeth. However, this animal had one thing never before seen in a reptile - it had feathers, including feathers on the long bony tail. Huxley based his hypothesis of the relationship of birds to dinosaurs on his detailed study of the skeleton of Archaeopteryx.

One of the leading scholars of the bird-dinosaur relationship is John Ostrom of Yale University, who has summarized all the details of the skeletal similarities of Archaeopteryx with small, bipedal Jurassic dinosaurs such as Compsognathus. Compsognathus belongs to the group of dinosaurs that includes the well-known Velociraptor, of Jurassic Park fame, and Deinonychus, which Ostrom called the ultimate killing machine. The skeleton of Archaeopteryx is so similar to that of Compsognathus that some specimens of Archaeopteryx were at first incorrectly classified as Compsognathus. Ostrom regarded Archaeopteryx as being on the direct line of descent of birds from reptiles.

New fossil specimens from Mongolia, China, Spain, Argentina, and Australia have added to our knowledge of the early history of birds, and many paleontologists now reckon that the turkey on our Thanksgiving tables is a descendant of the dinosaurs.

Evolution of mammals

The oldest reptiles having mammal-like features, the synapsids, occur in rocks of Pennsylvanian age formed about 305 mya. However, the first mammals do not appear in the fossil record until Late Triassic time, about 210 mya. Hopson (1994) noted, "Of all the great transitions between major structural grades within vertebrates, the transition from basal amniotes [egg-laying tetrapods except amphibians] to basal mammals is represented by the most complete and continuous fossil record.... Structural evolution of particular functional systems has been well investigated, notably the feeding mechanism... and middle ear, and these studies have demonstrated the gradual nature of these major adaptive modifications."

A widely used definition of mammals is based on the articulation or joining of the lower and upper jaws. In mammals, each half of the lower jaw is a single bone called the dentary; whereas in reptiles, each half of the lower jaw is made up of three bones. The dentary of mammals is joined with the squamosal bone of the skull. This condition evolved between Pennsylvanian and Late Triassic times. Evolution of this jaw articulation can be traced from primitive synapsids (pelycosaurs), to advanced synapsids (therapsids), to cynodonts, to mammals. In mammals, two of the extra lower jaw bones of synapsid reptiles (the quadrate and articular bones) became two of the middle-ear bones, the incus (anvil) and malleus (hammer). Thus, mammals acquired a hearing function as part of the small chain of bones that transmit air vibrations from the ear drum to the inner ear.

Evolution of whales

During the 1990s our understanding of whale evolution made a quantum jump. In 1997, Gingerich and Uhen noted that whales (cetaceans) "... have a fossil record that provides remarkably complete evidence of one of life's great evolutionary adaptive radiations: transformation of a land mammal ancestor into a diversity of descendant sea creatures."

The trail of whale evolution begins in Paleocene time, about 60 mya, with a group of even-toed, hoofed, trotting, scavenging carnivorous mammals called mesonychians. The first whales (pakicetids) are known from lower Eocene rocks, that formed about 51 mya; the pakicetids are so similar to mesonychians that some were misidentified as belonging to that group. However, the teeth of pakicetids are more like those of whales from middle Eocene rocks, about 45 mya, than they are like the teeth of mesonychians. Pakicetids are found in nonmarine rocks and it is not clear how aquatic they were.

In 1994, Ambulocetus natans, whose name means "walking whale that swims," was described from middle Eocene rocks of Pakistan. This species provides fossil evidence of the origin of aquatic locomotion in whales. Ambulocetus preserves large forelimbs and hind limbs with large hands and feet, and the toes have hooves as in mesonychians. Ambulocetus is regarded as having webbing between the toes and it could walk on land as well as swim; thus, it lived both in and out of the water.

From late Eocene time onward, evolution in whales shows reduction of the hind-limbs, modification of the forelimbs and hands into flippers for steering, development of a massive tail, etc.; all of these changes are modifications for the powerful swimming of modern whales. The fossil Rodhocetus from the upper Eocene rocks, about 38 mya, of Pakistan already shows some of these modifications.

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