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
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
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
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!
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
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
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.