Fossil Evidence of EvolutionFossil Evidence of Evolution

What is a fossil?What is a fossil?• …physical evidence of an organism that

lived long ago.• Examples: skeletons, shells, leaves,

seeds, imprints, tracks, and even fossilized feces and vomit.

• The vast majority of fossils are the remains of the hard parts of extinct organisms.

How do fossils form?How do fossils form?• Fossils form when body parts or

impressions are buried in rock before they decompose.

• The evidence is preserved in the rock through geochemical processes. Fossils are not usually the actual bodily remains.

• Fossilization is an extremely rare event.• Most ancient species are not represented

in the fossil record.

What is the fossil record?What is the fossil record?• …the collection of fossils that represents

the preserved history of living things on earth.

• It shows that Earth’s organisms have changed significantly over extremely long periods of time.

What is the fossil record?What is the fossil record?• The fossil record provides the dimension

of time to the study of life.(each layer = period of time)

The fossil record is not perfect...but:The fossil record is not perfect...but:

• It abundantly documents continuouschange.

• It is sequential in nature.• It contains numerous examples of

evolutionary transitions.• It is continually growing as new fossils are

discovered.

General Patterns in the Fossil RecordGeneral Patterns in the Fossil Record• Deeper rock layers were laid down before

the layers above them. Thus, fossils in lower layers are older than those in upperones.

• Fossils occur in a definite sequential order, from species that appear “primitive” to “modern” appearing ones.

• The species representing different lines ofdescent become more similar to each other as they approach their common ancestors.

• Comparison of the earliest members of four families of odd-toed ungulates.

(a) Hyracotherium (Horses) (b) Hyrachyus (Rhinos)(c) Heptodon (Tapirs)(d) Eotitanops (Brontotheres)

Fossils document the evolution of the modern camel from ancestral forms existing in much earlier geologic ages.

Because we can consistently trace lineages like this backwards in time, evolutionary descriptions of earth’s history fit the facts of the geologic record.

Fossils Form in Sedimentary RockFossils Form in Sedimentary Rock

The Geologic Time Scale organizes Earth’s history into four distinct ages:1. Precambrian2. Paleozoic3. Mesozoic4. Cenozoic

The boundaries between these major periods of geologic time are defined by major changes in the types of fossils found in the rocks deposited during these eras.

Geologic Time Scale

See page 337

Geologic Time Scale

Dating the Fossil RecordDating the Fossil Record• The discovery of radioactivity enabled scientists to

accurately determine the ages of fossils, rocks, and events in Earth’s past.

• Determining the age of a rock involves using minerals that contain naturally-occurring radioactive elements and measuring the amount of decay in those elements to calculate approximately how long ago the rock formed.

Age Determination Using Radioactive IsotopesAge Determination Using Radioactive Isotopes

•Radioactive isotopes are useful in dating geological materials because they convert or decay at a constant, and therefore measurable, rate.•Age determinations using radioactive isotopes have reached the point where they are subject to very small errors of measurement, now usually less than 1%.

Intermediate FormsIntermediate Forms• So many “transitional” fossils have been found that

it is often hard to tell when the transition actually occurred.

• Actually, nearly all fossils can be regarded as intermediates because they are connections between their ancestors and their descendants.

The Transition to LandThe Transition to LandIchthyostegaIchthyostega

EusthenopteronEusthenopteron

PanderichthysPanderichthys

AcanthostegaAcanthostega

TiktaalikTiktaalik

~385 million years ago~385 million years ago

~365 million years ago~365 million years ago

??

Direct Ancestor or Close Relative?Direct Ancestor or Close Relative?• Ancestor-descendant relationships can only

be inferred, not directly observed.• No matter how long we watch, no two

fossils will ever breed—we must look for other ways to determine relatedness.

• Because genetically similar organisms typically produce similar physical features, we can use fossils to help us recognize related species in the history of life.

Archaeopteryx: Archaeopteryx: An Intermediate Form Between An Intermediate Form Between

Reptiles and BirdsReptiles and Birds

Archaeopteryx: An Intermediate FormArchaeopteryx: An Intermediate Form• While considered the earliest

bird, it retained many distinctly reptilian features.

• A mosaic of 24 distinct anatomical features– 3 bird-like– 17 reptile-like– 4 “intermediate”

• Are dinosaurs still alive?

Feathered Dinosaurs from the Liaoning Fossil Beds in ChinaFeathered Dinosaurs from the Liaoning Fossil Beds in China

CaudipteryxCaudipteryx zouizoui

SinornithosaurusSinornithosaurus milleniimillenii

Mei longMei long

Video: The Liaoning Forest

MicroraptorMicroraptor guigui

SinosauropteryxSinosauropteryx primaprima

Reptile to Mammal Transition Reptile to Mammal Transition • 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.

• Evolution of this jawarticulation can be traced from primitive synapsids(pelycosaurs), to advanced synapsids(therapsids), to cynodonts, to mammals.

• Two of the extra lower jaw bones of synapsidreptiles (the quadrate and articular bones) became two of the middle-ear bones, the incus (anvil) and malleus (hammer).

• Thus, mammals acquired a hearingfunction as part of the small chain of bones that transmit air vibrations from the ear drum to the inner ear.

Evolution of BatsEvolution of Bats• Until recently, the oldest known bats in the fossil

record, like modern bats, could fly and echolocate.• Scientists long wondered which ability came first,

and they predicted the existence of fossil species that had one, but not both, of these abilities.

Palaeochiropteryx tupaiodon~47 mya

Icaronycteris index~50 mya

Prediction Confirmed!Prediction Confirmed!• Flying evolved first, echolocation

came after.• Onychonycteris finneyi is the most

primitive known species of bat– Lacks evidence of echolocation.– Short, broad wings with claws on all

five fingers (modern bats have no more than two claws).

– Longer hind legs and broader tail than modern bats.

– Shorter forearms than modern bats suggest less efficient flying. Onychonycteris finneyi

~52.5 mya

Evolution of WhalesEvolution of Whales• The evolution of whales and dolphins is

one of the best-documented transitions in the fossil record.

• Fossil, morphological, biochemical, vestigial, embryological, biogeographical, and paleoenvironmental evidence all support the inference that whales evolved from four-legged land-dwelling mammals.

• The descent of whales from land-dwelling mammals is well documented by transitionalfossils.

• The tentative reconstruction shown here is based on extensive fossil evidence.

• Many of these transitional fossils have features that were exactly what paleontologistshad predicted they would find in ancient whales.

• For instance, the fossils show transitions in dentition (teeth), the ear canal, the loss of hind limbs, the development of the tail fluke, and the transition of the nostrils to the blowhole.

The fossil record shows that whales and dolphins The fossil record shows that whales and dolphins probably evolved from a group of hoofed mammals called probably evolved from a group of hoofed mammals called ArtiodactylsArtiodactyls. Evidence suggests that these were the . Evidence suggests that these were the same ancestors of a wellsame ancestors of a well--known group of hoofed known group of hoofed mammals called mammals called MesonychidsMesonychids. . MesonychidsMesonychids had had notched, triangular teeth similar to those of early notched, triangular teeth similar to those of early predatory whales. Paleontologists previously considered predatory whales. Paleontologists previously considered MesonychidsMesonychids ancestral to whales, but they now consider ancestral to whales, but they now consider them to be a them to be a ““sister groupsister group”” instead.instead.

Mesonyx, a primitive mesonychid

~60 million years ago

Artist’s visualization of Sinonyx, another primitive Mesonychid

Later fossils in the series show the Later fossils in the series show the PakicetidsPakicetids,, a group of a group of carnivorous land mammals with peculiarities in the bones carnivorous land mammals with peculiarities in the bones of the ear that have only been found in whales. of the ear that have only been found in whales. PakicetidPakicetidteeth look a lot like those of fossil whales, but are unlike teeth look a lot like those of fossil whales, but are unlike those of modern whales. The shape of their teeth those of modern whales. The shape of their teeth suggests that they were adapted for hunting fish.suggests that they were adapted for hunting fish.

Pakicetids

~50 million years ago

Artist’s visualization of Pakicetus, a Pakicetid

Later, a species existed that had front forelimbs and powerful Later, a species existed that had front forelimbs and powerful hind legs with large feet that were adapted for paddling. This hind legs with large feet that were adapted for paddling. This animal, known as animal, known as AmbulocetusAmbulocetus, may have moved between , may have moved between water and land. Its fossilized vertebrae show that this animal water and land. Its fossilized vertebrae show that this animal could move its back in a strong up and down motion, which is could move its back in a strong up and down motion, which is the method modern whales and dolphins use to swim and the method modern whales and dolphins use to swim and dive. It also had a nose adaptation that enabled it to swallow dive. It also had a nose adaptation that enabled it to swallow underwater, the ability to hear underwater, and teeth similar underwater, the ability to hear underwater, and teeth similar to primitive whales.to primitive whales.

Ambulocetus

~47-48 million years ago

Artist’s visualization of Ambulocetus natans

Rodhocetus

~46-47.5 million years ago

A later fossil in the series, A later fossil in the series, RodhocetusRodhocetus, shows an , shows an animal with smaller animal with smaller functional hind limbs and functional hind limbs and even greater back flexibility. even greater back flexibility. The ankle bones are similar The ankle bones are similar to existing hoofed land to existing hoofed land mammals such as the mammals such as the hippopotamus. The forefeet hippopotamus. The forefeet of of RodhocetusRodhocetus had hooves had hooves on the central digits, but the on the central digits, but the hind feet had slender toes hind feet had slender toes which may have supported which may have supported webbing. This suggests webbing. This suggests that that RodhocetusRodhocetus was was predominantly aquatic.predominantly aquatic.

Artist’s visualization of Rodhocetus

BasilosaurusBasilosaurus fossils represent fossils represent a recognizable whale, with a recognizable whale, with front flippers for steering and front flippers for steering and a completely flexible a completely flexible backbone. This animal had backbone. This animal had small hind limbs, although small hind limbs, although they are thought to have been they are thought to have been nonfunctional.nonfunctional.

Basilosaurus

hind limbs

~35 - 45 million years ago

Artist’s visualization of Basilosaurus

DorudonDorudon was a primitive whale was a primitive whale that also had small hind limbs. that also had small hind limbs. When they were first found in When they were first found in the same deposits as the same deposits as BasilosaurusBasilosaurus, the two animals , the two animals were so similar that were so similar that DorudonDorudonwere thought to be baby were thought to be baby BasilosauriBasilosauri. They are, in fact, . They are, in fact, different species, and now different species, and now baby baby DorudonDorudon are also well are also well known.known.

Dorudon

~37 million years ago

Artist’s visualization of Dorudon

Evolution of Modern WhalesEvolution of Modern Whales• Toothed whales have full sets of

teeth throughout their lives.• Baleen whales only possess

teeth during an early fetal stage and lose them before birth.

• Fossil evidence indicates that the ancient whale Janjucetus,with skull features that make it the earliest known baleen whale, also had a full set of teeth.

Artist’s visualization of Janjucetus

Both Teeth and Baleen?Both Teeth and Baleen?• The skull of an ancient toothed whale

called Aetiocetus has holes for blood vessels that were likely used to nourish baleen.

Artist’s visualization of Aetiocetus

Evolution of the BlowholeEvolution of the Blowhole

Nostrils further back

Rodhocetus~47 million years ago

Nostrils at front of skull

Pakicetus~50 million years ago

Nostrils at middle of skull

Aetiocetus~25 million years ago

Nostrils at top of skull

Beluga WhaleToday

Transitional Forms?

ConclusionConclusion• Many critical gaps in our knowledge

remain.• These gaps may or may not be filled by

new evidence in the future.• However, it is certain that important

discoveries will continue to be made that will likely intrigue us, possibly surprise us, and definitely enrich our understanding of the evolutionary history of life.