Fossil Evidence of Evolution

Contemporary Scientific History of the Universe billion years in 30 volumes - each volume = 450 pages -each page = 1 million years Big Bang, p. 1 Origin of Earth Life BeginsComplex Animal Life Dinosaurs pp Modern Humans p. 450, last sentence

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? 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? …the collection of fossils that represents the preserved history of living things on earth. The fossil record provides the dimension of time to the study of life. It shows that Earth’s organisms have changed significantly over extremely long periods of time.

(each layer = period of time)

The fossil record is not perfect...but: It abundantly documents continuous change. 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 Record Deeper rock layers were laid down before the layers above them. Thus, fossils in lower layers are older than those in upper ones. Fossils occur in a definite sequential order, from species that appear “primitive” to “modern” appearing ones. The species representing different lines of descent 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 Rock

The Geologic Time Scale Earth’s history is organized into four distinct ages: 1. Precambrian 2. Paleozoic 3. Mesozoic 4. 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 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 Isotopes Radioactive isotopes are useful in dating geological materials because they convert or decay at a constant, and therefore measurable, rate. Age determinations using multiple radioactive isotopes are subject to very small errors of measurement, now usually less than 1%.

Step 1: List ALL of the long-lived radioactive nuclides. 10 Be1.6 x 10 6 yes - P 40 K1.25 x 10 9 yes 50 V6.0 x yes 53 Mn3.7 x 10 6 yes - P 87 Rb4.88 x yes 93 Zr1.5 x 10 6 no 97 Tc2.6 x 10 6 no 98 Tc1.5 x 10 6 no 107 Pd~7 x 10 6 no 115 In6.0 x yes 123 Te1.2 x yes 129 I1.7 x 10 7 yes - P 135 Cs3.0 x 10 6 no 138 La1.12 x yes 144 Nd2.4 x yes 146 Sm7.0 x 10 7 no 147 Sm1.06 x yes 150 Gd2.1 x 10 6 no 152 Gd1.1 x yes 153 Dy~1.0 x 10 6 no 174 Hf2.0 x yes 176 Lu3.5 x yes 182 Hf9 x 10 6 no 187 Re4.3 x yes 190 Pt6.9 x yes 192 Pt~1.0 x yes 205 Pb3.0 x 10 7 no 232 Th1.40 x yes 235 U7.04 x 10 8 yes 236 U2.39 x 10 7 yes - P 237 Np2.14 x 10 6 yes - P 238 U4.47 x 10 9 yes 244 Pu8.2 x 10 7 yes 247 Cm1.6 x 10 7 no

Step 2: Order Nuclides by half-life Listing of nuclides by Half-Life 50 V6.0 x yes 144 Nd2.4 x yes 174 Hf2.0 x yes 192 Pt~1.0 x yes 115 In6.0 x yes 152 Gd1.1 x yes 123 Te1.2 x yes 190 Pt6.9 x yes 138 La1.12 x yes 147 Sm1.06 x yes 87 Rb4.88 x yes 187 Re4.3 x yes 176 Lu3.5 x yes 232 Th1.40 x yes 238 U4.47 x 10 9 yes 40 K1.25 x 10 9 yes 235 U7.04 x 10 8 yes 244 Pu8.2 x 10 7 yes 146 Sm7.0 x 10 7 no 205 Pb3.0 x 10 7 no 236 U2.39 x 10 7 yes - P 129 I1.7 x 10 7 yes - P 247 Cm1.6 x 10 7 no 182 Hf9 x 10 6 no 107 Pd~7 x 10 6 no 53 Mn3.7 x 10 6 yes - P 135 Cs3.0 x 10 6 no 97 Tc2.6 x 10 6 no 237 Np2.14 x 10 6 yes - P 150 Gd2.1 x 10 6 no 10 Be1.6 x 10 6 yes - P 93 Zr1.5 x 10 6 no 98 Tc1.5 x 10 6 no 153 Dy~1.0 x 10 6 no

Nuclide Half-Life In Nature? (years) 50 V6.0 x yes 144 Nd2.4 x yes 174 Hf2.0 x yes 192 Pt~1.0 x yes 115 In6.0 x yes 152 Gd1.1 x yes 123 Te1.2 x yes 190 Pt6.9 x yes 138 La1.12 x yes 147 Sm1.06 x yes 87 Rb4.88 x yes 187 Re4.3 x yes 176 Lu3.5 x yes 232 Th1.40 x yes Step 3: Eliminate nuclides continually produced by ongoing decay processes FACT: Every nuclide with a half-life of less than 80 million years is missing from our region of the solar system, and every nuclide with a half-life of greater than 80 million years is present. Every single one! Nuclide Half-Life In Nature? (years) 238 U4.47 x 10 9 yes 40 K1.25 x 10 9 yes 235 U7.04 x 10 8 yes 244 Pu8.2 x 10 7 yes 146 Sm7.0 x 10 7 no 205 Pb3.0 x 10 7 no 247 Cm1.6 x 10 7 no 182 Hf9 x 10 6 no 107 Pd~7 x 10 6 no 135 Cs3.0 x 10 6 no 97 Tc2.6 x 10 6 no 150 Gd2.1 x 10 6 no 93 Zr1.5 x 10 6 no 98 Tc1.5 x 10 6 no 153 Dy~1.0 x 10 6 no

Intermediate 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.

Example: The Transition to Land Ichthyostega Eusthenopteron Panderichthys Acanthostega Tiktaalik ~385 million years ago ~365 million years ago ? ? Video

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 reproduce—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: An Intermediate Form Between Reptiles and Birds

Archaeopteryx: 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 China Caudipteryx zoui Sinornithosaurus millenii Mei long Video: The Liaoning ForestThe Liaoning Forest Microraptor gui Sinosauropteryx prima

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 jaw articulation can be traced from primitive synapsids (pelycosaurs), to advanced synapsids (therapsids), to cynodonts, to 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.

Turtles have a shell and no teeth, both unique traits among reptiles. Scientists predicted that the oldest turtles should show evidence of these changes. November 2008: The oldest known turtle, Odontochelys, has an incomplete shell and teeth. Evolution of Turtles Odontochelys semitestacea – 220 mya

Evolution also predicted intermediate forms between lizards and snakes. Adriosaurus, a fossil lizard with hindlimbs, reduced forelimbs, and an elongated body. Evolution of Snakes Snakes are tetrapods with no legs. Evolution predicted primitive fossil snakes with evidence of limbs. Ball python PachyrhachisEupodophisNajash

Evolution 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! 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 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 transitional fossils. The tentative reconstruction shown here is based on extensive fossil evidence. Many of these transitional fossils have features that were exactly what paleontologists had 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 probably evolved from a group of hoofed mammals called Artiodactyls. Evidence suggests that these were the same ancestors of a well-known group of hoofed mammals called Mesonychids. Mesonychids had notched, triangular teeth similar to those of early predatory whales. Paleontologists previously considered Mesonychids ancestral to whales, but they now consider them to be a “sister group” instead. Mesonyx, a primitive mesonychid ~60 million years ago

Artist’s visualization of Sinonyx, another primitive Mesonychid

Later fossils in the series show the Pakicetids, a group of carnivorous land mammals with peculiarities in the bones of the ear that have only been found in whales. Pakicetid teeth look a lot like those of fossil whales, but are unlike those of modern whales. The shape of their teeth suggests that they were adapted for hunting fish. Pakicetus ~50 million years ago

Artist’s visualization of Pakicetus, a Pakicetid

Later, a species existed that had front forelimbs and powerful hind legs with large feet that were adapted for paddling. This animal, known as Ambulocetus, may have moved between water and land. Its fossilized vertebrae show that this animal could move its back in a strong up and down motion, which is the method modern whales and dolphins use to swim and dive. It also had a nose adaptation that enabled it to swallow underwater, the ability to hear underwater, and teeth similar to primitive whales. Ambulocetus ~47-48 million years ago

Artist’s visualization of Ambulocetus natans

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

Artist’s visualization of Rodhocetus

At about the same time, a species known as Maiacetus also existed. This species had big teeth that were well-suited for catching and eating fish, suggesting that they made their living in the sea. However, other evidence suggests that they may have came onto land to rest, mate, and give birth. ~47.5 million years ago Maiacetus

Artist’s visualization of Maiacetus

Artist’s visualization of Protocetus

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

Artist’s visualization of Basilosaurus

Dorudon was a primitive whale that also had small hind limbs. When they were first found in the same deposits as Basilosaurus, the two animals were so similar that Dorudon were thought to be baby Basilosauri. They are, in fact, different species, and now baby Dorudon are also well known. Dorudon ~37 million years ago

Artist’s visualization of Dorudon

Evolution 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? 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 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 Whale Today Transitional Forms?

Evolution of Echolocation Fossils demonstrate that whales acquired underwater hearing in stages. Pakicetus lacked the fat pad extending to the middle ear which modern whales have. Basilosaurus, transmitted sound to the middle ear as vibration from the lower jaw. Today’s toothed whales can echolocate, the melon directs sound outward and the lower jaw works as a receptor. Pakicetus ~50 million years ago Basilosaurus ~35-45 million years ago Tursiops Bottle-nosed Dolphin Melon

Giving Birth Modern whales are born tail first to prevent drowning in the birth canal. Fossil evidence shows a Maiacetus baby with its head facing the birth canal, suggesting that this species still gave birth on land.

Other Transitional Fossil Series Primitive fish to sharks and rays. Primitive fish to bony fish. Amphibians to reptiles. Land mammals to manatees. Five-toed ancestors to horses. Bipedal apes to humans.

Conclusion 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.