AR103 - Readings pgs. 105-115 - Week 8 Notes - Nov. 1, 2011.
Chapter Five: Macroevolution: Processes of Vertebrate and Mammalian
Pages # 105 -115
What Are Fossils and How Do They Form?
- Fossils are traces or remains of organisms found in geological beds on the earth’s surface.
- The oldest fossils date as far back to 3 billion years ago, because they are remains of
microorganisms, they are extremely small and are called mircofossils.
- After the organism died, these “hard” tissues were further impregnated with other minerals,
being eventually transformed into a stone-like composition. This process is called
- Mineralization is the process in which parts of animals (or some plants) become
transformed into stone-like structures. Mineralization usually occurs very slowly as
water carrying minerals, such as silica or iron, seeps into the tiny spaces within a bone.
In some cases, the original minerals within the bone or tooth can be completely
replace, molecule by molecule, with other minerals.
- Sometimes insects were trapped in tree sap, which later became hardened and chemically
altered. Because inside hardened amber there was little or no oxygen, the insects have
remained remarkably well preserved for millions of years, even with soft tissue and DNA still
- Leaf imprints in hardened mud, or similar impressions of small organisms, or even the traces
of dinosaur feathers are fossils, as well as dinosaur footprints and hominin tracks.
- Whether a dead animal will become fossilized and how much of it will be preserved depends
partly on how it dies, but even more on where it dies.
- Taphonomyis the study of how bones and other materials come to be buried in the earth and
preserved as fossils.
- Taphonomists try to understand the processes of sedimentation, including the action of
streams, preservation properties of bone, and carnivore disturbance factors.
Vertebrate Evolutionary History: A Brief Summary
- Geologists have formulated the geological time scale
- Geological Time Scale is the organization of earth history into eras, periods, and epochs;
commonly used by geologists and paleoanthropologists.
- For the time span encompassing vertebrate evolution, there are three eras: the Paleozoic, the
Mesozoic, and the Cenozoic.
- The earliest vertebrate fossils date to early Paleozoic at 500 mya, and their origins are
probably much older.
- During the Paleozoic, several varieties of ﬁshes, amphibians, and reptiles appeared.
- At the end of the Paleozoic, close to 250 mya, several varieties of mammal-like reptiles were
- The evolutionary history of vertebrates and other organisms during the Paleozoic and
Mesozoic was profoundly inﬂuenced by geological events.
- The positions of the earth’s continents have dramatically shifted during the last several
hundred thousand million years ago.
- This process is called the continental drift, and is explained by the geological
theory of plate tectonics, which states that the earth’s crust is a series of
gigantic moving and colliding plates.
- Continental drift is the movement of continents on sliding plates of the earth’s
surface. As a result, the position of large landmasses have shifted drastically
during the earth’s history.
1 AR103 - Readings pgs. 105-115 - Week 8 Notes - Nov. 1, 2011.
- Such massive geological movements can induce volcanic activity, mountain
building, and earthquakes.
- During the late Paleozoic, the continents came together to form a single colossal landmass
- During the Mesozoic, the southern continents began to split off from Pangea, forming a large
southern continent called Gondwanaland.
- The northern continents were consolidated into northern landmass called Laurasia.
- Gondwanaland and Laurasia continued to drift apart and to break up into smaller
- By the end of the Mesozoic the continents were beginning to assume their current
- Groups of animals became effectively isolated from each other by oceans signiﬁcantly
inﬂuencing the distribution of mammals and other land vertebrates.
- These continental movements continued in the Cenozoic and indeed are still happening,
without such dramatic results though.
- During most of the Mesozoic, reptiles were the dominant land vertebrates, and they exhibited a
broad expansion into a variety of ecological niches, which included aerial and marine habitats.
- The most famous of these highly successful Mesozoic reptiles were the dinosaurs,
which themselves evolved into a wide array of sizes and species and adapted to a
variety of lifestyles.
- Many dinosaurs were “warm-blooded”
- Many species became extinct because of major climate changes to the earth’s
atmosphere from collisions with comets or asteroids; not all dinosaurs became
entirely extinct and have many descendants still living today.
- The Cenozoic is divided into two periods: the Tertiary, about 63 million years in duration, and
the Quaternary, from about 1.8 mya up to and including the present.
- Epochs are categories of the geological time scale; subdivisions of periods in the Cenozoic
era, including the Paleocene, Eocene, Oligocene, Miocene, and Pliocene (from the Tertiary)
and the Pleistocene and Holocene (from the Quaternary).
- Studies using fossils and DNA of living species, suggests that all the living groups of mammals
had diverged by 75 million years ago.
- Later, only after several million years following the beginning of the Cenozoic, did the
various current mammalian subgroups begin to diversify.
- Cenozoic could be called the Age of the Mammals because during this era that, along with
birds, mammals replaced earlier reptiles as the dominant land-living vertebrates.
- Several characteristics relating to learning and general ﬂexibility of behaviour are of
prime importance to the growth of mammals in the Mesozoic and Cenozoic eras
- To process more information, mammals were selected for larger brains than
those typically found in reptiles.
- The cerebrum became generally enlarged, especially the outer covering,
the neocortex, which controls higher brain functions. In some mammals,
the cerebrum expanded so much that it came to comprise most of the
brain volume; the number of brain convolutions also increased, creating
more surface area and thus providing space for even more neurons.
- For such a large and complex organ as the mammalian brain to develop, a
longer, more intense period of growth is required. Slower development can
occur internally (in utero) as well as after birth.
2 AR103 - Readings pgs. 105-115 - Week 8 Notes - Nov. 1, 2011.
- Other forms (ﬁsh reptiles birds) lay eggs, and “prenatal” development
occurs externally, outside the mother’s body. Mammals with few
exceptions, give birth to live young.
- There’s considerable variation among the major groups in how
mature the young are at birth, and in placental mammals, in
utero development goes the farthest.
- Placental is a type (subclass) of mammal. During the Cenozoic,
placentals became the most widespread and numerous
mammals and today are represented by upward of 20 orders
including the primates.
- A distinctive feature of mammals is the dentition.
- Many living reptiles have similar shaped teeth, mammals have differently
- This varied pattern, termed heterodont dentition, is reﬂected in
the ancestral mammalian arrangement of teeth, including 3
incisors, 1 canine, 4 premolars, and 3 molars in each quarter of
the mouth. A total of 44 teeth in ancestral mammalian dental
- Heterodont is having different kinds of teeth; characteristic of
mammals, whose teeth consists of incisors (cutting), canines
(grasping), premolars (crushing), and molars (grinding).
- The heterdont arrangement allows mammals to process a wide
variety of foods.
- Regarding teeth, their disproportionate representation in the fossil
- As the hardest, most durable portion of a vertebrate skeleton,
teeth have the greatest likelihood of becoming fossilized, since