A long long time ago, in a galaxy not too far away, was a little blue
planet called Earth, and on this world not a single mammal lived. However a lot
of time has past since then and we now have lots of furry creatures that are
collectively called mammals. How did they get their? Where did they come from?
These are the kinds of questions that led me to my subject of choice. I will
endeavor to provide examples, using specific transitional fossils, to show that
mammals have evolved from a group of reptiles and were simply not placed here by
unknown forces.


Before I begin, I would like to define some terms so that nobody gets
left in the dust. The term transitional fossil can be used in conjunction with
the term general lineage, together they help explain the how one species became
another.

"General lineage":

This is a sequence of similar genera or families, linking an older to a very
different younger group. Each step in the sequence consists of some fossils that
represent certain genus or family, and the whole sequence often covers a span of
tens of millions of years. A lineage like this shows obvious intermediates for
every major structural change, and the fossils occur roughly (but often not
exactly) in the expected order. However, usually there are still gaps between
each of the groups. Sometimes the individual specimens are not thought to be
directly ancestral to the next-youngest fossils (e.g. they may be "cousins"" or
"uncles" rather than "parents"). However they are assumed to be closely related
to the actual ancestor, since the have similar intermediate characteristics.

Where Does It All Begin ?

Mammals were derived during the Triassic Period ((from 245 to 208
million years ago) It began with relatively warm and wet conditions, but as it
progressed conditions became increasingly hot and dry.) from members of the
reptilian order Therapsida. The therapsids, members of the subclass Synapsida
(sometimes called the mammal-like reptiles),generally were unimpressive in
relation to other reptiles of their time. Synapsids were present in the
Carboniferous Period (about 280 to 345 million years ago) and are one of the
earliest known reptilian groups. Although therapsids were primarily predators by
nature, some adaptations included a herbivorous species as well, they were
generally small active carnivores. Primitive therapsids are present as fossils
in certain Middle Permian deposits; later forms are known from every continent
except Australia but are most common in the Late Permian and Early Triassic of
South Africa.

The several features that separate modern reptiles from modern mammals
doubtlessly evolved at different rates. Many attributes of mammals are
correlated with their highly active lifestyle; for example, efficient double
circulation of blood with a completely four-chambered heart, anucleate and
biconcave erythrocytes (blood cells), the diaphragm, and the secondary palate
(which separates passages of food and air and allows breathing during
mastication (chewing) or suckling). Hair for insulation correlates with
endothermy (being warm-blooded), the physiological maintenance of individual
temperature independent of the environmental temperature, and endothermy allows
high levels of sustained activity. the unique characteristics of mammals thus
would seem to have evolved as a complex interrelated system.

Transitions to New Higher Taxa

Transitions often result in a new "higher taxon" (a new genus, family,
order, etc.) from a species belonging to different, older taxon. There is
nothing magical about this. The first members of the new group are not bizzare,
they are simply a new, slightly different species, barely different from the
parent species. Eventually they give rise to a more different species, which in
turn gives rise to a still more different species, and so on, until the
descendents are radically different from the original parent. For example, the
Order Perissodactyla (horses) and the Order Cetacea (whales) can both be traced
back to early Eocene animals that looked only marginally different from each
other, and didn't look at all like horses or whales. (They looked more like
small, dumb foxes with raccoon-like feet and simple teeth.) But over the
following tens of millions of years, the descendents of those animals became
more and more different, and now we call them two different orders.

Major Skeletal Differences (derived from the fossil record)

The mammalian skeletal system shows a number of advances over that of
reptiles. the mode of ossification (process of bone formation) of the long bones
is one characteristic. In reptiles each long bone has a single centre of
ossification, and replacement of cartilage by bone proceeds from the centre
toward the ends. In mammals secondary centres of ossification develop at the
ends of the bones. Mammalian skeletal growth is termed determinate, for once the
actively growing zone of cartilage is used up, growth in length ceases. As in
all bony vertebrates, of course, there is continual renewal of bone throughout
life. The advantage of secondary centres of ossification at the ends of bones
lies in the fact that the bones have strong articular surfaces before the
skeleton is mature. In general, the skeleton of the adult mammal has less
structural cartilage than does that of a reptile.

The skeletal system of mammals and other vertebrates is broadly
divisible into axial and appendicular portions. The axial skeleton consists of
the skull, the backbone and ribs, and serves primarily to protect the central
nervous system. the limbs and their girdles make up the appendicular skeleton.
In addition, there are skeletal elements derived from gill arches of primitive
vertebrates, collectively called the visceral skeleton. Visceral elements in the
mammalian skeleton include jaws, the hyoid apparatus supporting the tongue, and
the auditory ossicles of the middle ear. The postcranial axial skeleton in
mammals general has remained the rather conservative during the course of
evolution. The vast majority of mammals have seven cervical (neck) vertebrae,
and do not have lumbar ribs, both characteristics are unlike reptiles.

The skull of mammals differs markedly from that of reptiles because of
the great expansion of the brain. The sphenoid bones that form the reptilian
braincase form only the floor of the braincase in mammals. In mammals a
secondary palate, that is not present in reptiles, is formed by processes of the
maxillary bones and the palatines. The secondary palate separates the nasal
passages from the oral cavity and allows continuous breathing while chewing or
suckling.

The bones of the mammalian middle ear are a diagnostic of the class. The
three auditory ossicles form a series of levers that serve mechanically to
increase the amplitude of sound waves reaching the tympanic membrane, or eardrum,
produced as disturbances of the air. The innermost bone is the stapes, or
"stirrup bone." It rests against the oval window of the inner ear. The stapes is
homologous with the entire stapedial structure of reptiles, which in turn was
derived from the hyomandibular arch of primitive vertebrates. The incus, or
"anvil", articulates with the stapes. The incus was derived from the quadrate
bone, which is involved in the jaw articulation in reptiles. The malleus, or
"hammer", rests against the tympanic membrane and articulates with the incus.
The malleus is the homologue of the reptilian articular bone. The mechanical
efficiency of the middle ear has thus been increased by the incorporation of two
bones of the reptilian jaw assemblage. In mammals the lower jaw is a single bone,
the dentary.

The mammalian limbs and girdles have been greatly modified with
locomotor adaptations. The primitive mammal had well developed limbs and was
five-toed. In each limb there two distal bones (radius and ulna in the forelimb;
tibia and fibula in the hindlimb) and a single proximal bone (humerus; femur).
The number of phalangeal bones in each digit, numbered from inside outward, is
2-3-3-3-3 in primitive mammals and 2-3-4-5-4 in primitive reptiles.
Modifications in mammalian limbs have involved reduction, loss, or fusion of
bones. Loss of the clavicle from the shoulder girdle, reduction in the number of
toes.

The Transition

This is a documented transition between vertabrate classes. Each group
is clearly related to both the group that came before, and the group that came
after, and yet the sequence is so long that the fossils at the end are
...