Transitions of Reptiles to Mammals


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