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Evolution of Snakes
by Lenny Flank
The fossil history of snakes is very poorly known,
since snake skeletons are very delicate and do not
fossilize easily. By carefully examining the fossil
materials which have been recovered, and by making
comparisons of the anatomy of living snakes and their
relatives, biologists have been able to reconstruct
something of the evolutionary history of snakes. Snakes,
like all living things, are the product of the process of
evolution, which allows species to change over time in
response to environmental factors to produce entirely new
species. The engine of evolution is "natural
selection", in which those individual animals that
possess superior survival traits tend to live longer than
others and reproduce, in turn passing those same traits
on to their offspring.
According to most paleontologists, reptiles evolved
from the large group of ancient amphibians known as
Labrynthodonts, which received their names from the
distinctive structure of their teeth. In the
Labrynthodonts, the enamel of the tooth was folded in on
itself to form a complex mazelike pattern.
The evolutionary advance that set the reptiles apart
from the amphibians was the development of the amniote or
shelled egg which could be laid on land, freeing the
reptiles from the necessity of returning to the water as
adults for reproduction. The oldest known fossil egg was
found in Texas, and dates to the lower Permian period of
the earth's history, over 275 million years ago. It is
not known from which particular group of Labrynthodont
amphibians the reptiles developed; several different
families of ancient amphibians seemed to have been
developing characteristics at the time similar to those
of reptiles.
The oldest fossil which can be definitely recognized
as a reptile is a small lizardlike animal known as Hylonomus,
whose skeletons have been found inside petrified tree
stumps in Nova Scotia. During the period of time in which
Hylonomus lived, the earth was a different place
than it is now. The continents were all joined into one
large super-continent near the equator, known as Pangea
("all earth"), and even such places as
Antarctica and northern Canada had warm, humid climates
with lush tropical forests. Since then, the Pangea land
mass has broken up into a series of "plates"
which move slowly atop the earth's mantle, a process
known as "plate tectonics". As we shall see,
the breakup and movement of these plates has had
noticeable consequences for the evolution and
distribution of modern snakes.
Hylonomus was a member of a group of very ancient
reptiles known as the Cotylosaurs, or "stem
reptiles", which are believed by paleontologists to
be ancestral to all of the reptile families alive today.
The Cotylosaurs first appeared during the Permian, the
period of time that immediately preceded the rise of the
dinosaurs. During the next few million years, the
Cotylosaurs diverged into three distinct groups of
reptiles which are distinguished from each other by their
differing skull structures. The earliest of the
Cotylosaurs were Anapsids, which means that they lacked
any arches or openings between their skull bones. The
Anapsids eventually went on to produce the modern
turtles. Later, another group of Cotylosaurs developed a
single arch in the skull, between the postorbital and
squamosal bones, through which the jaw muscles passed.
These reptiles are known as Synapsids, and they went on
to evolve into the modern mammals. The third group of
reptiles, the Diapsids, diversified to produce the
extinct dinosaurs as well as the modern lizards and
snakes. Thus, although snakes are not direct descendants
of the dinosaurs, they are evolutionary cousins of Tyrannosaurus
and Triceratops. (Modern birds are also
descended from Diapsid reptiles, and are thus distant
evolutionary cousins of the modern snakes.)
One of the earliest snakes to appear in the fossil
record has been given the scientific name Lapparentophis
defrenni. It was found in the Saharan Desert and has
been dated to the early Cretaceous period, about 130
million years ago. Although the fossil consisted of only
a few back bones and was missing all the ribs and the
entire skull, the structure of the vertebrae is
characteristic of that of snakes. Recently another
fossil, consisting of just two vertebrae, have been found
in Spain that are a few million years older than Lapparentophis.
This fossil has not yet been named.
Another very early snake has been found in marine
deposits in North Africa and Europe. This snake, which
lived about 100 million years ago, has been called Simoliophis.
Although it appears to have been at least partially
aquatic, Similiophis does not appear to be
related to any of the modern sea snakes, and may not be
related to any living snakes. Both Lapparentophis
and Similiophis appear to have gone extinct some
time before the end of the Cretaceous.
The most complete skeleton of a fossil snake was found
in Upper Cretaceous rocks in Argentina. Most of the skull
was preserved as well as a large number of vertebrae and
ribs. The six foot skeleton was named Dinilysia
patagonica, and it shares many anatomical
characteristics with the modern boas and pythons, which
are usually considered to be the most primitive of the
living snakes. Another fossil snake, Gigantophis,
that was found in Egypt, had an estimated length of over
fifty feet, and is the largest of all the known snakes.
It was also related to the modern boids.
One of the most interesting snake fossils is the
extinct boid Paleryx, found in Germany. Fossils
of this ancient snake have been found which still contain
the impressions of the scaled skin.
Based on these fossil finds, as well as on anatomical
study of modern reptiles, scientists have concluded that
the snakes probably evolved from a family of lizards
during the time of the dinosaurs. Snakes and lizards
share a number of distinct features in the structure of
their skull; both, for instance, possess a moveable
quadrate bone at the back of the jaw, and both are
missing the quadratojugal bone at the rear of the skull.
In particular, the Varanid family of lizards, which
includes the monitors, are very similar to snakes in
their skull structure. The most snake- like of the living
monitors is the Earless Monitor, a burrowing semi-aquatic
lizard found in Borneo. The Earless Monitor has movable
eyelids, but the lower lid sports a clear
"window" which allows the monitor to see even
when its eyes are closed, protecting it from water and
dirt. This is very reminiscient of the snake's brille or
eyecap, which is formed in embryonic snakes when the
transparent upper and lower eyelids fuse together. The
Earless Monitor also has a number of snakelike features
in its skull architecture and, as the name implies, it
lacks any trace of an external ear, just as in snakes. It
is probable that the Earless Monitor more closely
resembles the saurian ancestor of the snakes than any
other living lizard.
Based on these similarities, some herpetologists have
theorized that an ancient group of monitor-like lizards
began to follow a burrowing way of life, tunneling
through loose dirt and sand in search of earthworms and
other prey, just as some lizards do today. Over a period
of millions of years, these burrowing lizards lost their
limbs and their external ears--to help them burrow more
easily--and also replaced their eyelids with a clear
brille or spectacle to protect their eyes while digging.
At about the time that the dinosaurs reached their apex,
one group of these burrowing lizards then gave up its
subterranean lifestyle and emerged to the surface, where
they developed a new legless mode of locomotion and
rapidly diversified to invade a large number of
ecological niches. Today we classify the various
descendants of these legless lizards as snakes.
The "burrowing ancestors" theory has,
however, come under some attack recently. Several
herpetologists have pointed out that the Dinilysia skull
does not show many features adapted to a burrowing
existence. Some biologists have theorized that the
snake's unique features are the result of a largely
aquatic or semi-aquatic lifestyle, as illustrated by the
Earless Monitor. In this interpretation, the lack of
ears, the covered eyes and the long limbless bodies
allowed the first snakes to move efficiently through
water or wet marshy areas in search of prey. It was only
later that snakes moved from an aquatic environment to
invade the dry land. During the time that snakes
developed, the Varanid family did contain a number of
semi-aquatic and marine species, including the giant
Mososaurs.
In any case, the first of the modern terrestrial
snakes to appear seem to have been relatives of the
living boids, or boas and pythons. These were large
heavy-bodied snakes with a rather primitive and heavy
skull structure. The living boas and pythons all have
tiny clawlike toes protruding from either side of their
cloaca--these are the remnants of the legs that their
ancestors once had, and are thus an evolutionary relic
tying the snakes directly to their lizard ancestors.
After the dinosaurs disappeared, the boids were the
dominant snake family on earth, and became widespread and
very diverse. About 36 million years ago, however, a
group of smaller, faster snakes appeared which competed
with the boids for food and living space. These were the
colubrids, the family which we think of today as
"typical snakes". The colubrids were unable to
outcompete the boids and remained a small group of snakes
until about 20 million years ago, when the continental
plates began to reach their present positions. As the
tectonic plates moved away from the equator, the climate
cooled dramatically, and the boids, unable to cope with
the lower temperatures, disappeared from many areas and
were greatly reduced in number and diversity. The
colubrids quickly moved into the empty environmental
niches that had been occupied by the boids, and soon
dominated the snake world. Today, the colubrids make up
over two-thirds of all the living species of snakes.
One family of the colubrids, however, added a new
twist to the snake's survival arsenal. About 15 million
years ago, snakes began appearing which had a number of
greatly enlarged teeth at the rear of their jaw. These
teeth had shallow grooves running down one side. Today,
such snakes are referred to as opisthoglyphs or
"rear-fanged" snakes. In the rear-fanged
snakes, the enlarged teeth are used to pierce the skin of
prey after it has been seized and partially swallowed,
allowing venom (composed of highly modified saliva) to
flow out of the Duvernoy's gland and dribble down the
grooved teeth into the wound. Since it is difficult for
these snakes to inject their venom until after they have
partially swallowed their victim, it is unlikely that the
snake's venom apparatus was originally developed as a
defensive weapon. More probably it appeared as an
effective way of quickly killing and subduing food. A
large number of rear-fanged snakes are still alive today.
Shortly after the opisthoglyphs appeared, another
group of snakes developed a more refined venom apparatus.
These snakes are known as proteroglyphs, and are
classified as the Elapids. Instead of having fangs at the
rear of the jaws, the proteroglyphs have short fixed
fangs which have migrated (by reducing the size of the
maxillary bone) to the front of the mouth, where they can
be used to bite and strike at enemies as well as food.
The grooves in the fangs have become deeper and meet at
the edges to form a hollow tube. These hollow fangs are
connected to venom glands in the cheeks, which can inject
venom through the fangs like hypodermic needles when the
snake bites. Living descendants of the Elapids include
the cobras and the sea snakes.
By about 10 million years ago, the most highly
specialized of the snakes appeared in the fossil
record--the solenoglyphs, commonly known as vipers. In
the vipers, the fangs are extremely long, much larger
than in the Elapids. In fact, they are so long that the
snake cannot close its mouth if they are erected. Thus,
the solenoglyphs use a rotating maxillary bone to fold
the fangs up against the roof of the mouth, where they
are ready to spring into position when the snake bites. A
short time after the vipers appeared, a group known as
the pit vipers developed a number of heat-sensitive pits
on the front of the face, which they used for finding
their warm-blooded prey at night (this feature has also
been independently developed by the venerable old Boid
family). Finally, just a few million years ago, a group
of pit vipers developed a structure at the end of their
tail, made up of interlocking pieces of unshed skin,
which could be loudly rattled and used as a warning
device against predators. The rattlesnakes are generally
thought to be the most specialized of all the living
snakes.
The fossil record of snakes, however, is patchy and
incomplete, with large gaps. Newer techniques using
molecular biology may give us a more complete picture of
snake evolution. Using methods such as immunological
responses and DNA-DNA hybridization, the precise genetic
"distance" between living species can be
determined, and a rough picture of when and in what order
they evolved can be drawn. The study of snakes using DNA
techniques is still in its infancy, but has already
revealed a few surprises. Preliminary results indicate
that the vipers are not, as was formerly thought, the
most recent of the snakes, but instead diverged from the
ancestral boid stock before both the elapids and the
colubrids.
If this finding is confirmed, it means that we have to
completely re-think our view of how snakes evolved. It
appears that the snakes underwent a rapid radiation in
their initial burst of evolution, with a number of
different lifestyles appearing at once and then
developing independently and in parallel afterwards. Much
work remains to be done on the evolution of snakes.
Excerpted from
"Snakes: Their Care and Behavior". (c)
copyright 1997 by Lenny Flank, Jr.
Last Updated: February 12, 2007
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