The most primitive order of reptiles that we can
agree on so far is that of the cotylosaurs which were lizardlike,
insect eating, heavily limbed reptiles with solid roofed skulls
and labyrinthine teeth and were very similar to amphibians of
the time. Yet they were different enough that we have mostly agreed
to start from this rootstock. Sometimes change was fairly quick
in evolutionary terms and other times it might take millions of
years for any really significant change to manifest itself.
Of course we know that one major difference that
appeared early on was that reptiles, with their ability to lay
eggs out of water were able to better colonize the land. But several
other behavioral and anatomical changes had to be made before
these differences were profound.
From these roots came all the later families of
reptiles. Some became today's turtles, snakes and lizards. Some
of their other descendants turned back into the sea and became
the plesiosaurs and ichthyosaurs which totally colonized the oceans
as well as much of the fresh waters. Interestingly some of the
earliest reptiles refused to leave the water at all. The Mesosaurs
are perhaps the oldest group that we know of which decided not
to colonize the land but instead stay in the waters until they
eventually died off.
There were essentially two waves of evolutionary
changes that radiated out from these first reptiles. The first
were the "mammal like reptiles" or synapsids. These were later
replaced by the Diapsids. These two names are given to differentiate
a single opening in the skull for attachment of jaw muscles for
the Synapsids and two openings for the Diapsids. Of such small
improvements or differences does evolution decree success or failure.
The synapsida also differed from earlier reptiles in that they
had efficient killing teeth at the front of their mouths and shorter
teeth for cutting or shearing at the back. Perhaps more importantly
was the fact that unlike earlier reptiles their legs were slung
underneath them and supported at the top of the leg, unlike earlier
reptiles who had sprawled legs that bowed out to the side.
Starting from this first radiation reptiles went
into different variations as the selection process of evolution
started rewarding successful adaptations and causing extinctions
for "dead ends" for those that could not handle the rigors of
The first radiation or "wave" of mammal-like synapsid
reptiles were called pelycosaurs. The major distinguishing characteristic
of the first group was a longer and stronger jaw. The two classic
examples of this first group are the "sail fin" Dimetrodon which
was a carnivore and the Edaphosaurus which was an herbivore or
plant eater. From these lines later arose the warm-blooded mammals
and in fact some of these mammal like reptiles may well have already
developed endothermy or warm bloodedness. At the least these reptiles
were experimenting with different types of lifestyles based on
body temperatures and social structures.
These mammal-like reptiles became firmly in charge
of the planets land ecosystems by 280 mya during the Permian and
early Triassic. But by 260 mya they became the second wave by
evolving into the "Therapsids." They were mostly carnivorous reptiles.
Within 30 million years they had dominated most niches until they
became extinct at the end of the Permian at 225 mya and gave way
to the Diapsids.
Earliest (that we have found so far) of the Diapsids
is the Petrolacosaurus which was a very lizard like creature similar
to the kind that are still found today. From this creature or
others similar to it came the two most successful reptilian families:
the Lepidosaurs ("scaly reptiles") and the Archosaurs ("ruling
reptiles"). The Lepidosaurs eventually evolved into present-day
snakes and lizards. The Archosaurs, became what we know of as
Major differences between the two families were
that the Lepidosaurs generally stayed small, which allowed them
to feed on the always numerous insects, and had very flexible
skulls (such as in modern snakes) while the Archosaurs had proportionally
large heads with deep rooted teeth, a tendency to have armored
backbones, and a leg and hip arrangement that allows for a variable
gait. Thus we have the ancestors of the most successful modern
day reptiles in the Lepidosaurs and the most successful of the
ancient reptiles the dinosaurs arising from the Archosaurs. From
these same Archosaurs likely later arose the birds.
The history of life on this planet has seen many
mass extinctions. They seem to play a very large role in shaping
the types and species of animals that are successful. These mass
extinctions could have been caused by many factors; asteroid impact
with the planet, radical change in climate, evolution of different
types of plants causing a die off of animals dependent on them,
solar flares, new and deadly diseases, and many other possible
scenarios. Again a certain amount of guesswork is required. The
result of a mass extinction around 225 million years ago gave
the Archosaurs and other reptiles a big advantage that they were
able to exploit better than all their competing relatives including
the mammal-like reptiles and early mammals.
About the time that Archosaurs ancestors were just
starting to diversify came the crocodiles. One of the first was
Protosuchus (first crocodile) which was probably a land carnivore.
The next advance in crocodillians came with the Mesosuchia. The
modern Crocodillian lines started appearing at the end of the
Jurassic Period but reached their peak by the late Cretaceous
about 80 mya with the Eusuchia and are still with us today. If
you believe that "going the distance " is a sign of species success
than crocodiles are definite winners. Maybe the reason that they
were such winners is that biologically speaking they were very
conservative. They changed very little in the many million years
they have been on the planet. This is similar to the strategy
taken by the Lepidosaurs which later became snakes and lizards:
find a niche and hold on to it rather than try to exploit all
The history of the Crocodillians has actually been
pretty hard to trace from the fossil records. We know they came
from the thecodontians but so did the flying pterosaurs, dinosaurs,
and other diverse lines. We believe that the original land dwelling
carnivores, the pseudosuchians, are the most likely candidates
for crocodillion ancestors. At about 212 mya these creatures were
only about 3 feet in length and looked very much like a lizard.
Their remains have been found in North and South America, Europe,
and parts of South Africa and Eastern Asia.
At about 190 mya we have found well advanced fossils
of Mesosuchaisn descendants which now started to show adaptations
for aquatic existence. By 150 mya fossils showed that a great
diversification had taken place. Examples are the Gonopholididae,
Pholidosauridea, and Atoposauridea.
The more modern families all appear to have their
start in the Eusuchians after the separation of the continents.
We do not know exactly when the crocodile family and the alligator
family separated. But by 80 mya it was clear that they were two
distinct families in North America. No one knows the exact origin
of the third family; the Gavialinae though future fossils may
answer this mystery.
The direct ancestors of today's crocodillians are
mostly unknown or guessed at. The following are best guesses.
Alligator olseni was the likely immediate progenitor of the American
alligator. Eocaimen cavernensis is likely the South American caiman's
ancestor. And Crocodylus lloidi is now the Nile and possibly the
mugger crocodile. The fossil records are still too incomplete
to guess at the other species closest relatives.
Turtles have always been a very successful line.
They were as many and varied then as they are now and branched
off early form the Cotylosaurs as far back as the Permian. Examples
are Archelon (ancient turtle) which grew to enormous sizes, Proganochelys
(first turtle), Podocynemis, and Protostega.
At about this time another temporary non-dinosaurian
success was the Pterosaurs or "winged reptiles." Examples are
Pterodactylous (wing finger) and Eudimorphodon (true two-form
tooth). The most spectacular species of the flying reptiles found
so far were Pterodaustro (southern wing) which was much like present
day flamingos in life style and Quetzalcoatilus (feathered serpent)
which had a wing span equal to some of modern airplanes. Both
the Pterosaurs and the crocodiles were either part of or evolved
from Archosaur type reptiles.
There were other non-dinosaurs around during the
reptiles' golden age also. Ichthyosaurs were the "porpoises" of
the dinosaurian age. There were many species and they were found
throughout the ancient seas. Sizes ranged from just a couple of
feet to over 50 feet. We believe that they gave birth to live
young at sea in much the same way that porpoises do. This leads
to speculation as to whether they may have been warm blooded and
perhaps raised their young in families unlike most other reptiles
with which we are familiar. Porpoises of today are very social
and this may well have been the case with some of these sea-going
Mosasaurs (reptiles from the Meuse) were large aquatic
marine reptiles that may be closely related to modern monitor
lizards. They ranged primarily through shallow inland seas and
Plesiosaurs (ribbon reptiles) are the arch-typical
sea monsters of Hollywood movies. Most were fish eating marine
dwellers with long necks, small heads full of sharp teeth, short
tails, wide bodies, and very well developed paddles instead of
hands or feet. But there were also shorter neck versions with
large heads and powerful jaws that probably were similar to the
killer whales of today.
As we have seen, lizards and snakes are both contemporaries
of the dinosaurs as well. Again, because of the puzzles of the
fossil records we do not have definitive answers to all the questions
on their origins. The Lizards because of their skull adaptations
talked about earlier and perhaps other unknown factors have been
very successful from perhaps 200 million years ago up to the present.
Fossils are limited but clear on that.
Snakes are the newest of the reptiles and that they
appeared somewhat after the first lizards. Very few fossils have
been found and most of these are fragmentary so a lot of guesswork
is needed here. They were probably around for along time living
a fossorial or burrowing existence and then returned to the surface
toward the end of the dinosaurs' reign.
The trouble with most history stories of life on
this planet is that once you come to the extinction of the dinosaurs
everyone now goes on to the mammals. Whoa there, did we miss something?
Did all the rest of the reptiles also suddenly die out? Not! Let's
go back and carry the tread of our story onward.
Many reptiles survived the mass extinction of 65
mya. Still existing in the class Reptilia are the Orders: Rhynchocephalia
one species the Tuatara; the Chelonia includes 244 species of
turtles and tortoises; the Crocodylia including 22 species of
alligators and crocodiles; Amphisbaenia including 144 species
of worm lizards; the Suborder Sauria and order Squamata containing
3,751 species of lizards; and the Suborder Serpentes, order Squamata
containing 2389 species of snakes for a total of at least 6,547
different species of reptiles alone.
Let's look at what little is known about the surviving
reptiles evolutionary history primarily from the end of the Cretaceous
Period starting 65 million years ago and continue our story line:
Paleontologists and zoologists have not been able
to determine exactly where the ancestor of modern serpents evolved
from. The two most popular theories are that either snakes came
from lizards (possibly like the earless monitor lizard Lanthanotus
borneensis or a gecko-like lizard or evolved independently from
a common ancestor such as the Amphisbaenia or wormlizard. So far
we have found fossils dating to the early Cretaceous Period in
North Africa that are about 140 million years old.
There are indications that the species may be as
old as 200 million years but since snakes are often "fossorial"
or underground dwellers they make poor candidates for fossilization.
By 60 million years ago it is known that snakes were found worldwide.
The earliest fossils that we can attribute to any
modern snake family are from the Boidae or boas. They date to
at least 60 mya and quite possibly 85 mya. At about 55 to 60 mya
we have also found fossils from the family Aniliidae which are
The next oldest fossils found were from the families
Leptotyphlopidae, Anomalepidae, and Typhlopidae which are small
wormlike burrowing snakes.
By the Eocene (36 to 55 mya) many fossils were starting
to show up in the records. It is clear that at this point boids
dominated the snake groups. The modern Colubrids started showing
up as early as 50 mya but didn't start showing up in numbers until
the Oligocene (22 to 36 mya). Since the vast majority of snake
fossils have shown up only in North America and Western Europe
we have a very limited idea of what really went on in the evolution
of snakes. We do know that there was a rapid evolution of species
from 22 to 5 million years ago. Venomous snakes appeared in the
fossil record for the first time during that period and possibly
as late as 25 mya. Those fossils already show an advanced venom
delivery apparatus so they may have evolved even earlier, though
that is strictly conjectural at this point.
Most of the modern species of snakes were present
by the Pliocene (1.8 to 5,5 mya) and that amounted to likely 3,000
or more species that is down today to about 2700.