Reptile History
Dragon History

 

Evolutionary History

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 survival.

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 the dinosaurs.

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 others.

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 reptiles.

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 the like.

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:

Snakes

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 mostly burrowers.

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.

 

© 2008 - 2016 all rights reserved