THE EVOLUTION OF REPTILES

Reptiles existed on the earth for roughly 100 million years before the first dinosaur appeared.

As you can see from the partial geological time scale to the above, reptiles evolved from amphibians sometime during the Carboniferous Period.

What's the difference between amphibians and reptiles? In other words, what specific evolutionary development changed an amphibian into a reptile? Oddly enough, it was not some adaptation to the animal's body, but a change in reproduction - the development of the amniotic egg.

Prior to the development of the amniotic egg, amphibians were "chained" to the ocean or some other large body of water, because they had to lay their eggs in water. If the eggs were removed from water and placed on land, they would simply dry out, obviously killing the egg. Imagine a map with only areas within about a mile or so of the ocean, rivers and lakes marked in blue. This would be about the limits of the colonization of the land by early amphibians.

The solution to this problem (the amniotic egg, see figure below) is basically a little environmental pod, like a spacecraft. The egg has the amniotic sac which contains the embryo immersed in amniotic fluid, a food supply (the yolk), a storage area for solid waste (allantois), and of course, the shell. But the shell is really the major development in a number of ways. Since there is no supply of air within the egg (as you would need in a space ship), the shell must allow oxygen in and gaseous respiratory waste (carbon dioxide) out. To do this the shell is actually porous on a microscopic scale. However, simply making the shell porous does not solve the problem,

because we actually end up back where we started since a porous shell would also allow the loss of water along with the carbon dioxide. The result would be a hard shell with a dried out interior (dead egg)! The solution to this is the thin layer of tissue called the chorion (this is the "skin" you see when you peel a hard-boiled egg). This tissue allows oxygen and carbon dioxide to pass through by osmosis, but is impermeable to water, so the egg retains its moisture. With all these features, an amniotic egg can be buried in the sand, or placed in a nest in a tree or wherever, and still survive. The amniotic egg opened up any portion of the earth's land suitable for reptiles for their colonization.

THE DIVERGENCE OF REPTILES - SYNAPSIDS AND DIAPSIDS

Sometime during the Permian two major clades of reptiles diverge (or evolve) from the original Anapsid reptiles. These are the Synapsids (one temporal fenestra below the squamosal and postorbital bones of the skull) and the Diapsids (two temporal fenestra - one above and one below the squamosal and postorbital bones). Both groups are extremely important because the Synapsids will eventually lead to the evolution of mammals, while the diapsids will give rise to the dinosaurs.

The Synapsid Reptiles

There are two main groups of Synapsid reptiles - the Pelycosaurs and the Therapsids.

Starting in the early Permian Period the Pelycosaurs become a very important group, with the carnivorous Dimetrodon, as seen above, being the most well known example. These reptiles are commonly referred to as the "sailbacks" for obvious reasons. In general, they are still a very primitive reptile group, with both the fore- and hind-limbs sprawled outwards to the side of the body, and with undifferentiated teeth (Dimetrodon has all fangs, big and little, but still all fangs; while Edaphosaurus, a herbivorous pelycosaur has all small peg-like teeth). The main evolutionary novelty developed by this group is the "fin" or "sail" on its back, which is formed by an extreme elongation of the neural spines of the animal's vertebrae. Some paleontologists have suggested that the purpose of the fin was to make the animal look larger in order to scare away attackers (in the same way a cat puffs up when it is frightened); while others have speculated that the fin may have been brightly colored for mate attraction.

The most generally accepted hypothesis, however, is that the fin was an early evolutionary attempt at thermoregulation. The pelycosaurs were still cold blooded ectotherms, but the sail is believed to have acted as a large solar panel. Each of the elongate neural spines which formed the framework of the fin had a deep groove which is believed to have held a large vein. If this is the case, the skin between the elongate spines would have held large numbers of blood vessels making it a thin, blood charged membrane. If the animal turned so that sunlight hit the fin all along one side, the result would have been a fairly rapid warming of the animals blood, allowing the animal to warm up quickly. On the other hand, if the animal had become overheated through exertion, it could face into the wind and the fin would act just like a fin on your car's radiator, allowing for rapid shedding of heat, and the cooling of the body.

     Note that the animal would still be an ectotherm, but this use of the fin for thermoregulation may have been one of the first cases of modifying a part of an animal's body into a large thin membrane to help control body temperature. The same principle is still in use among modern animals like the elephant, whose large thin ears allow it to shed excess body heat and prevent overheating.

THERMOREGULATION

Thermoregulation simply means an animal's ability to control its body temperature. Fish, amphibians and reptiles are ectothermic (cold blooded) because they have a low rate of metabolic heat generation and therefore need to take heat from their external environment in order to warm up their bodies to a functional temperature. This is why reptiles are often seen in the morning basking in the sunlight. Without that heat from the sun they would remain lethargic all day and would be unable to function. Since their body temperature is to a large extent dependent on the temperature of their environment, the opposite condition also applies - if they get overheated they must hide in the shade in order to cool down, otherwise they would die from heat stress.

Birds and mammals on the other hand have a high rate of metabolic heat generation which allows them to maintain a constant body temperature despite fluctuations in the temperature of their environment. They are called endothermic (warm blooded). In these animals overheating is prevented by losing heat through sweating, panting, etc., although a nice shady tree is also a big help.

The obvious advantage to endothermy is that you are never at other than full operating temperature (unless the environmental conditions turn extreme). In other words you never end up in a situation where you see a carnivore coming at you, but you're just too sluggish to move. The down side is that if you are going to supply your own heat through a high metabollic rate you need lots of fuel - endotherms need to eat much greater amounts of food than ectotherms. An alligator needs to eat about once a month. If a lion ate once a month you would end up with a dead lion.

 Therapsid Reptiles - The "Mammal-like" Reptiles

By the middle of the Permian Period a new group of Synapsids had appeared. These were the Therapsids, also known as the "mammal-like" reptiles. A quick look at Lycanops (above) will allow you to understand why these reptiles are called "mammal-like". This animal could probably be mistaken for a cat or dog by an average person. They are much more advanced than their cousins the pelycosaurs in that they are developing an upright stance - the legs are tucked under the body, not sprawled out to the sides. Also we begin to see differentiation of the teeth.

The therapsids diversify rapidly in the Permian, and fill numerous ecologic niches including

that of large herbivores. Moschops (above) is an example of a large herbivorous therapsid. It's barrel-like chest and peg-like teeth clearly identify it as a plant eater.

The main importance of the therapsids is that they are within the clade which will eventually give rise to the mammals.

Thermoregulation in Therapsids??

Some people have speculated that some of the therapsids may have been endothermic, and even gone so far as to depict therapsids with hair or fur. There is no direct evidence to support this. However, there is some circumstantial evidence. During the Permian the therapsids extended their range far to the south (their remains are found today in South Africa which was much farther south during the Permian). The best evidence we have indicates that these regions were high temperate to subarctic, and no other reptiles are found there. As a result some scientists have speculated that the therapsids may have been warm blooded (a necessary condition to survive in that climate). The fur or hair would have been necessary as insulation. But it is important to remember that, as yet, there is no hard evidence to support this hypothesis.

The Diapsids - the Other Side of the Family Tree

While the therapsids dominated the Permian, another group - the Diapsids - had evolved from the original Anapsid reptiles. Following the massive extinction event that marks the end of

the Permian Period, the diapsids start to radiate into large numbers of empty niches, although the therapsids are still dominant.

The diapsids include a large number of reptiles and lizards that you are familiar with (snakes, flying reptiles, dinosaurs, etc.), but unlike the synapsids there is no easy classification system for the diapsids of the Permian and Triassic.

  In general, we group these animals among the Archosaurs and define subgroups on the basis of the tarsals (ankle bones). Four types of ankle are defined: primitive mesotarsal (PM), crocodile normal (CN), crocodile reversed (CR), and advanced mesotarsal (AM). The differences in the shape and size of these bones result in different hinging movements between the ankle and the leg. Note the line X - X for each of the examples to the left. As the hinge changes, so does the stability and flexibility of the ankle. To a large degree the ankle structure correlates to the stance of the animal. Note in the next figure (archosaur cladistics) that the PM ankle is found in early diapsids with a sprawled gate (humerus and femur extend laterally away from the body) and in more advanced animals with a semi-sprawled gate. However, the CN ankle is found in animals with the "pillar erect" gate (the pelvis overhangs the femur), like the crocodiles; while the CR ankle is found only in a small group of "typical erect gate" animals known as the Ornithosuchidae. The AM ankle is found only in the "typical erect gate" Pterosaur (flying reptiles) and dinosaur groups.

In general, these archosaur groups went through a major diversification through much of the Triassic, and include the origin of the dinosaurs roughly 224 million years ago. But by the end of the Triassic, despite the original dominance of the therapsids and the diversification of other archosaurs, the dinosaurs had risen to dominance. How did this happen?

The Great Triassic Takeover Controversy

For most of this century, it was simply believed that there was something special about the dinosaurs which made them better than all the other reptiles. In a sense, the dinosaurs rose to dominance as a result of their own manifest destiny. Now, there are many scientists who

doubt this model. The result is that there are two models for the rise of the dinosaurs to dominance during the Triassic. The old model is called the Competitive Displacement Model, while the new one is the Opportunistic Model.

In the figure to the left, the Competitive Displacement Model is illustrated in (a). Here, the idea is that because of some dinosaur traits which made them better adapted for survival during the Triassic, they gradually out competed the other reptiles, and took over the world (something like Bill Gates and Microsoft). What were those traits? Supporters of this model point to the upright stance of the dinosaurs ( in some cases they were even bipedal). In general, animals with an upright stance do have an advantage over those with a sprawled stance due to Carrier's Constraint (sprawled animals cannot run and breathe at the same time). So proponents of the Competitive Displacement Model argue that the dinosaurs had superior locomotion and respiration, and also probably had higher metabolisms and maybe thermoregulation (endothermy).

Those who favor the opportunistic model (b) argue that as a result of a mass extinction near the end of the Triassic, large numbers of reptile species were eradicated and the dinosaurs were simply the first and fastest to take advantage of the situation and diversify to fill all the then vacant niches. That's why their model has the sharp vertical boundaries - no gradual displacement , but a very rapid extinction followed by opportunistic diversification.

Proponents of the Opportunistic Model make the following arguments. First, the fossil record does not support the Competitive Displacement Model. The dinosaurs appear to diversify rapidly in the late Triassic only after the extinction event results in major losses among the therapsids and the archosaurs (other than dinosaurs). Second, the superior adaptations argument is weak because other archosaurs and therapsids also appear to have developed an erect gate. Further, endothermy in early dinosaurs is by no means a proven fact. Third, the idea that competition among species can have a major, long-term effects may very well be an extreme oversimplification. Lastly, they point to a possible smoking gun - the large crater in Manicougan, Quebec (roughly 65 km in diameter) which dates from about 215 million years ago. A near perfect match with the timing of the Late Triassic extinction event. This, they claim is evidence that the dinosaurs took over only after the extinction (due to the impact) of the other animals.

So who's right? Right now, the Opportunistic Theory appears to receive the most support, but that may change. The idea that an impact could cause such massive worldwide effects is still being debated. Even the end Cretaceous impact (which supposedly killed off the dinosaurs) is not unanimously accepted as the cause for dinosaur extinction, and that crater is at least 125km in diameter.