The Evolution of Modern Reptiles

Instructor: Stephanie Gorski

Steph has a PhD in Entomology and teaches college biology and ecology.

In this lesson, we will discuss the major hurdles that reptiles had to overcome so that they could colonize dry land. We will also discuss how reptiles are classified.

Reptile Eggs

Imagine that you want your children to colonize other planets. What do you need to do to make them ready for these other planets? You might imagine that your children would need structural changes to deal with the different gravity on other planets. You might imagine that other planets could be hotter or colder, and try to give your children ways to regulate heat. But first and foremost, you would design them a really nice, safe, cozy spaceship.

A reptile's egg is very much like your spaceship. It was the structure of the reptilian egg that allowed vertebrates to colonize land far away from oceans or rivers.

Have you ever eaten caviar or fish roe? Imagine leaving it open on the counter. If a soft, gooey egg from a fish or amphibian is left in the open air, it will dry up and the embryo inside will die. These delicate eggs, so sensitive to dehydration, are a major reason that you would never see an amphibian in the middle of the desert.

The evolution of the amniotic egg in early reptiles is one major step that allowed our evolutionary ancestors to colonize dry areas. Amniotic eggs are covered with a hard shell that protects the developing embryo inside from predators, pathogens, destruction, and, perhaps most importantly, water loss. These shells allow oxygen to pass through, so the fetus can breathe, but they don't allow water to pass through. These hard shells allow reptiles to keep their eggs anywhere- burying them in the sand like a sea turtle, for example. Once these shells developed, new and strange worlds were available for colonization by reptiles.

The amniotic egg is structured with four small sacs inside the shell.

  1. The chorion allows for the exchange of gases, oxygen and carbon dioxide. If you've ever noticed the thin skin on a hard-boiled egg, that's the chorion.
  2. The amnion holds water.
  3. The allantois holds waste. Since eggs can't excrete waste, you can think of the allantois as a garbage bag that holds the embryo's excretions until it is ready to hatch.
  4. Finally, the yolk holds nutrients that feed the embryo as it develops.

Snake eggs hatching
Snake eggs hatching

Sprawling and Erect Gait

Early vertebrates that lived on land inherited the bone and muscle structure of their fishy ancestors. If you know how to swim, you know that moving on land is much different than moving on water. On land, you have to hold your own weight in a way that water doesn't require.

Early vertebrates have what is known as a sprawling gait, meaning their limbs sprawl out to the side, as opposed to an upright gait like you have. The problem with the sprawling gait is that it requires an animal to flex its trunk to the left and right as it walks. Thus, every time an animal takes a step with its left foot, its right side compresses, and vice versa. When the right side compresses, the right lung compresses. That means an animal with a sprawling gait can't run and breathe at the same time.

This doesn't mean that animals with a sprawling gait can't run at all. You may have seen a lizard run, and lizards have sprawling gaits. A lizard can run for a short time using stored oxygen, and then it will use anaerobic glycolysis to produce energy without oxygen. But a lizard can't run for a sustained period of time without going into oxygen debt. This problem is known as Carrier's Constraint.

Descendants of reptiles such as mammals and birds overcame Carrier's Constraint by developing an upright gait, which allows them to run without collapsing their trunk. This made it beneficial for these animals to have a higher metabolism, which eventually led to warm-blooded animals.

However, reptiles that still exist today have a sprawling gait. If you think of reptiles as being lazy and relatively inactive, this has a lot to do with Carrier's Constraint.

Catching prey the way a lion or tiger does requires a lot of oxygen and a lot of energy. That's why you don't really see reptiles that hunt actively. Reptilian predators that exist today ambush their prey. For example, a chameleon will flick out its tongue at a passing insect. Crocodiles will hide under the water and wait for prey to come by. Komodo dragons can sprint for short distances in search of prey, but prefer prey that is very close by.

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