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One of the more ‘recent’ evolutionary developments in vertebrates is a terrestrial egg.

This adaptation allowed for even greater diversity in this branch of the Animal Kingdom, giving rise to organisms that could now live their entire lives on land.

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A Brief History of Amniotes

When you hear the word ‘animal,’ the images that come to mind are likely of vertebrates. These are animals with a backbone, and they’re the ones we are most familiar with: mammals, reptiles, fish, amphibians, and others.But take a look at that list again, and you’ll start realizing how diverse the vertebrates really are.

They sure have come a long way from their primitive chordate ancestors – animals like the tunicates and lancelets. These two animals may not look much like us (or much like animals!), but like all chordates, they have four distinguishing features in common: a dorsal, hollow nerve cord, a notochord, pharyngeal slits, and a post-anal tail.There have been many important events along the vertebrate evolutionary path. A long time ago, some chordates developed heads. This branch from the evolutionary tree created a new group called the craniates, which are simply chordates with heads. Following this, some craniates developed that backbone that makes a vertebrate a vertebrate. Next, jaws evolved in some of these vertebrates, which allowed for even greater diversification in this section of the animal kingdom.

Jawed animals were able to eat new things and to become more specialized in their feeding.After jaws, some vertebrates also developed legs and moved onto land. These animals are known as the tetrapods because they have four limbs (‘tetra’ means ‘four’).

This was a major accomplishment! Tetrapods took diversity to a whole new level, because life was no longer confined to aquatic environments.Tetrapods had it pretty good, but some of them took it one step further. This group, called the amniotes, are tetrapods with a terrestrially adapted egg. This may not sound like a big deal, but this major development meant that these tetrapods could lay their eggs on land instead of having to return to an aquatic environment to do so. In this group, we find mammals and reptiles (including birds).

Because of this unique adaptation, these guys could officially call land ‘home!’

Amniotic Eggs

The eggs of amniotes really are something special. In fact, it’s where amniotes get their name. The amniotic egg has specialized embryonic membranes which are not part of the embryo’s body.

The amnion of these eggs is a fluid-filled sac surrounding the embryo inside the egg. Outside of this is the yolk sac, which provides nutrients for the embryo. This is the very same yolk sac that you find when you crack open a chicken egg for breakfast.Surrounding all of this is the chorion, which eventually develops into the placenta in mammals. Filling the rest of the space, amniotic eggs also have that typical egg white called the albumen.But what made these eggs land-friendly was their waterproof outer shell.

Other tetrapods, like frogs, have to return to water to lay their eggs, because their eggs do not have this outer shell. Without water, their eggs dry out, and the embryos can’t survive.The protective shell was crucial for this evolutionary step in vertebrates. In fact, it’s so beneficial that we see this same type of development in the evolution of plants. Seeds are like egg shells for plants – they surround and protect the plant embryo and food source inside. This ‘survival packet’ allows for plant embryos to develop in a wide variety of environments instead of being dependent on water.

Examples of Amniotes

As you can imagine, there are many different amniotes on Earth today. Reptiles are numerous and found in a wide variety of environments. Snakes, lizards, turtles, and crocodiles are just a few examples of these 4-legged creatures. But birds are also included in this group, and there are just too many of those to name in this short lesson! Dinosaurs aren’t around any longer, but they, too, were terrestrial tetrapods with amniotic eggs.Reptiles have other land-living adaptations that make them different from their amphibious counterparts. For example, the skin of reptiles is covered with waterproof scales that prevent them from drying out on land. They also can’t ‘breathe’ through this tough skin, so they take in most of their oxygen by breathing through their lungs.

Reptiles are also ectothermic, meaning that they do not generate their own body heat. The word comes from the Greek ‘ektos,’ which means ‘outside’ and ‘therm,’ which means ‘heat.’Birds are included in the reptile category, but they are a bit different because they have feathers and are adapted for flight. Part of what makes flight possible is the adaptation of the front two limbs into wings.

Just like the wings of an airplane, bird wings facilitate lift and movement in the air.Another major difference is that birds are endothermic, meaning that they do generate their own body heat (‘endo’ means ‘inside’). Birds come as small as hummingbirds and as large as ostriches and are found on all seven continents, making them one of the most diverse groups of animals on Earth.

Like birds, mammals are also endothermic amniotes. But mammals are easily distinguishable from reptiles, because they have hair and produce milk. Most mammals also don’t ‘hatch’ from eggs; rather, they’re born live. In fact, there are only two mammals that do lay eggs: the spiny anteater and the duck-billed platypus. Otherwise, the mammalian egg stays inside the mother during development, receiving nutrition from her blood until it is ready to be born.

Lesson Summary

A far cry from their ancient ancestors, amniotes are a diverse and wide-spread group of animals. Named for their distinct shell-covered eggs, amniotes are tetrapods with a terrestrially adapted egg. Being able to lay eggs outside of water meant that these animals could thrive and diversify on land in new and exciting ways.The amniotes are reptiles, birds, and mammals. It’s clear that reptiles are adapted to land life with their protective, thick skin that keeps them from drying out.

Birds are reptiles with feathered wings. These appendages are adapted from the two original tetrapod front limbs, and their feathers and wing shape facilitate flight. Mammals are amniotes that produce milk and have hair. But what makes mammals especially unique is that their babies develop inside the mother instead of hatching from an egg outside her body.

Learning Outcomes

After reviewing this lesson, you’ll have the ability to:

  • Identify the four features that all vertebrates have in common
  • Define amniotes and summarize their evolution
  • Describe three types of amniotes: reptiles, birds, and mammals

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