Aminotes - Online Tutor, Practice Problems & Exam Prep

Birds are endothermic vertebrates with feathers and lightweight skeletons, classified as modern dinosaurs within the monophyletic group of reptiles. Amniotes are divided into sauropsids (reptiles and birds) and synapsids (mammals). Mammals possess mammary glands, hair, three middle ear bones, and a neocortex for advanced cognition. They include monotremes (egg-laying), marsupials (pouch-bearing), and placental mammals (long gestation with a placenta for nutrient transfer). Understanding these classifications highlights evolutionary adaptations and the diversity of life forms.

Birds and Synapsids

I want to briefly mention birds, which are endothermic vertebrates that have feathers, beaks, and lightweight skeletons we often call hollow or bird bones. And the reason I want to bring these guys up is that they are actually part of the monophyletic group, dinosaurs. That is to say, birds are in fact dinosaurs, and dinosaurs are part of the monophyletic group of reptiles. So these guys, even though they look very different, they've evolved to have a very different morphology, are part of that same monophyletic group. So, essentially, birds are dinosaurs. They are the living embodiment of dinosaurs today. And hopefully, you can see that, to a degree with this picture of a feathered dinosaur on the way to becoming a bird next to this roadrunner from today. And yes, it's a roadrunner just like the bird from the cartoon. That's what they actually look like, and they don't go "me me". Also, very quickly, you can see a feathered dinosaur here on its way to adapting birdlike flight as we see over here. Now, amniotes come in 2 basic flavors. Right? There are these sauropsids, which we just talked about. Those are reptiles and birds. And then there are these organisms called synapsids. And these are the group of amniotes that include mammals, and they happen to be distinct from other amniotes, or rather distinguished from other amniotes due to certain features of the skull. You don't really need to worry about that. I am not going to get into it. All I really want to do here is convey how, in a qualitative way, get you to think about how these sort of reptilian looking creatures, these synapses, which, you know, to the untrained eye, these guys the early forms of these creatures really do just look like reptiles. Right? But, hopefully, you can see, through these images I've provided, how they would transition from being more reptilian, like you see here, to something more akin to mammals you're familiar with today. Right? So, the growing of hair is certainly one of those big changes. And also, just the change in, you know, face shape, losing that sort of reptilian look. So here again, we have some more, kind of primitive early synapsids, and they're going to, you know, eventually lead to later synapsids like this guy who, you know, these guys obviously look very reptilian. This guy is kind of a weird transition organism. Right? It sort of has a reptilian look to it, also has hair, one of those weirder transition organisms. And then eventually we're going to, you know, get something more like this which certainly looks like it's on its way to becoming a mammal. Right? Has the fur, has the shape, but there's still something about it that looks, you know, harks back to its more ancestral form. And then, of course, ultimately, we're going to get, you know, rodent-like mammals. These are the earliest mammals to hit the scene. And to be clear, I'm not saying that they are rodents. I'm saying they have a rodent-like appearance. Those are what the earliest mammals look like. So hopefully these pictures can give you an idea of how these early synapsids, which were very reptilian looking, would eventually become mammals.

Mammals and Monotremes

After all these strange organisms we've looked at, we finally arrive at mammals, animals that look like us. These are endothermic amniotes, and they have mammary glands. That's what gives them their name. These are milk producing glands that feed the offspring. Mammals also characteristically have hair and 3 middle ear bones. This one seems kind of random. It's just one of the defining features. Don't, you know, I wouldn't stress about it too much. It's, the 3 bones used to convey sound in our ear. And, the last feature of mammals is the neocortex. This one is important. Right? This is what allowed for improved cognition and eventually for us to develop these big noodles that we have. So, we said that mammary glands produce milk to feed the offspring, most of that milk is coming from fat. It or rather, milk is, mostly, water and fat and sugar. So good nutrients to bulk up a baby. Here you can see a bunch of pups nursing on, this mother. Aren't corgis the cutest? I love corgis.

Alright. So let's talk about mammals. First of all, monotremes. These are kind of the weirdest mammals that there are. These are the strangest ones out there. They're egg-laying mammals. Now, generally when we think of mammals, we think that mammals give live birth. These guys don't, they actually lay eggs, that's why they, you know, are often, looked at as sort of an early evolutionary offshoot from mammals. Right? Whereas later mammals would give birth to live offspring, these guys don't. There are very few monotremes in the world. There are platypi. So here we have platypus, and then over here we have an echidna. And there are about 4 species of echidna, if I'm correct, and, only one in existence. So, very few monotremes in the world, and they are all found in Australia.

With that, let's turn the page.

Marsupials and Placental Mammals

Marsupials are another one of those strange types of mammals. They actually birth underdeveloped offspring that have to remain in a pouch, which contains the mammary gland nipple. So basically, these offspring come out and they are not fully developed like the live birth offspring of most mammals that you're used to. They look almost like they are still partly like a fetus in many ways, and that's why they have to remain in that protective pouch. You can see this little baby kangaroo poking its head out of the pouch there; well, that is a much more developed kangaroo than what it looks like when they first come out. And just to give you some examples of marsupials, we have a koala bear here, a kangaroo here, and an opossum here. Now, most marsupials are only found in Australia, like the koala bear and the kangaroo. Opossums, on the other hand, made it to the Americas.

So moving on, finally, we get to placental mammals. These are mammals like you are probably most used to. These mammals give birth to live developed offspring after a long gestation period, what we normally call pregnancy. And the defining feature of these organisms is the placenta, this organ pictured here, that connects the developing fetus to the uterus wall and allows the fetus to get nutrients from the mother to do gas exchange, as in breathe. This is how the fetus breathes, essentially. And, it also allows for waste disposal from the fetus. So here we have an example of a developing fetus in the uterus with placenta pictured. And here is an example of a placental mammal, a three-toed sloth. By far the most regal, if not also one of the nicest placental mammals out there. That's all I have for this video. I'll see you guys next time.

Here’s what students ask on this topic:

What are the main differences between sauropsids and synapsids?

Sauropsids and synapsids are two major groups of amniotes. Sauropsids include reptiles and birds, characterized by features like scales, feathers, and lightweight skeletons. Synapsids, on the other hand, include mammals and are distinguished by their skull structure, hair, mammary glands, and three middle ear bones. While sauropsids evolved into modern reptiles and birds, synapsids transitioned from reptilian ancestors to mammals, showing significant changes such as the development of fur and more complex brain structures like the neocortex.

How are birds classified as modern dinosaurs?

Birds are classified as modern dinosaurs because they belong to the monophyletic group of reptiles, which includes dinosaurs. This classification is based on evolutionary lineage, where birds evolved from theropod dinosaurs. Key features like feathers, beaks, and lightweight skeletons (hollow bones) are shared traits that link birds to their dinosaur ancestors. Thus, birds are considered the living descendants of dinosaurs, representing a direct evolutionary continuation.

What are the defining characteristics of mammals?

Mammals are defined by several key characteristics: they are endothermic amniotes with mammary glands that produce milk to feed their offspring. They also have hair or fur, three middle ear bones for sound transmission, and a neocortex, which is a part of the brain responsible for higher-order brain functions like cognition. These features distinguish mammals from other amniotes and highlight their evolutionary adaptations.

What are monotremes and how do they differ from other mammals?

Monotremes are a unique group of egg-laying mammals, including species like the platypus and echidna. Unlike most mammals that give live birth, monotremes lay eggs and are considered an early evolutionary offshoot. They possess mammary glands but lack nipples; instead, milk is secreted through openings in the skin. Monotremes are primarily found in Australia and exhibit a mix of reptilian and mammalian traits, making them distinct from marsupials and placental mammals.

What is the role of the placenta in placental mammals?

The placenta is a crucial organ in placental mammals, facilitating nutrient exchange, gas exchange, and waste disposal between the mother and the developing fetus. It connects the fetus to the uterine wall, allowing for the transfer of oxygen and nutrients from the mother’s bloodstream to the fetus while removing waste products. This organ supports the fetus throughout the gestation period, ensuring proper development until birth.