Most of us start learning about mammals before we can even talk. Books for babies and young children are filled with pictures of cute furry mammals from Aardvark to zebras. Many of them from dogs and cats to elephants and lions are so familiar to us that they feel like old friends, but how much do we really know about mammals? Let's start with the basics. Mammals are vertebrate animals with four legs; usually, fur or hair; most of the time and that makes milk; at least the females do. We all know that much, but mammals are also distinguished in ways that may be less familiar. Mammals have sweat glands that allow them to secrete water onto the surface of their skin. As the water evaporates, it cools the skin, helping the animal to control its body temperature. Mammals have jaws that consist of a single lower mandible bone called the dentary, which is connected by a hinge to the upper part of the skull, the cranium. The bones that formed the jaw hinge in the ancestors of mammals evolved into the bones that form the middle ear in mammals and help transmit sound to the inner ear. This is part of what makes some mammal species so good at hearing. Mammals have specialized teeth with different sizes and shapes that reflect their different diets. Unlike reptiles, mammals can have teeth with different sizes and shapes, even within the same individual. Here we have some skulls from a variety of different types of mammals. This is a dog skull. Take a look at the different types of teeth. Here in the front, these short flat teeth are the incisors and those are useful for scraping. The long pointy canines, whose name means dog, are for puncturing, enemies or prey. The premolars here getting back towards the back of the mouth, they have cusps that are used for both cutting and chewing while the molars, all the way at the back here are mostly for chewing. Mammals are the only animals that have so many different kinds of teeth all in the same mouth. We can see some other examples in some of the other skulls that we have here. This is a pig and we can see if we take the jaw off of the cranium here, is a similar situation where you've got these flat molars here towards the back of the mouth and you can see it really well here on the mandible, you've got molars in the back, you've got these premolars and then once again, very pointy canines, and then the incisors in the front for scraping. Pigs are omnivores, so like dogs, they need to be able to use their teeth for a lot of different types of things. This is an interesting one. This is a Virginia Pawsome and they have, first of all, a lot of teeth and second of all, they're all really pointy, but we still have incisors, canines, premolars, and molars. Although here, even the molars are quite sharp, intimidating-looking set of teeth here. This is a really cool one. This is a type of rodent. It's a muskrat and like all rodents, their front incisors are very elongated. They're actually reinforced with minerals that give them this orange color, and that's for scraping away on nuts and seeds or wood or whatever. Then in the back, they have these very flat molars. Mammals belong to a group known as synapsids and today mammals are the only living synapsids, but this group was once much more diverse. In fact, during the Permian period, synapsids were some of the dominant land animals. Let's head over to the Houston Museum of Natural Sciences to take a look at some fossils that help tell the story of mammal evolution. Here in the Paleontology Hall at the Houston Museum of Natural Science, there are several fossil synapsids on display. This one is Dimetrodon. I had a toy Dimetrodon as a kid and I used to think it was a dinosaur. Now I know that Dimetrodon isn't a dinosaur. It's a synapsid. One of the distinguishing characters of synapsids is the hole in the skull located just behind the eye. This hole is called the temporal fenestra. Synapsids all have a single temporal fenestra. In fact, because we humans are synapsids, we have a temporal fenestra. If you rub your temples, the spot just behind your eye, you're touching the place where the muscle that controls your jaws attaches to the skull. That muscle passes through a hole created by the zygomatic arch or cheekbone and connects to the jaw. There's also a nice replica of what Dimetrodon may have looked like when it was alive. Looking at this replica of Dimetrodon, it's easy to see why it sometimes gets confused with dinosaurs. Early Synapsids did look a lot more like reptiles than mammals. They didn't have any hair and they laid eggs. But they're actually more closely related to you and me, than they are to T-Rex or any other dinosaur. The first true mammals evolved during the Jurassic period. That means they were alive at the same time as some dinosaurs. In the aftermath of the mass extinction event at the end of the Cretaceous period, 66 million years ago, mammals became much more diverse. Several species of living mammals have retained some of the characteristics found in the earliest mammals. The monotremes or lineage of just five species of mammals that are restricted to Australia and New Guinea. They include the platypus and four species of echidnas. Platypus and echidnas have hair like all mammals. The females produce milk, but not from nipples. Rather, the milk is produced by glands in the skin and drips onto the fur where the babies lap it up. But unlike most other mammals, monotremes lay eggs rather than giving birth to live young. In that sense, they resemble their reptilian ancestors. Another way that monotremes are like reptiles is that they have a cloaca, a single opening for their urinary, defecatory, and reproductive systems. Another major lineage of mammals with about 324 species is the marsupials. Unlike monotremes, marsupials give birth to live young. However, they're born at a very early developmental stage and complete their development inside a pouch on their mother's body. I once had the opportunity to pet a Joey, a baby kangaroo that was inside its mother's pouch at a zoo in Australia. The inner lining of the pouch was the softest thing I've ever touched. Inside the pouch is also a nipple, where the Joey can drink its mother's milk. Having a nipple makes it easier for the Joey to drink. Marsupials first evolved in the Cretaceous period when South America, Africa, Antarctica, and Australia were joined together to form the super continent Gondwana. Marsupials were widespread on this enormous landmass. But once it began to split up, some populations became more isolated than others. The marsupials living on what became the continents of South America and Australia were especially isolated as these landmasses had very little contact with other continents. Today, marsupials are only found in Australia and the surrounding islands, and in North and South America. Today, we think of Australia as the land of marsupials, like kangaroos and koalas. Indeed, nearly all of the mammals native to Australia, about 250 species are marsupials. They thrived and diversified due to Australia's isolation from other major land masses where other mammals live that could have competed with the marsupials. As Australia's marsupials diversified, they evolved the bodies and lifestyles that allowed them to occupy many ecological niches and ways that paralleled the evolution of mammals elsewhere in the world. The only North American marsupial is the Virginia Opossum, a species that migrated from South America after the Panamanian land bridge formed about 3 million years ago. In contrast, South America is home to about 120 species of marsupials. It had even more before the Panamanian land bridge connected it to North America. While Virginia Opossums traveled across the land bridge from South to North, other species traveled from North to South. These included many eutherian or placental mammals, which competed with the native marsupials, causing many of them to become extinct. In most places where marsupials have come into contact with eutherian mammals, the eutherian mammals have had the advantage. The major innovation of the eutherian mammals was the evolution of the placenta. We'll talk more about the placenta, and how it came into existence in another lecture. For now, let's consider how the eutherian mammals have diversified to become the dominant mammals on Earth today. Let's head over to the Houston Zoo to check out some interesting eutherian mammals. One of my favorite groups of eutherian mammals are the xenarthrans. These include the armadillos, sloths, and anteaters. I'm at the Pantanal Exhibit at the Houston Zoo, where they have a couple of giant anteaters. This species is native to tropical South America. I've often come across them in the arid grasslands of Central Brazil, and the Wetlands of the Pantanal in Southern Brazil. Because they're eutherian mammals, the females undergo a longer gestation or pregnancy than marsupials. That means that their young are born at a later developmental stage, and are therefore less vulnerable. When baby giant anteaters are born, they look like miniature versions of adults already covered in fur. But they still spend the first year of their lives very close to their mothers. In fact, they spend most of that first year clinging to their mother's back. Giant anteaters have one of the most unusual body shapes of all eutherian mammals. Its skull has an elongated snout with a tiny mouth at the end. Like many xenarthrans, anteaters have no teeth. They don't need them since they feed on ants and termites using their sticky tongue, which is two feet long. They use their long sharp claws to dig into termite nests. The claws are also used for defense. Just because they can't bite doesn't mean they're defenseless. Another major group of eutherian mammals are the hoofed animals with an odd number of toes, known as the Perissodactyla. They include horses and zebras, which have one toe, and tapirs and rhinoceroses that have three toes. Behind me is a white rhinoceros. Rhinos are among the largest of all living land animals, second only to elephants. Unfortunately, despite being protected, rhinos are regularly killed for their horns, which are sold as medicine, even though there's no evidence to suggest that they have any medicinal properties. After all, their horns are made of keratin, the same substance as our fingernails and toenails. The hoofed animals with an even number of toes called the Artiodactyla form a separate lineage. These include deer, antelope, cattle, sheep, and giraffes, all of which have two toes, and pigs and hippos which have four toes. Interestingly, the cetaceans, aquatic mammals that include whales and dolphins, are closely related to the artiodactyls. The fossil record shows how the ancestors of modern cetaceans gradually lost their hind limbs as they evolved to be better adapted to a fully aquatic lifestyle. Their front legs evolved into paddle-like fins, while their tails evolved into a powerful fluke made of cartilage that propels them through the water. They also lost most of their hair, relying on thick layers of fat beneath the skin to stay warm. The cetaceans subsequently split into the toothed whales; the group that consists of the dolphins, porpoises, and sperm whales, and the baleen whales; which are filter feeders. One species of baleen whale, the blue whale, is the largest animal that has ever lived, larger than any dinosaur. They can reach a length of nearly 100 feet. One of the most diverse groups of mammals is the rodents. Rodents include not only the mice and rats, but also squirrels, beavers, porcupines, and the enormous South American capybaras. They have large incisors that are useful for gnawing and scraping. There are more than 2,000 species of rodents. Bats are another very diverse group of mammals with some 1,400 species classified in the order Chiroptera. Bats evolved unique bodies that allow them to fly. Their wings are modified forelimbs containing the same bones that form our hands and fingers connected by a thin membrane. Bats have diverse diets, many feed on insects while some are strict herbivorous. Some bats serve as important pollinators, including of crop plants like mangoes and bananas.
