Category Archives: monotremes

Episode 119: Before the Dinosaurs

What kinds of animals lived before dinosaurs evolved? What did they evolve into? Let’s find out!

Dimetrodon! Not a dinosaur! Not even actually a reptile:

Cotylorhynchus had a teeny head. I am not even exaggerating:

Moschops had a big thick skull:

Lisowicia was the size of an elephant but looked like…well, not like an elephant:

Show transcript:

Welcome to Strange Animals Podcast. I’m your host, Kate Shaw.

Lots of people know about dinosaurs. Dinosaurs are really interesting. But do you know what animals lived before dinosaurs evolved? Let’s find out.

If you’ve heard of dimetrodon, you may think it’s just another dinosaur. It’s the animal that looks sort of like a huge lizard with a sail-like frill down its back. But not only was dimetrodon not a dinosaur, it went extinct 40 million years before the first dinosaur evolved.

Dimetrodon lived almost 300 million years ago and was a synapsid. Synapsid is a catchall term for a group of animals with both reptilian and mammalian characteristics, also sometimes called proto-mammals. The term synapsid also includes mammals, so yes, you are related to dimetrodon verrrrrrry distantly. You are more closely related to dimetrodon than you are to any dinosaur, let’s put it that way.  Dimetrodon was an early synapsid, which are referred to as pelycosaurs.

The largest species of dimetrodon grew up to 15 feet long, or 4.6 meters, with some probably growing even larger. It had serrated teeth, a long tail, short legs, and a massive sail on its back. The sail is formed from neural spines, which are basically just really long prongs of bone growing from the vertebrae. The spines were connected with webbing, although possibly not all the way to the tip of the spines. Ever since the first fossil remains of dimetrodon were discovered in 1878, scientists have been trying to figure out what the sail was for.

For a long time the most popular theory was that the sail helped with thermoregulation. That is, it helped dimetrodon stay warm in cool weather and cool in warm weather by absorbing sunshine or releasing heat, depending on where dimetrodon was. If dimetrodon was chilly, it would angle its body so that lots of sunlight reached its sail, but if dimetrodon was hot, it would find a patch of shade or turn its body so that minimal sunlight reached its sail, allowing the blood vessels covering the sail to release heat into the atmosphere.

This is a pretty good guess, since many modern animals use something similar to help regulate body temperature. That’s why African elephants have such large ears. But more recent studies of dimetrodon’s sail show that it didn’t have a lot of blood vessels, as it would if it was for thermoregulation. These days paleontologists suggest the sails may have mostly been for display. Different species had differently shaped sails, and there’s some evidence that male and female dimetrodons of the same species may have had differently shaped sails too. It’s possible the sails were brightly colored or patterned during the breeding season.

But dimetrodon wasn’t the only early synapsid with a sail. Secodontosaurus had one too and resembled dimetrodon in many ways, including having a long tail and short legs. But where dimetrodon was chunky with a massive skull, secodontosaurus was much more slender with long, narrow jaws. It may have eaten fish. It probably grew up to nine feet long, or 2.7 meters, and it lived around 275 million years ago. It was related to dimetrodon, but paleontologists aren’t sure how closely it was related.

The largest pelycosaur, or early synapsid, was cotylorhynchus [ko-tillo-rinkus], which lived around 275 million years ago in what is now North America. It was a weird-looking animal. Weird, weird weird. Seriously, it was very strange. It grew to almost twenty feet long, or 6 meters, with a barrel-shaped body, great big legs, and a long tail. But its neck was very short and its head was tiny.

Some researchers think cotylorhynchus lived in the water. Its forefeet may have been paddle-shaped. It ate plants, which is why its body was so big, since it needed room to hold lots of plants while they digested. It may have dug for roots as well, since its forefeet had long claws. Weird as it was, if you think of it as shaped sort of like a giant tortoise, its small head and big body make more sense.

Dimetrodon and other pelycosaurs lived in the early Permian era. By the mid-Permian, a group of synapsids called therapsids started evolving to become more mammal-like. The legs of therapsids were positioned more beneath the body instead of sprouting out from the sides, which is the difference between a dog’s body and a lizard’s body. This allowed therapsids to run more efficiently and breathe more efficiently when moving fast.

We know that at least some of these early therapsids had fur because paleontologists have found coprolites, which as you recall are fossilized poops, with fur embedded in them. Since this was long before mammals evolved, it had to be therapsids with fur. In fact, it was the therapsids that eventually evolved into mammals, so technically you are also a therapsid.

Therapsids were probably warm-blooded and probably had whiskers. But they wouldn’t have looked like mammals today. They probably resembled reptiles in a lot of ways, especially early therapsids. The tails of many therapsids would have looked like reptile tails, long, thick, and pointed. The heads would have looked much more like a lizard head than a mammal head, with no external ears.

Some therapsids would have looked really weird. For instance, moschops [mo-shops], which lived around 260 million years ago in what is now southern Africa. Moschops was a type of therapsid that ate plants, and it was massively built. It was around 8 feet long, or 2.5 meters, and had a thick skull and short snout with strong jaw muscles. The back sloped downward from the shoulders to a short tail. Its relatively short legs were sturdy to hold up the weight of the broad and massive body. The front legs were much farther apart than the hind legs. Its teeth were strong but not sharp; instead, they had chisel-like edges that helped it bite through tough vegetation.

Moschops had such a thick skull that many researchers think it fought other moschopses by butting heads. The small brain was extremely well protected by a skull that was as much as 6 inches thick, or 15 cm, and new research shows that the head was usually held forward instead of up. This makes sense in a grazing animal, and would also make sense if males were butting heads to impress potential mates, or if individuals fought over territory or food. If moschops did butt heads, it’s possible that it lived in groups with a certain amount of social organization.

Toward the end of the Permian, a group of therapsids called dicynodonts became widespread and lived well into the Triassic era. Dicynodonts were probably warm-blooded, probably had fur or hair, and some may have had feet that were more paw-like than reptilian, with fleshy pads. But while all these features are mammalian, most dicynodonts had a horny beak like a turtle and either no teeth at all, or only a pair of teeth in the front of the jaw that grew like tusks. Some paleontologists think only males had these tusks. Most dicynodonts were herbivorous and some dug burrows.

About 250 million years ago, there was a mass extinction event called the Permian-Triassic extinction, or sometimes just the Great Dying. Researchers aren’t sure what caused it, but like the later extinction that ended the dinosaurs, it may have been caused initially by a massive meteor impact that sent the earth’s climate into a tailspin. 96% of all marine species went extinct and 70% of land animals. This was the event that led to the rise of the dinosaurs ultimately. But some therapsids survived.

The biggest dicynodont evolved after the great dying and it was the size of an elephant. Lisowicia lived in what is now Poland around 230 million years ago, but it was probably more widespread than that sounds. We only have a single specimen of lisowicia that was discovered in south Poland in 2008. It probably stood 8 ½ feet high, or 2.6 meters. All four of its legs were positioned under the body like modern mammals, whereas most dicynodonts were similar to moschops, where the hind legs were under the body and the forelegs were more widely spaced and sprawling. But it probably didn’t look much like a modern mammal beyond that. Its head would probably have looked quite reptilian since it had a horny beak like other dicynodonts. Its tail was short.

Dicynodonts went extinct by the late Triassic, but the related cynodonts persisted. Cynodonts are the direct ancestors of mammals. You are definitely also considered a cynodont. The first cynodonts evolved in the late Permian and had a lot of traits that are still retained by mammals, such as fur, whiskers, warm-bloodedness, and teeth that are differentiated into different types like molars and incisors. They also developed what’s called a secondary palate, or as we call it, the roof of the mouth. All mammals still have this feature, which allows us to breathe and chew at the same time. But cynodonts also still probably laid eggs. Eventually cynodonts developed into monotremes like the platypus and echidna, which many researchers consider to retain many cynodont features.

Probably the largest cynodont was cynognathus, which lived around 240 million years ago. Cynognathus was a predator that grew almost four feet long, or 1.2 meters, not counting its long tail. It was widespread throughout the southern hemisphere, with cynognathus fossils of various species found in modern-day southern Africa, South America, and Antarctica. It had already evolved the secondary palate, and its head and jaws were both long and wide, with sharp teeth.

Because cynodonts lived alongside dinosaurs for millions of years, they evolved into animals that were generally quite small, no larger than a rat, and frequently nocturnal. But they were still incredibly successful, spreading out across the world and evolving into animals that looked more and more like mammals that we’d recognize today. The haramiyids were probably insectivores and lived in trees, with some species able to glide like flying squirrels or the colugo. Many cynodonts lived in large shared burrows, suggesting increasingly complex social behavior.

But not all early mammals were tiny and ran away from dinosaurs. Repenomamus [re-penno-may-mus] lived around 125 million years ago and grew over three feet long, or 1 meter. In shape, it somewhat resembled a badger with a long tail. We know it ate small animals, including hatchling dinosaurs, because fossil remains of a baby psittacosaurus [sit-acko-saurus] was found in the stomach area of a fossil repenomamus. The psittacosaurus remains were in chunks, which suggests that repenomamus had bitten it into pieces to swallow it.

Repenomamus was considered a Eutriconodont, a type of early mammal, but the eutriconodonts went extinct at about the same time as the dinosaurs.

But by then, the therapsids were fully evolved into what we have termed mammals. And they were poised to take over. Or, I should say, we mammals were poised to take over. And we have.

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Thanks for listening!

Episode 045: Monotremes

At last, it’s the episode about the platypus, a monotreme! Only two kinds of monotremes remain: the platypus and the echidna. Monotremes are mammals that lay eggs! Not even making that up.

The echidna:

Do not eat:

A platypus and another platypus:

Show transcript:

Welcome to Strange Animals Podcast. I’m your host, Kate Shaw.

This week we’re finally, finally going to look at the platypus and its relations, called monotremes. I’ve been promising a platypus episode for months and now, it is time.

There is so much weird about the platypus, it’s hard to know where to start. So let’s pull back for a second and look at the bigger picture.

Hopefully most of my listeners are familiar with what traits make an animal a mammal instead of a bird or a fish or what have you. At some point in elementary school, you either had to memorize a list of mammalian traits or you will have to memorize one. The list will be something like this: mammals are warm-blooded, grow hair, and feed their babies with milk. Boom, that’s a mammal. There are more differences than that, and some minor exceptions in the growing hair category, but those are the big differences. But even a little tiny baby who doesn’t know anything knows the difference between a bird and, say, a cat or dog. Birds have feathers, mammals never do. Birds lay eggs, mammals never do.

But wait. That’s wrong. Not the feather thing, but the egg-laying. Some mammals lay eggs. Specifically, the monotremes.

There are three main types of mammals. The biggest is the placental mammal group, which includes humans, dogs, cats, mice, bats, horses, whales, giraffes, and so on. A female placental mammal grows her babies inside her body in the uterus, each baby wrapped in a fluid-filled sac called a placenta. During birth, the placenta tears open and the baby is born first, followed by the placenta, which is frequently called the afterbirth. Placental mammals are pretty well developed when they’re born, with considerable variation. Baby deer and horses, for instance, can stand and run within a few hours of birth, while kittens and puppies don’t even have their eyes open yet. But they’re all mostly done cooking, so to speak.

The second type is the marsupial mammal group, which includes possums, kangaroos, koalas, wombats, sugar gliders, and so on. A female marsupial has two uteruses, and while her babies initially grow inside her, they’re born very early. A baby marsupial, called a joey, is just a tiny little pink squidge about the size of a bean that’s not anywhere near done growing, but it’s not completely helpless. It has relatively well developed forelegs so it can crawl up its mother’s fur and find a teat. Some species of marsupial have a pouch around the teats, like possums and kangaroos, but other species don’t. Either way, once the baby finds a teat, it clamps on and stays there for weeks or months while it continues to grow.

The third and rarest type of mammal is the monotreme group, and monotremes lay eggs. But their eggs aren’t like bird eggs. They’re more like reptile eggs, with a soft, leathery shell. The female monotreme keeps her eggs inside her body until it’s almost time for them to hatch. The babies are small squidge beans like marsupial newborns, and I’m delighted to report that they’re called puggles. Echidnas have pouches and after a mother echidna lays her single egg, she tucks it in the pouch. The platypus doesn’t have a pouch, so after she lays her one to three eggs, a mother platypus holds them against her belly with her flat tail to keep them warm.

Monotremes show a number of physical traits that are considered primitive. Some of the traits, like the bones that make up their shoulders and the placement of their legs, are shared with reptiles but not found in most modern mammals. Other traits are shared with birds. The word monotreme means “one opening,” and that opening, called a cloaca, is used for reproductive and excretory systems instead of those systems using separate openings.

It wasn’t until 1824 that scientists figured out that monotreme moms produce milk. They don’t have teats, so the puggles lick the milk up from what are known as milk patches. Before then a lot of scientists argued that monotremes weren’t mammals at all and should either be classified with the reptiles or as their own class, the prototheria.

It’s easy to think, “Oh, that mammal is so primitive, it must not have evolved much since the common ancestor of mammals, birds, and reptiles was alive 315 million years ago,” but of course that’s not the case. It’s just that the monotremes that survived did just fine with the basic structures they evolved a long time ago. There were no evolutionary pressures to develop different shoulder bones or stop laying eggs. Other structures have evolved considerably.

There are only two types of monotremes still living today, the platypus and the echidna, also called the spiny anteater. We only have one species of platypus but four species of echidnas. All monotremes live in Australia and New Guinea these days, but once they were common throughout much of the world. The oldest platypus fossil found is from South America. But around 60 or 70 million years ago, the ancestors of today’s marsupials started to outcompete the monotremes. Researchers think the ancestor of the platypus and echidna survived because it spent a good part of its life in water. Marsupials are not water animals because their joeys will drown.

Let’s talk about the echidna first. The echidna is a land animal, unlike its ancestors, although it swims well and likes water. It looks a lot like a big hedgehog with a long nose. It’s around a foot to 18 inches long, or 30 to 45 cm. Its short fur is brown or black in color with paler spines, and like a hedgehog it will curl up when threatened so that its spines stick out. It has short, powerful legs that it uses to dig burrows and dig up insects, worms, and especially termites, which it slurps up with a sticky tongue. It doesn’t have teeth. It lives in forests and is a solitary animal most of the time.

Now, the platypus. When I was a kid, pretty much all my knowledge of the platypus, and Australia in general, came from the cartoon Dot and the Kangaroo, so I’m upset to report that the scientific name of the platypus has been changed from Ornithorhynchus paradoxis to Ornithorhynchus anatinus. Sometimes people say duck-billed platypus, which isn’t wrong but since there’s only one species of platypus there’s no reason to be that specific, guys. The snout looks like a duck bill but it’s actually very different. It’s soft and rubbery and it’s packed with electroreceptors that allow the platypus to sense the tiny electrical fields generated by muscular contractions in its prey.

When the first platypus skin was brought to Europe in 1798, scientists immediately decided it was a big fakey fake. Obviously some wag had taken a dead beaver and stuck a duck’s bill on it. It wasn’t until 1802 that a scientist was able to dissect a recently killed platypus and realized that not only was it a real animal, not a hoax, but it was really weird. And they didn’t even know until 1884 that it laid eggs.

The platypus is about 20 inches long, or 50 cm, and it has brown fur that’s short and very dense. The platypus spends a lot of time in the water, and its fur traps air close to the skin to help keep the animal warm while not allowing water in. It has a strong flattened tail that acts as a rudder when it swims, along with its hind feet. It actually swims using its big webbed forefeet. It lives in eastern Australia along rivers and streams, and digs a short burrow in the riverbank to sleep in. The female digs a deeper burrow before she has her babies, sometimes over 65 feet long, or 20 meters. At the end she makes a nest out of leaves.

The platypus eats pretty much everything it can catch, from worms and fish to crustaceans and insects. Platypus puggles are born with a few teeth, but lose them as they grow up. Adults don’t have teeth at all, just hardened skin. While it’s underwater, the platypus closes its eyes, nostrils, and ears, and instead navigates and hunts by means of its well-developed electrolocation abilities. The echidna has some electroreceptors but only a fraction of the number the platypus has. Males of both the echidna and the platypus have sharp spurs on their hind legs, but only the platypus can inject venom. Researchers still aren’t sure why.

The platypus is difficult to keep in captivity, so unless you visit Australia you probably won’t get to see one. The echidna is less difficult to keep in captivity but won’t breed in captivity. So if you live in the same parts of the world where the last living monotremes live, consider yourself lucky because you might catch a glimpse of the only mammals that still lay eggs.

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