The Truth about the Orders of Mammals

In school, we were fed the narrative that there are five types of vertebrates: mammals, reptiles, amphibians, birds, and fish. However, that is bullshit. I mean, it’s technically true. But it’s also immensely oversimplified. Not even acknowledging that by “fish” they’re lumping together jawless fish (lampreys, hagfish), cartilaginous fish (sharks, rays), and bony fish (what most people would actually call a fish) which are all much more distantly related than mammals, reptiles, amphibians, and birds are to each other. Don’t believe me? All those tetrapods I just mentioned are technically included under bony fish. (remember our fish ancestors that came to land, which is generally regarded as a bad move). If they acknowledge the existence of other types of mammals at all, it is only placentals and marsupials.

That’s not even what I’m angry about right now. I’m angry because teachers lie to their own students with the preposterous claim that mammals give live birth. Haven’t they ever heard of a platypus? Because platypuses somehow manage to have fur and lay eggs at the same time, something these teachers must think defy the very laws of reality.

There are in fact three types of mammal; marsupials, monotremes, and the placental mammals. The three factions likely existed as early as the dinosaurs, but for some reason, were isolated onto three major land masses.

The only thing which can quell my burning rage is to explain the three types of mammal to you now.

• Table of Contents:
• Monotremes
• Marsupials
• Placentals

Monotremes; the Weirdest Mammals

Starting off with Australia. You may have been expecting me to say that marsupials evolved in Australia, and you would be wrong. The marsupials came to Australia from Antarctica, which was connected then and not quite as cold. So why is Australia famous for having lots of crazy marsupials like kangaroos and Tasmanian devils? It’s because the mammals that did evolve in Australia were so shit that almost all of them died. That’s particularly humiliating since marsupials are also pretty shit and are known for getting wiped out by invading Placental mammals. 

So what exactly did live in Australia that was so shit? Monotremes! Only five species of monotreme are left today. In Australia there is the Platypus and the Short Beaked Echidna. The island of Papua New Guinea also has three species of Long Beaked Echidna. You know things are bleak when three of your five surviving members are Papuan knockoffs of your other member. They only survived because they had niches that are so hard to adapt to that their shittiness doesn’t matter.

It’s like being the only programmer for a company but also an asshole with no personal hygiene. They’d love to fire you, but you’re only person on earth who has any idea how your code works. Untangling your Gordian knot of bullshit should cost more than just putting up with you and your bullshit. That’s why I always ignore all formatting guidelines, never leave comments or documentation, and only write in C++. Definitely not because I’m an awful programmer.

The Monotremes branched off from other mammals a very long time ago. They still retain a lot of the primitive features of the very first mammals. For instance, their limbs are attached to the sides of their bodies like lizards instead of underneath. They also still lay eggs, thus proving my 3rd grade science teacher wrong. 

“Monotreme” is Greek for “one hole”, so named because Monotremes only have one hole instead of two. I’ll let you guess which two. Here’s a hint; for them, anal isn’t just kinky. It’s mandatory. This hole is known as a cloaca. While this may seem odd, it also describes marsupials, reptiles, amphibians, birds, and pretty much everything that isn’t a placental. So actually we are the weird ones for not giving birth through our ass. It also means that the name “monotreme” is not a very descriptive name.

Platypi are well adapted to living in the water. That’s why they have a duck bill and feet combined with a beaver tail. There were aquatic marsupials that could’ve easily defeated the platypodes, but none could’ve walked all the way to Australia. Walking isn’t exactly their strong suit since they’re, y’know, aquatic.

Echidnas are covered in spines which render them virtually immune to predators, much like the hedgehog or porcupine. Thanks to this echidnas can live in the wild for around 45 years. That’s a pretty long time for an animal of their size. Long beaked echidnas don’t have teeth. Instead, they have spines on their tongues which they use to grind food against the roof of their mouth. Echidnas also have four headed penises. Yes that’s right, Knuckles was born requiring a quadruple circumcision. And you know the best part? Their freaky four headed penis? Also covered in spines. So is your cat’s. Isn’t science fun?!

Why did the Monotremes Die?

So why are monotremes so shit? What killed them all? Did they just fall off the bottom of the earth? Well, no one really knows for certain. There are almost no surviving monotreme fossils from before the time the proto-kangaroos came and kicked their shit in. 

My hypothesis is because Australian ecology has a weird obsession, bordering on fetishistic, with venom. I am serious when I say that practically everything to evolve in Australia is toxic in some way. And that includes the Monotremes.

That cuddly male platypod your holding? You better hope it’s not mating season because he has venomous spikes on his ankles. This ain’t your grandma’s bee stinger venom. Platypus venom will give you crippling pain for weeks which can’t be stopped by morphine. My colleague Dr. Doofenshmirtz should have been grateful that Agent Perry the Platypus went easy on him. It is thought that most monotremes also had similar ankle spurs.

Echidnas have ankle spurs too but they aren’t venomous. That would be a bit overkill since you’d need to get past their two inch spines. I’m guessing they were so caught up in this weird venom arms race that they never evolved the physiological adaptations that marsupials did. Dumping all your evolution points into venom or poison never works out. Trust me; sharks with snake fangs? Not any deadlier than normal sharks, but way more expensive. Just spend that money on giving the sharks lasers.

Marsupials: are the Most Fecund Mammals

Long ago there were Marsupials in Laurasia, which was a continent that has since broken into North America and Asia. But then all the marsupials in Laurasia died for unknown reasons. This might have also been because of placentals, but that is the subject of debate. 

Luckily, some of the marsupials managed to go to South America which then became separated, allowing the marsupials a relatively easy environment to evolve in without any placental mammal predators. 

Eventually South America would be connected to Australia via Antarctica. Then the Marsupials invaded Australia and killed off most of the monotremes. Antarctica at the time was warm enough that they could do that. Then Antarctica stopped being warm enough which killed all the marsupials living there. 

Then the Great American Interchange happened, which is when South America and North America became connected through panama. North America was a much harsher environment than South America. It was also full of Placental Mammals who proceeded to invade and fuck over the South American Marsupials. Thus adding to the long, proud history of North Americans invading and fucking over South Americans. Though some Marsupials did manage to survive this, some even counter invaded North America like the Virginia Opossum. The surviving groups of South American marsupials include the Opossums, the Shrew Opossums, and the Colocolo Opossums, and that’s it. Despite their names these three groups aren’t closely related. It’s still telling that they are all so similar to all be called that in the first place. 

But don’t think the marsupials are out of the ring yet. The Australian marsupials are currently being fucked over by a number of invading placentals yet again. Namely rats, cats, rabbits, and most of all humans.

So associating marsupials with Australia is a bit like how the imperial system started in Europe and managed to spread to other continents, but was replaced by the metric system back home because it was so shitty. Now it’s just associated with America and Europeans make fun of us even though they’re the ones who came up with it in the first place and some still use it a bit.

Why are the Marsupials Dying?

So why can’t marsupials catch a break and manage to not get fucked over by placental mammals? We can look at the four major differences between marsupials and placentals. There are actually a lot more differences but I am arbitrarily choosing these four: 

1. Marsupials still have kneecaps made of soft cartilage instead of bone like placentals do. Which is cool, I guess. 
2. Marsupials have cloacas. That probably isn’t all that important, though I don’t envy the joeys which have to climb out there.
3. They have pouches which allow partially undeveloped fetuses to crawl out of the cloaca and finish growing in the pouch. That actually is pretty good. It allows the mother to get pregnant again while the joey is still growing, allowing them to reproduce faster. Female marsupials also have two wombs and two vaginas, all of which are stuffed into their cloaca. And yes, I know you were wondering, male marsupials do have two headed penises to match. Marsupials are basically baby factories giving them the potential to have a significant advantage over placentals reproduction wise.
4. Marsupials don’t have a corpus callosum. The function of the corpus callosum is too technical to get into here. Basically, it’s the part of the brain that makes placental mammals be less dumb.

While of course you can’t argue that the fact that marsupials tend to be pretty stupid compared to placentals is the only reason why marsupials keep getting outcompeted, it definitely is a contributing factor. 

Koalas are infamous for being one of the most mind numbingly stupid animals that actually has a brain. That said, I wouldn’t bet on a Koala outsmarting a jellyfish. A Koala would rather starve to death than eat anything other than eucalyptus leaves that are still on the branch. If you take the leaves off and put them on a plate the Koala won’t even know it’s food anymore. Sloths, on the other hand, are basically pothead Koalas but even they can figure out what a leaf is. Both animals have very similar lifestyles. They both eat only tree leaves which gives them so little energy that they have to run their brains at half power. And yet the Sloth, while dumb, at least has some basic semblance of a problem solving capability. That’s because sloths have a corpus callosum. The inability of Koalas to comprehend the existence of anything edible that isn’t a eucalyptus leaf that’s still connected to the plant is the major reason why Koalas are currently endangered, thanks to humans cutting down and burning all the eucalyptus. Whereas *most* species of Sloth are not endangered despite their trees also getting cut or burned.

Of course, this is an extreme example. Most marsupials aren’t nearly as dumb as Koalas. My point is that a placental can do a lot more given the same amount of brain matter. We may think of placental animals like rats, cats, rabbits, humans, etc, as being stupid but compared to things like marsupials they’re practically Einstein. That said, Drop Bears are more powerful than any placental mammal. 

There’s also probably some other reasons, but no one wants to hear about the intricate nuance of animal ecology so let’s just move on.

Placentals; the Most Successful Mammals

Lastly are the Placental Mammals. I don’t think I really need to talk much about them, do I? You know what a Placental Mammal is; you are one! Unless you are reading this in the future and are an alien, robot, or elevated corvid. Placental mammals give birth with a placenta, hence the name. We have a corpus callosum and ossified kneecaps. We’re the overdogs. The other types of mammals ain’t shit compared to us.

So which of the types of mammals are your favorite? Argue about it in the comments. Do not be civil. You should absolutely take this even more seriously than you would sportsball teams.

Further Reading:

Prevosti, F.J., Forasiepi, A. & Zimicz, N. The Evolution of the Cenozoic Terrestrial Mammalian Predator Guild in South America: Competition or Replacement?. J Mammal Evol 20, 3–21 (2013). https://doi.org/10.1007/s10914-011-9175-9

O’Leary, Maureen A et al. “The Placental Mammal Ancestor and the Post–K-Pg Radiation of Placentals.” Science 339 (2013): 662 – 667. https://pubmed.ncbi.nlm.nih.gov/23393258/

All photos obtained from https://www.photosforclass.com/.