Megatherium the weird badass sloth, at NHM

Megatherium was a complete badass. It's unlike anything that exists today. It lived in South America for a huge amount of time (1.9million years). It was huge, six metres long, as heavy as an elephant, one of the biggest mammals ever. Even so it could walk on its back legs, just like a bear. But it's not a bear, it's a sloth. No-one is scared of sloths, but Megatherium would have been scary. When it was first described in 1795 all museums around the world wanted to have a skeleton (this website is good on the history)

 It looks like a savage predator but it was actually a herbivore (plant eater). It looks like a dinosaur, but it only died out 10,000 years ago (dinosaurs died out 65,000,000 years ago). When you look at the history of the earth, it came close to still being alive today, and no-one really knows why it isn't.

I had seen Megatherium before when I visited the dinosaur museum in Paris in 2011, so I was looking forward to seeing Megatherium when I visited the Natural History Museum in London in June. They are really very proud of it and say it has been on display ever since 1850, but they have stuck it in a corner of a corridor away from everywhere else like a third cousin at a wedding reception. But I like that, I think it's good to see it by itself and not get distracted by other exhibits around it.

Here's the whole exhibit:

The first thing that you notice is, is looks like someone who was bad at drawing skeletons had drawn it. If you just saw a drawing of it, it's hard to believe an animal like this existed. Everything about it looks wrong ! The hips look much too big. The ribcage looks much too wide at the bottom. The hands look too big and the tips of the fingers look too thick. The skull looks bonkers,  the back legs look much too short, the tail looks too thick and the scapulas look like they are on the wrong side.

But the first thing is, Megatherium's skeleton looks different from other skeletons because Megatherium was so completely badass it isn't like any other animal today. The other thing to remember is it existed for 1,900,000 years, so it must have been pretty good at doing what it did.

The other thing to remember is that the NHM's Megatherium skeleton isn't really a Megatherium skeleton, because it isn't "a" Megatherium and it isn't a skeleton.

It's actually made from plaster-casts of  two half skeletons, not one whole one, but that's okay because I know how hard it is to find all the bones of an animal that died six months ago let alone 10,000 years ago.

What happened was he NHM had half a skeleton from Argentina, and the Royal College of Surgeons had another half, in case they had to teach doctors to operate on one or something, I don't know. In 1849 they made plaster casts of the bones, so they had a new replica skeleton, then they just filled in missing bones by looking at what they had. It was a good job they did because the Royal College of Surgeon's got bombed in world war II and their half of the Megatherium didn't survive that. This one is Megatherium americanum.

Here's an early diagram of a Megatherium cuvieri (which is the same thing, but a different name) skeleton from 1834. The artist wasn't very good (look at the spikes on the elbows !) and the hips are complete bonkers.

The Natural History Museum originally had their skeleton on all fours like this, and it looks pretty stupid, like it's waiting for a bus. Here's a picture of it from 1907 that shows it the same way. It looks pretty dopey like this:

Then scientists looked at the bones and worked out that it was adapted to walk on its back legs. The length and shapes of the bones can tell you how the animal was best adapted to. They worked out that Megatherium was best adapted to bi-pedal (walking on back feet) awesomeness. 

The femurs that look much too thick were so thick because they could support all its weight. No-one's quite sure how much it weighed, and I read almost four tonnes when I researched it, and NHM say 2.5 tonnes, which is still the same weight as two small cars, and if you could carry two small cars around you would have thick femurs with lots of room for muscle attachments too.

The femurs look much too short, but long femurs would have been stupid for Megatherium. It was so bigger than any other animal that it didn't need to run away fast. If it was balancing 2.5 tonnes on its back legs it would only take small steps, not big strides where it would lose its footing and fall over.

The tail looks too thick because this tail wasn't like most tails. It was a strong muscly tail that it could lean back on for support.(*) When we stand out legs are straight, but here Megatherium has its legs in a position like it's doing a poo, which is a difficult position to stand in unless you have support from behind.

There are three other things that are important in the picture above too. 

• There is the big, overdeveloped bone at the back of the foot (the talus ? I'm not sure) which supports it from the back.
• There's the short, thick tibia (shin bone) which shows it was a strong but slow moving animal.
• The feet are completely weird. If you were carrying two cars around you'd probably keep your feet flat on the ground. Megatherium had huge claws, so the feet couldn't go flat, the same way that Wolverine from X-Men can't put his hands flat on a table without the claws scratching the table. So it walked on the side of it's feet. This is bonkers for an animal that big. Think of an elephant which has big, wide, flat, feet. Even dogs and cats have evolved a way to retract their claws when walking (cat's don't retract entirely). So at some point a scientist came up with the theory that it walked on the side of its feet, and I bet the other scientists laughed at him. (But he was right, and they found fossilised footprints to prove it )

Here are the huge flared hips. They make sense if you think of them supporting all the body weight above.

Here's what I was talking about with the scapula (shoulder-blade). It is triangularish with a ridge (spina) across the middle like most scapulas. But this is the only scapula that I have seen where the ridge closest to the bum of the animal rather than the head (here's my post on other scapulas). That's why I said it looks like it is on the wrong side. It must must be like that because the way the muscles work, and make it stronger in pulling on one direction than the other, but I'm not exactly sure what.

The sticky-up bits on the spine on the shoulder vertebrae (the thoracic vertebrae) are big because muscles would be on either side and use the sticky-up bit to attach. The size of the sticky-up bit show how big the muscles were there. Megatherium's are big, but not huge. In proportion to the body it's about the same size as a cow's.

You can tell it had strong arms, though, because the scapula has a special bit over the shoulder socket which attaches to the collar-bone (humans have these too) and the collar-bone is short and thick and the same shape as a human one. This helps keep the shoulders strong. (Birds have a specially adapted version to keep their ribcage strong when they fly, I wrote about it a few weeks ago)

These look like strong arms, but they are much longer and thinner than the back legs. This isn't the best picture to show it (I'm going to sack dad, he took the pictures) but the one on the NHM website shows it better. The humerus is actually oval shaped, wide when you see it from the front but thin from the side. This was probably the clue that it could balance on two legs as well as four:

The hands look odd. They have the same bones as humans (but no thumb, which is why Megatherium never hitchhiked) but the distal phlanges (fingertip bones) are thicker, not thinner than the rest. That's because the big claws attached here.

It is hard to describe the skull. The front looks like a camel. The lower jaw is a completely weird shape, thicker in the middle for more muscle attachments I guess. It has a small hook at the back of the jaws, similar to hedgehogs, again for muscle attachments. Part of the top of the skull overlaps the jaw. It looks like it could bite hard which is unusual. Most planteaters (like deer or sheep) have thin lower jaws which can move from side to side to grind. The top big of the skull would stop the bottom bit moving from side to side.

I found this video about a Megatherium attack, which I think came from a BBC programme but I'm not sure. It shows you how weird Megatherium must have looked with a huge bum and a small head. Megatherium probably didn't care how he looked, if he could scare away moas and sabre-tooth tigers. No-one really knows whether it scavenged meat. It was so big it could probably just do what it wanted.

So why did Megatherium die out ? No-one is really sure, but it's only predator was man, so it's possible it was hunted to extinction. It's a real shame. The world would be a better place with Megatherium in it.

(*) I thought it was unusual that an animal used its tail to support its weight, so I asked on Twitter if there were any others. Turns out there are loads of them, like kangaroos, wallabies, armadillos, otters, ardvarks, some monkeys, pangolins and meerkats !

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