Atlasaurus imelakei: The Sauropod on Stilts

A small herd of Atlasaurus. Just look at those long legs, but why the short neck? 

OK, there are some pretty weird sauropods out there. Amargasaurus cazaui seemed to have had a pair of spikes going down its neck, Nigersaurus taqueti had a duck-like snout and a constant supply of teeth in its jaws, and nobody can forget the armored monster Agustinia ligabuei that gave stegosaurs and ankylosaurs a run for their money. But I think that one sauropod should be added to this list. One that you've all probably heard of somewhere, but never got the chance to learn about; Atlasaurus imelakei.

Atlasaurus was named after two things, the Atlas mountains in Morocco where it was found, and its large body size, which was refers to the Greek Titan Atlas who holds up the world on his shoulders. It lived during the Mid-Jurassic and was a member of the Macronarians, a group of sauropods that included Brachiosaurus, Camarasaurus, and all the later Titanosaurs, however, where Atlasaurus fits in this group is uncertain, but many think it's a close relative of the Brachiosaurs.

Atlasaurus is known from a near-complete skeleton, only missing part of the skull and tail. It seems to have been about 50 feet long, which is mid-sized for a sauropod, but larger than nearly any other type of dinosaur. However, when scientists first found the leg bones, they estimated an animal  much bigger, around the 70 foot range. In fact, when the full specimen was unearthed, scientists were baffled to find that Atlasaurus had the longest leg-to-body ratio of any sauropod. They had discovered a sauropod on stilts!

The arms of Atlasaurus alone were about 12 feet tall, and the full leg was almost 15 feet tall. In fact,  the legs would've probably fit nicely onto a 70 foot Brachiosaurus, despite being three times more massive! Like Brachiosaurus legs, the humerus was nearly as long as the femur, and the other bones in the legs were also quite tall, pushing the shoulder level up much higher than the level of the hips. This gave both Atlasaurus and Brachiosaurus good height while feeding, and made them good tree-top feeders. You'd probably think that with Atlasaurus' longer legs though, it would give it a higher reach than other sauropods in it's size range. Well the interesting thing is, Atlasaurus has lost a lot of it's neck.

Along with having increasingly long legs, Atlasaurus seems to have lost much of the giraffe-like neck of other sauropods. The neck is considerably long compared to most animals but compared to giraffes and other sauropods, Atlasaurus was one of the shortest-necked sauropods, only beaten by Brachytrachelopan and some other short-necked dicraeosaurids. This makes Atlasaurus unique in the fact that it evolved longer legs to increase feeding height, while basically every other sauropod yet discovered simply evolved a longer neck to increase feeding height. However, it isn't so hard to imagine why most sauropods took the easy way up.

An interesting thing to note is that the combination of such long legs and such a short neck suggests that Atlasaurus was unable to reach the ground to feed or drink by simply using the "giraffe posture" some sauropods are thought to have exploited. The only way it could simply bend down to drink is by either submerging itself up to it's knees, or by bending it's elbow at a near 55° angle, spreading the legs apart into an old ceratopsian-like pose, and shifting its center of mass towards the front of the body. When you factor in that this animal is close to 14 tons, it becomes clear that this is an incredibly difficult maneuver to pull off. 

This long legs and short neck combination seems like more of a handicap than an advantage. In fact, even I have trouble trying to find out exactly what evolutionary pressure caused Atlasaurus to evolve such features either. Just finding an explanation for the long legs alone proves extremely difficult, but I've come up with a few ideas, although none seem legit.

My first idea was that, due to the handicap that Atlasaurus couldn't drink easily without being submerged suggests to me that it may be semi-aquatic, using it's long legs to sift through marshes and wetlands and walk along river bottoms. I quickly rejected that idea after reviewing the anatomy again, which seems even worse at such behavior than most other sauropods. And although evidence shows that these animals lived along coastlines, no marshes were present and the climate was mostly dry and arid. 

Perhaps the long legs were for increased speed. The long legs of a giraffe can propel adults across land at 31mph, which helps them escape from predators. However, a quick comparison to the legs of a giraffe and Atlasaurus certainly shows that these animals weren't runners, and the difference in length of the front and hind limbs would also hinder speed because the front legs would surpass the stride of the rear ones. Another interesting idea I had is that the legs could help keep their bellies from getting injured from thorny plants while moving through heavy brush. But in my opinion, it is hard to imagine what kind of spiky plants would cause such discomfort as to change their whole anatomy. And as I said before, fossil evidence suggest that these animals lived along semi-arid coastlines, not dense forests with spiky plants.

At the moment we have no idea as to why Atlasaurus evolved as it did, and it is certainly one of those oddballs on the dinosaur family tree that we may never have the answer too. So why do you think Atlasaurus evolved its long legs and shortened its neck? I'd definitely love to hear other people's opinions on why this creature evolved.

What little details can tell you.

Ahhh, Stegosaurus neck armor, all the things you can tell us....

Being the Paleontologist-in-training that I am, I'm very interested in how dinosaurs behaved and interacted with one another on a daily basis. I want to know how they caught their prey, escaped from predators, and what they were doing that helped them survive for however long they did, and much of a dinosaur's anatomy can tell you exactly that. I've developed a little talent where I can look at any part of a dinosaur's anatomy and come up with a number of ideas for the purpose of the limb, neck, tail, or skull. Then it can give me some ideas on how the animal might have lived.

For example, Stegosaurus is famous for its double row of back plates on its back, but many people don't know that it actually had a dense layer of pebbly armor on the underside of its neck. The most obvious idea is that the neck armor was a defence for the neck against theropods, but I can think of many more functions. Another is that the armor could have been used as a display, similar to what many now think the plate's function was. Some pebbly osteoderms seen in things like iguanas and other lizards are used as a form of display, and large scales seen around the head region in both lizards and birds are used for show.

Stegosaurus also notably lived in an environment filled with cycads, which produce large cones filled with seeds that are very nutritious. However, the cone is protected by the large spiny leaves surrounding the center of the plant, and could cause injury to animals trying to reach into it. So maybe the neck armor was used to help Stegosaurus reach into the cycads and feed on the seeds. Maybe Stegosaurus was a specialist cycad eater?!

Entire debates can spur up from little ideas like this, and can help explain many unexplained phenomena in the fossil record. Stegosaurs became less common by the beginning of the Cretaceous, but flowering plants also started to take over by this time as well. The evolution of flowering plants also coincided with the diversification of numerous groups of herbivores, among them the ornithopods and ankylosaurs, which would've caused competition with the Stegosaurs. Angiosperms also began out-competing cycads and forcing them to retreat to the tropics, so perhaps the cycad-eating stegosaurs went extinct as the cycads sterted to disapear. It could have happened.

(I literally came up with that theory within the last 20 minutes before writing this, but looking over it multiple times I can believe it. I'll probably look more into the idea, see if anyone else has suggested anything like it....)

I can do the same thing with other dinosaurs, take this for example: Small ornithopods like Hypsilophodon have long toes. It's true, Hypsilophodon and its relatives do have rather long toes. At first some thought this meant they may have been tree-climbers, a lot like tree kangaroos from Australia and New Guinea, which also have long toes. However, many now find this unlikely, since many of the proportions of the feet are not like those of tree-climbing animals, and are typical of ground-dwelling ones. They also had stiffened tails, not the flexible tails of most tree-dwelling animals, and suggest that they were fast runners. So why did they have long toes? 

Looking at modern animals, many ground-dwelling animals that have long toes live on marshy and uneven ground, since it gives them more grip when moving and helps prevent them from sinking in mud. Many of these species are also found in deposits that were marshy and wet at the time, and it seems likely to think that they were living in these environments. Even if many people agree that it had a certain function, opinions on how it was used may still differ. Camels, for example, also have long toes and broad feet to gain the same effect when moving across sand dunes. So maybe they were desert dwellers? (I find this unlikely) Some small animals also use long toes to help distribute weight while moving over snow, very similar to what snowshoes tend to do. Some small ornithopods like Leaellynasaura would've encountered snow during parts of the year, so perhaps their long toes helped them with moving across this kind of terrain.

As you can see, you can tell a bunch of stuff by just looking at a dinosaur's foot, tail, or skull, and little details like what I listed before can tell you all kinds of things about how it survived, interacted, and even went extinct. That's why every bone counts when talking about dinosaurs. So, I've decided that the next few posts I make are going to be about all kinds of interesting dinosaurs. I'm also thinking about making some topics about different dinosaur lifestyles, such as semi-aquatic ones, and different behaviors the would've exploited. Many people only think of spinosaurs when talking about semi-aquatic dinosaurs, but trust me, I've found many more examples than those fish-eating theropods.

So yea, you could say this is my make-up for the previous month of absence, but I was planing on doing this before then anyways. So, as always, I'll take requests for dinosaurs and as many ideas for topics as wanted.

RaptorX

Predators or Scavengers: Those Puny Little Arms

Don't worry T-Rex, I hate them too.

Tyrannosaurs had huge heads, terrifying jaws, and extremely powerful bodies, so it doesn't really make any sense that these predators have those puny little two-fingered arms now does it? Some have taken this as evidence of a loss of predatory lifestyle in the tyrannosaurid group, while many others find the small arms as being lost to compensate the enlarged jaws. Both, however, have their problems, and that's what my new Predator vs Scavenger topic is about! We will be reviewing the structure, musculature, evolution, ontogeny, and all the known possibilities for the strange arms of the tyrannosaurids to attempt and find out why tyrannosaurids evolved their tiny little appendages.

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Biomechanics of the Tyrant's Tiny Limbs

When Tyrannosaurus rex was first discovered in in the first few years of the 1900's, only the humerus (upper arm bone) was recovered, and the first mounted skeleton of T-rex in 1915 portrayed it with three fingers like those of Allosaurus and most other theropods. However, a year earlier the forelimb of Gorgosaurus was described and showed the typical two fingered hands characteristic of tyrannosaurids. It wasn't until 1989 that we had our first complete T-rex forelimbs, and proved just how short and tiny these arms were.

The arms were small relative to the animal's body size, only about as long as a average human's. The humerus of T-rex was roughly 15 - 16 inches long, and was made up of extremely thick cortical bone (a type of bone with very little openings and high density), making it able to withstand heavy loads. The radius and ulna (lower arm bones) were both only about 8 inches long, and weren't quite a strong as the humerus. The hands were made up of the two small carpal bones, one reduced metacarpal that represents the lost third finger, and two functional digits.

It's true these arms were tiny, but they certainly weren't vestigial or useless. Like I said, the arms were made up of thick bone, and was built to withstand strong forces. Their are also areas on the arms and the long shoulder blades for large muscle attachments. In fact the biceps of a full-grown Tyrannosaurus was capable of lifting over 400 pounds, not even counting the four other kinds of muscle present in the arm.

Even though the arm was particularly strong, it had a reduced range of motion, and was only capable of allowing 40 degrees of motion at the shoulder and 45 degrees at the elbow. Compared to humans this is very small range, but even other theropods such as carcharadontosaurids, allosaurids, and especially members of the maniraptora all have a range that greatly exceeds that of the tyrannosaurids.

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Evolution and Ontogeny of the Puny Little Arms

The tyrannosaurs didn't always have their tiny arms though, early tyrannosaurs like Guanlong, Dilong, and the (really) recently discovered Yutyrannus all have arms that are either in the typical size range of most other theropods, or actually longer. It was only the later, larger tyrannosaurs that evolved their abnormally small forelimbs, though exactly when these tiny arms evolved in the first place remains unknown.

In 2010 the mystery was thought to have been solved by the discovery of Raptorex, which was thought to be a small tyrannosaur from the early cretaceous that had already evolved the two-fingered limbs. However, it was later studies on the specimen showed that Raptorex wasn't actually from the early cretaceous, but actually dated to the masstrichian stage of the late cretaceous. Not just that, but the remains are strikingly similar to known Tarbosaurus juveniles, and the specimen itself is already known to be immature. Many now consider Raptorex to be a nomen dubuim, and most likely represents a juvenile Tarbosaurus. Thus, the mystery as to where these stubby little arms emerged remains anonymous.

At the moment, the transitional morph is still yet to be found, but based on my knowledge of these tyrants, I personally believe the small arms probably evolved about 85 mya in either North America or Asia. Both places have early tyrannosaurids present, in Asia it was Alectrosaurus and in North America it was Appalachiosaurus. Sadly the front limbs of both species are unknown, so whether or not they had short arms of animals like Tyrannosaurus or long arms of creatures like Yutyrannus is still a mystery. All the tyrannosaurids from 80 mya onwards had puny little two- fingered digits.

However, it does seem that at least one tyrannosaurid "bucked the trend" a bit when it came to the puny arms. Daspletosaurus torosus had the longest arms of any tyrannosaurid known, which were about twice as long as a tyrannosaurid of similar size. This is especially strange since Daspletosaurus is also known to have had really short legs, the shortest in the family, while most other tyrannosaurs had rather long legs for theropods. (I have a theory as to why this might have happened in Daspletosaurus, but I will talk about it farther down the post)

One of the most interesting things about tyrannosaurid arms though are the changes they experience as the animal grows. Young tyrannosaurs, and especially members of Tyrannosaurus itself, tend to have very long arms and claws compared to their parents; the size of a juvenile's arm was about the same length as the adult's arm, despite being 10 - 15 times less massive. Not just that, the arms of the juveniles have the same proportions as some allosaurids, and they aren't just long, but very strong and much more useful for grasping.

Just try comparing the tiny stubs of Sue...

...to the longer arms of Jane.

The change in the arm lengths might reflect behavioral changes as tyrannosaurs grew, and also suggests that juveniles used their arms more than adults did. I have even heard some suggestions that the adult's tiny arms were there simply because of their juvenile stage. Although this idea may have some truth to it, I still think another theory for their function may be more likely.

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The Function

So after that little overview on the forelimbs, let's try and see which of these theories is most logical:

Head over Hands:

Why does Rex have three fingers?

Of course, the first theory is that the arms were simply useless appendage that were lost as the skull grew to it's enormous size to keep it in balance. It's an interesting idea, but abelisaurids have tiny arms, even smaller than that tyrannosaurids' stumps, yet the skulls of these animals are actually smaller compared to body size than many large theropods. At the polar opposite, some carcharodontosaurids have skulls that are actually larger than most tyrannosaurs of similar size, and heads just as heavy. Yet they have typical three-fingered, long arms of most large theropods. With this logic, why didn't they lose their arms?

It's also interesting to note that theropod arms aren't very heavy in the first place, and shouldn't cause too much of a weight problem. So this theory doesn't really hold up very well. However, even if the head wasn't responsible for such a thing, it could be behavioral reasons that caused the arms to become reduced. The head was massive and built for crushing bone, which could have made the arms redundant simply because they didn't have a use anymore. This could also explain the abelisaurid arms, as the skulls of these animals were built for gripping, so the arms probably hadn't much use either.

However, as I explained earlier, even though they were small, the arms were heavily constructed and powerfully muscled. So no matter what people say, they were probably used for something. I will note, however, that their are many tyrannosaur specimens have been found with broken front limbs, injuries caused during life. A few specimens even show limbs that were nearly or completely torn off, but had healed over. The fact that we find these specimens survived losing these limbs suggests that these animals could go without using their tiny arms for extended periods of time, if not at all.

All for Love:

Uhhhh, more three-fingered Rexes....

Some interesting theories suggest that the arms might have had some kind of mating purpose, either for grasping mates, as display organs (such behavior is seen in ostriches), or for keeping your mate happy by "scratching their itch."

The idea that they were used for grasping mates was first proposed by Henry Osborn (the one who described T-rex) in 1905, and has become one of the most popular theories for their use. I have to admit that I find much of the evidence stacking in this theory's favor.  Being so big means that you have a harder time trying to mate. Tyrannosaurs were very large animals that primarily used their heads and jaws to secure a meal, thus the arms were mostly lost, but still kept to help right themselves when mounting a female.

I mentioned earlier that Daspletosaurus had the longest arms of any tyrannosaurid, but the shortest legs. This is consistent with the copulation theory. Since Daspletosaurus was so close to the ground, it would've had a much harder time grasping a mate during copulation. Thus, as the legs got shorter and the animal got closer to the ground, longer arms were probably favored to help with gripping onto mates.

The other two theories are also somewhat believable, but they are based on ideas with no evidence for. Nobody can test whether or not a female Gorgosaurus thought a male Gorgosaurus looked "sexier" with bright colorful armpits. Neither do we know weather or not Tarbosaurus really scratched their mates to make them feel better, or weather or not they were even social enough to exploit such behavior. They're all very interesting ideas, but they're nothing more; just ideas.

Prehistoric Forks:

Ok, I will point out the numerous fingers, but I'm more worried about the feet...

A rather odd theory that I've come across many times over the years is the idea that the tiny arms were used like a carving fork; holding onto the sides of a prey item as the jaws dispatched it and tore it apart. Other than the fact the arms had two sharp claws on them, their is basically nothing that suggests such a behavior, and the arm's design is not really built for it anyways.

For one, tyrannosaurid arms were very small and had a limited range of motion. This is the exact opposite of an animal that uses it's arms to grapple onto prey, such as the allosaurids and dromeosaurids, which have arms that are longer and have a large range of motion to reach out and pull another animal in towards it. They're built more like a carving fork, for holding onto something at close range that doesn't struggle, which brings more evidence to the copulation theory, as a mate is less likely to pull away than a prey item.

Another interesting point is the arm's overall length. They were so small that if a tyrannosaur was to grab onto a prey item, the animal would be literally pressed right up against the chest of the predator. Wouldn't you thing it'd be rather hard for something like a T-rex to dispatch a dinosaur that would basically be underneath it's body? Try eating something tied to your chest with just your mouth....

Also, note that evidence from the last few years have shown that tyrannosaurid hands faced inwards like all dinosaurs, not straight down like they were portrayed for so many years. Why do I point this out? Well one thing that you have to remember is that tyrannosaurs were often the largest animals in their local ecosystems. So if they were to try to catch something with their arms, wouldn't you expect the fingers to point downwards towards the smaller prey item? Not inwards which would've made it harder to hold on and grip.

Getting Up in the Morning:

Hey, it's a picture of me in the morning...

This is another rather interesting idea, that the arms were braces the were used to right the animal as it got up. This was first proposed in 1970 by Barney Newman, and became very popular at the time. During such activity, the forelimbs of the animal would have been extended in an action somewhat reminiscent of a push-up. However, with recent biomechanical analyses, such behavior seems impossible for a tyrannosaur to preform.

For one, as I mentioned above, the fingers weren't pointing downwards, they faced inwards. The shoulder joints were also too inflexible to allow the hand to face downwards, and prohibited a holding use for the claws to push on. This means that for the animal to be capable of pushing itself up, it would needed to do so with the sides of it's arms, and with very little surface area to actually push. 

Even so, it doesn't seem too hard for a tyrannosaur to actually get up without the aid of it's arms. All you need to do is have your legs right underneath your center of mass, then it's just a mater of lifting yourself up. Think about modern day bipedal animals, like ostriches, which get up just fine without the aid of arms. Us humans can also easily get up from a crouching position without our arms. Unlike ostriches or humans though, Tyrannosaurs could have also used their tails to help right themselves through counterbalancing.

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Conclusions

In my opinion, the only practical theory put forth so far is, ironically, the first to be proposed: copulation. It works well with what we know about large bodied animals, and these extinct tyrants seem to be no different. Although the arms may have been used for more than just copulation, I think that that was the only reason why they were retained for the last 20 million years of the Cretaceous. I find it interesting to think about what these animals would look like if they had survived past the extinction. Perhaps the giant Tyrannosaurs would keep their arms, but if smaller species evolved they may have lost them without any of the size restrictions.

But now here's an interesting little thought, note I said almost nothing in the post about whether or not the tiny arms suggest Tyrannosaurs were scavengers or a predators. That's because they don't indicate either. Many animals lack arms and are very efficient predators, such as Anacondas, Sharks, and Birds of Prey. So are many active scavengers, like Vultures and Piranha (despite popular media these are actually quite timid fish). Thus, it doesn't necessarily mean anything...

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P.S. Sorry it has been so long since my last update. I've been busy with a science fair project at school and was unable to be on here at all. Luckily, I finished it earlier today and am hoping to make up for lost time with a special post about what I've been working on.

RaptorX