Monday, May 28, 2012

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.

No comments:

Post a Comment