Sunday, 8 September 2013
Mosasaurs- A Journey Into Prehistoric Seas
One of the unfortunate side-effects of the fame of dinosaurs as a group is that the spotlight often gets taken away from other Mesozoic creatures who were just as impressive. It catches some people off guard to find out that, between the Triassic and Cretaceous, dinosaurs were not the only animals alive at the time. When you consider the diversity of living things in the world today, it's ridiculous to think that, over the span of hundreds of millions of years, only a certain group of Archosaurian vertebrates existed. Even within the Sauropsida (the group that contains all reptiles and birds), there were amazingly diverse and amazingly large creatures roaming the earth at the same time as the dinosaurs. While it is certainly true that dinosaurs were the largest terrestrial animals at the time, the oceans were teeming with all kinds of unrelated marine reptiles. Today, we'll focus on one of them- the mosasaurs.
I'm sure most readers are wondering what the heck a mosasaur is. Well, if I may flog this point a little more, they were not dinosaurs. Let me explain (this may take while...)- you can split 'reptiles', birds and mammals into three general groups based on how many holes, called temporal fenestrae, they have in their skulls behind the orbital holes. These holes in the skull allow for attachment of the jaw muscles. Animals with no temporal fenestrae are called anapsids. This group contains many weird extinct creatures, as well as, perhaps, the turtles and tortoises. Animals with one fenestra are called synapsids. These are the mammals and their ancestors. The last group is animals that have two fenestrae, which is what we're concerned with. These are called the diapsids. Now, if you're still with me, you can divide the diapsids into a further two groups. One of them is the archosaurs. These creatures, in their primitive conditions, are united by having a hole in the skull in front of the orbit called the antorbital fenestra. This is the big, triangular hole just before the eyes you often see in many dinosaurs. Archosaurs also have a hole in their lower jaws called the mandibular fenestra. They also tend to have relatively large skulls, round non-overlapping scales, s-shaped necks, and an upright erect posture. As well as a few early primitive animals, the main groups contained within the Archosauria are the crocodiles, pterosaurs, and avian and non-avian dinosaurs. Still with me? Now, going back to the split in the diapsids, the other group that parallels the Archosauria is the Lepidosauria. The lepidosaurs are the tuataras, lizards, and snakes. Lepidosaurs lack many of the skull holes that characterize archosaurs, but they tend to have more kinetic skulls. This means the bones in the skull can move a little to a lot relative to each other. Lepidosaurs also generally have a sprawling posture, which gives them a more primitive, side-to-side form of movement. They also have a sliding joint in the pectoral girdle for locomotion, and keratinous scales, he outer layer of which they shed as they grow. This is the group that the mosasaurs belong to. They bear all the features of the lepidosaurs and are actually classified as a kind of lizard.
tl;dr- mosasaurs aren't dinosaurs because of their skeletons and other things.
Okay so, I've exhaustively gone over what mosasaurs are to science. But what exactly are they? I mentioned something about them being lizards, and that's true. However, it's much cooler than that. Picture a modern day Komodo dragon, which is a close relative of our animals of focus. Now, picture a Komodo much, much bigger, living in the Mesozoic oceans, with limbs evolved into flippers and a long, powerful tail for aquatic propulsion. As happens with many different tetrapod groups, the ancestors of mosasaurs were terrestrial animals that returned to the oceans. These creatures ranged from the modestly sized Platecarpus at 14 ft long, to the massive Tylosaurus, which may have reached lengths of around 50 ft. Mosasaurs had elongated, lizard-shaped bodies, with long, streamlined skulls and nostrils places on the top of the snout for easy access to air at the water's surface. Unlike the long, paddle-shaped flippers of plesiosaurs, mosasaurs had short, round flippers adapted for aquatic steering. These flippers were formed by the skin between the digits (yes, mosasaur skeletons still show five fingers on each hand) growing around the digits to form a fleshy flipper. Unlike the long crushing teeth of Tyrannosaurus, mosasaurs had shorter, conical teeth that were slightly curved. This allowed them to grasp slippery, wriggling prey. The jaws of mosasaurs are also pretty neat- there was a kinetic joint in the middle of the lower jaw, on each side, that would have allowed the lower jaws to expand out to the sides, thus increasing the gape of the animal and allowing it to swallow very large prey items. This is basically how modern snakes are able to expand their jaws to swallow big animals. This similarity, among others, has led some palaeontologists to suggest snake are, perhaps, descendants from the mosasaurs.
Being equipped with very long tails leads scientists to presume, as stated earlier, that mosasaurs swam primarily with their tails. This contrasts the other group of large Mesozoic reptiles, the plesiosaurs, that had very short tails and propelled themselves with their long, oar-like flippers. It was assumed for a long time that the way that mosasaurs swam was much like how a modern day snake swims- by wiggling its body from side to side, generating a sort of wave down the whole of its body which propels the animal forward through the water. This is called anguilliform locomotion and is also seen in animals like eels. However, new evidence suggests that some later, more highly derived mosasaurs had a downward-kinked tail, with a fleshy sort of fin sticking out of where the kink in their tail bones occurs. This gave them, essentially, tail flukes that formed a crescent-shaped structure on the end of their tails, much like a shark. And, like a shark, the mosasaurs that had these elaborate tail flukes probably held their bodies quite rigid and propelled themselves by powerful thrusts with their tail fins. This kind of adaptation is also seen in ichthyosaurs, so it could very well have evolved again independently in mosasaurs.
The way that mosasaurs lived their lives is no less fascinating itself. It seems as though the smaller species fed on mostly fish, while larger mosasaurs could eat fish, sharks, aquatic birds, and other marine reptiles. There's also direct evidence from tooth marks on fossils that mosasaurs would, from time to time, attempt to feed on ammonites, and would even scavenge off the bodies of dinosaurs that had died and drifted out to sea.
One obstacle in their lives they would have had to face was that of giving birth. It's impossible for reptile eggs, with their semi-permiable shells, to be laid in water and survive. The offspring would die. Modern sea turtles get around this problem by hauling themselves onto land (a very difficult process for them) and laying their eggs there. This solution is feasible for the earlier, smaller species of mosasaurs, but for the massive 50 ft Tylosaurus, it would have likely been impossible for such a heavy, bulk animal to get onto land with its small, underdeveloped limbs. Its likely that such a gigantic animal, not being able to support its own weight, would end up crushing itself on land. There is a solution though- fossil specimens of ichthyosaurs have been found in the midst of giving birth to live young. It seems likely that ichthyosaurs, who could also not have gotten themselves onto land, kept their eggs inside their bodies until the young were ready to hatch. The baby ichthyosaurs would then hatch inside their mother and be "born" into the surrounding water. Many scientists now think that mosasaurs adopted this process for giving birth, freeing them from having to leave the water. Now, being descended from land dwelling reptiles (but unrelated to each other), neither ichthyosaurs nor mosasaurs could breathe in water and would have had to come to the surface for air. This poses problems towards giving live birth, as its a timely affair as any mother will tell you. But nature found a way around this- the ichthyosaur specimen found mid-birth shows the offspring coming out of the mother tail-first. This means that, for the whole time its being born, the young ichthyosaur could still breathe until, at last, its head emerges. It would then, presumably, make a dash to the surface for its first gasp of air. Though we have no direct evidence, it seems likely that mosasaurs used this tail-first live birthing method themselves.
The prehistoric world was teeming with life that came and went long before human beings arrived on this planet. The incredible diversity of weird and fascinating creatures requires much more study to truly understand it, and it's worth remembering that we've still only discovered a fraction of the animals and plants that came before us. So when you think back to the time of the dinosaurs (as cool as they were), remember that they had a plethora of wonderful neighbours that deserve our admiration just as much.