Tuesday, September 18, 2012

The Skulls of Sauropods and Their Behavioral Significance

A fossil replica of Camarasaurus lentus' skull.
Camarasaurus is one of the few Sauropods we have complete skulls for.
Sauropods were the most successful group of land herbivores to have ever evolved. They lived and dominated the Earth since the dinosaurs' reign began in the Triassic, straight through to the very end of the Cretaceous. They evolved into a variety of unique shapes, yet kept their ingenious body plan in check in nearly all of them. They have been found on all continents, including Antarctica, and thus some might have even braved icy climates. They were also the only land vertebrates that regularly rivaled, and in the case of Amphicoelias and (possibly) "Bruthathkayosaurus," even exceeded the maximum known size of modern day whales. However, despite their amazing sizes, shapes, and complexity, the sauropod's skull was tiny compared to the rest of the animal, barely taking up 1/200th of the total body mass, and rarely getting preserved. This has led to many problems when it comes to constructing how these animals lived, ate, and survived. Luckily, the few skulls we do have have shown that many sauropods had some unique ways of adapting to many existing problems in their native environments.

All sauropods for which we have skulls show many differences from those we've collected of their relatives/ancestors, the prosauropods. For one, the skull of prosauropods is greatly elongated, while most sauropods have skulls that were shorter. In fact, and I mentioned this in one of my other posts, the skulls of sauropods resemble the fossil hatchlings of prosauropods, suggesting that prosauropods evolved by retaining juvenile traits into adulthood (a similar theory has been put forth on how birds evolved from theropods). The openings in the sauropod's skull, known as fenastrate, are also much less organised than in prosauropods. While prosauropods had their skulls' fenestrate in line along the horizontal axis, sauropods had all the skull openings towards the top of the skull and lie in an almost "zig-zag"pattern.

Where the nostrils of sauropods were thought to have been
as opposed to where we think they are now.
Image from palaeozoologist on Deviantart.
In many sauropods, specifically the members of the macronarian branch, the snout comes far forward of the external nares, the area that holds the nostrils. For a long time it was thought that this meant sauropods had their nostrils on top of their heads for snorkeling, as they were originally thought to be water-dwellers. In recent years, however, we have proven that sauropods indeed had their nostrils present on their snouts, and that the nostril openings were lead through a shallow depression of bone along the snout. Why did sauropods have this arangement? Well a number of theories have come forth as to why. Some have suggested it was for temperature control, others think that in some members with enlarged external nares like Giraffatitan and Camarasaurus, they could have been able to inflate skin around their nares to create a display. I, however, am fond of the idea that they might have been covered by a keratinous covering and used, along with their super-sized necks, to create low-frequency calls to communicate over long distances. Elephants, the largest land animals alive today, use low-frequency sounds to communicate over vast distances, why not sauropods?

Of course when you have a skull of an animal, one of the first things many people will examine is the teeth, and sauropods aren't lacking. We find many isolated teeth in the fossil record, and different groups of sauropods evolved different tooth designs. Primitive members of the sauropoda like Mamenchisaurus had teeth shaped almost like spoons, later diplodocoids like Apatosaurus and Diplodocus had teeth shaped almost like crayons, and macronarians like Brachiosaurus and Argentinosaurus kept the spoon-shaped teeth of earlier sauropods (well, that isn't necessarily true for the whole group, but I'll get into it further down). The teeth of sauropods were also much larger than earlier prosauropods, and took up a larger portion of the jaw. Many have less than 15 tooth positions on each side of the upper and lower jaw, basal members, however, could have up to 20, and the wonderfully weird rebbachisaurids could have dental batteries of over 30 tooth positions each with up to 8 replacement teeth underneath the present tooth!

A reconstruction of the head of a Diplodocus.
These teeth can tell us exactly what these animals were eating when you examine the amount of wear present on each tooth. When examined in this way, it seems diplodocids, like Diplodocus and Apatosaurus with their pencil-shaped teeth, had evolved to feed on trees, but were very selective, and might have preferred softer plants. In fact, a recent study using computer models found that Diplodocus' skull was best built to strip leaves right from branches. This is the first time such a method has been used on an herbivorous dinosaur, and might answer many questions as to how certain herbivores processed their food.

Not all diplodocoids likely used Diplodocus' method of feeding, however, such as the rebbachisaurid Nigersaurus, with its almost duck-like snout with close to 600 teeth all aligned across the front. Nigersaurus was likely a grazer, using its long neck to shift side to side cropping up large mouth fulls of low-growing plants. Another diplodocoid, Dicraeosaurus, had a very short neck and couldn't reach high into the trees, yet its teeth and skull suggest it was doing a similar thing as Diplodocus, perhaps to bushes instead of trees.

It also seems that many macronarians like Giraffatitan and Camarasaurus were doing something different than diplodocoids.  Macronarians and primitive sauropods, as stated above, had spoon-shaped teeth, and studies suggest they were messy eaters, and likely took in large amounts of conifers and other tough vegetation in their wide mouths. They were essentially just taking huge mouth fulls of vegitation, and when you take into account that a Giraffititan's head is about five feet long and about one and a half feet wide, they could have swallowed a small child if they wanted to! Many people forget this when studying sauropod dietary needs, and make unreasonable estimates that they might have needed to feed for 24 hours straight to gain enough food to keep themselves going. In truth, because of their huge mouths, it might have taken less time for a 30 ton Brachiosaurus to feed itself than a much smaller 5 ton elephant, which needs to sacrifice time chewing and is only able to pull up small bits of food in its trunk. Many primitive sauropods were doing similar things to macronarians, except a few might have instead fed on softer vegetation as opposed to tougher forms.

It's interesting to note, however, that a group of later Cretaceous titanosaurs, called the saltasaurids, evolved heads and body designs similar to late Jurassic diplodocids. In fact, the teeth also evolved into the crayon-shape of diplodocoids as well. Diplodocoids went extinct during the mid Cretaceous, likely being victims of a small extinction event that took place, but macronarians survived, suggesting that with diplodocoids absent, they started filling in the niches left by them and evolved similar ways of feeding.

So as you can see, sauropods seem to have made a decent living during the Mesozoic. Their skulls covered a variety of different shapes and forms in order to access their different food preferences. However, out of all different types of sauropods skulls that we have preserved, one species in particular seems to stand out. Uncovered in Argentina's Banjo de la Carpa formation in 2004, Bonitasaura salgadori has one of the oddest heads of any sauropod.

Bonitasaura's weird yet wonderful skull. Wish it was a frontal view though
since the head can be most admired from that angle. 
Known elements are in gray, reconstruction in white.
Bonitasaura's skull was extremely fragmentary when discovered, and the scientists who found the specimen at first didn't realize its significance. They did notice, however, that the skull was already a little weird; it was squared off and looked very similar to Nigersaurus' duck-like mouth, despite the fact Bonitasaura was a titanosaur, not a diplodocoid. This could be another good example of convergence between diplodocids and titanosaurs. Other than that though, the skull wasn't given much attention, and it wasn't until 2010 that paleontologists Pablo Gallina and SebastiĆ”n ApesteguĆ­a found something very interesting about the animal's lower jaw. It seemed that right behind the tooth rows, a large ridge was present that seemed to, in life, posess a keratinous beak!

The term "beak," however, should be used loosely. In truth, they were just large sheaths of keratin in the jaws that may have helped with oral processing. Still, this is a major discovery that needs more exploration, for example: Was Bonitasaura the only beaked sauropod, or did other species have beaks present and we just don't have their skulls yet? Why evolve a beak in the first place? Was the beak also present on the upper jaw, or only the lower one? Does the presence of a beak suggest that oral processing was becoming more important to sauropods towards the end of the cretaceous, possibly with angiosperms becoming more common? So far we can't answer any of these questions, and it will take much more complete specimens of this species in order to even come close to answering them.

Anyway, as always, feel free to share you're opinion and ask questions. As I said before, I also take requests for topics, since I want to tell you guys about what you want to know. The next post will be the one recommended by Mr. Gorsh, so be prepared for hadrosaurs, iguanodonts, and hypsilophodonts.

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