Interesting bone evolutions

I wanted to write about the new things that I have been learning this year particularly about bone fusion and its evolution through time. It all stems from the benefits that osteological (bone) fusion can have for a subject. Quite a common usage of increased bone surface area is for muscle attachment. For muscles to be effective, they must latch on to the bones that they act on sufficiently to move them an adequate amount. Muscles themselves can only contract a set amount, approximately 60% of their length. Therefore, decreasing the distance a muscle has to contract to complete a set movement makes the movement far more powerful. This is exemplified in jaw and skull evolution particularly from mammal-like reptiles (MLRs) to modern day mammals. Early mammals began their lives with their lower jaws made up of several bones. What that has developed into is that a single bone (the dentary) now makes up the lower jaw of mammals, joining with the squamosal (lower back bone found on the skull). The additional bones that previously made up the lower jaw in MLRs have now shrunk and manoeuvred themselves to positions where they now are vital components of the inner ear. This has led to increasingly detailed audio information to be taken up by mammal predecessors as more vibrations can be taken up these bones. This is a really interesting topic, and I might go into more detail on this in a later post, but for now this page is a good starting point:
http://evolution.berkeley.edu/evolibrary/article/evograms_05
As for relation to muscles, mammals are part of a group or clade of animals known as synapsids from the prefix "syn-" which relates to fusion, but that's for a later date. What this means is that they have one temporal fenestrae. Deriving from the latin word for "window", these are holes in the skull. Dinosaurs and birds are examples of diapsids, with 2 (di-) temporal fenestrae, and turtles are anapsids, with no fenestrae at all. The inclusion of these fenestrae in the skull structure not only splits up the bones that make up the skull itself, but also the muscles that are related to it. We can see what we might think of as primitive traits in anapsids (no fenestrae) that employ one large set of jaw muscles, known as the adductor mandibulae. Mandibles are the name given to the mouth-parts of spiders and some insects, so that name is reasonably easy to remember. Adduction is the act of muscles pulling bones towards the midline of the body, as opposed to ABduction. Adductors can also be found in the thighs, and is quite a general term. However, with the inclusion of fenestrae, the adductor mandibulae is split into 2 muscles. This is where we start to see why fenestrae are a good idea. Because the previously single muscle group is split into 2 shorter groups, the effectiveness of these muscles is increased as they have to move less of a distance to complete their task. As we said earlier, muscles want to move a shorter distance, so many smaller muscles doing the work of one large muscle is energetically favourable. The whole concept of jaw movement is really interesting, and the more complex it gets, the more it allows for different styles of physical digestion (chewing) and so different lifestyles for the animals involved. Another good page to look at for jaw evolution:
http://www.mhhe.com/biosci/pae/zoology/kardong/instructor/jaws_muscle.pdf
In dinosaurs, we saw what might be the pinnacle of herbivorous eating mechanics in hadrosaurids (a type of duck-billed dinosaur descended from Iguanodon and other iguanodontids). The hadrosaurids developed what is known as kinetic jaw mechanics. First, a quick note about herbivorous eating habits. When digesting plant material, because plant cells have a large amount of a compound called cellulose in them, they are difficult to digest normally because cellulose is a tough material that makes up a plant cell wall, which animal cells don't have (no need to get any more technical yet). Therefore, herbivores want to chew their food as much as possible to break down the material so their digestive juices can do as thorough a job as possible at taking out nutrients from the plants. So, back to kinetic jaw mechanics. This allow hadrosaurids to not only grind their top and bottom layers of teeth forwards and backwards, but also from left to right. This gave a bigger range of motion for the grinding teeth of hadrosaurids so they could squeeze out as much nutrition from the plant matter as possible. This is a really good animation of the motion to visualise what I'm going on about
https://www.youtube.com/watch?v=DZkYcyYdZJU&list=PLBDCC26DA4441F47D
That's all for now, I hope you enjoyed and if you have any questions I'll try to answer. I'm no expert in the field, I just think this information isn't always fully available and appreciated by the public and I want to share my interests so I'll answer as best I can.
Cheers