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14. Mesozoic II.docx

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University of Toronto Mississauga
Jessica Hawthorn

MESOZOIC II The characters that unite ornithiscians, primitively they have leaf shaped teeth. This is teh tooth. It looks like teh laef of a tree. They way they work is that the upper ones and lower ones come down next to each other, and slide against each other. They also have a pone at the tip ofthe new called a predentary bone. The cleft chin is hte remnant of the embryological fusion of our lower jaw. We have one lower jaw. Reptiles retain for the most part either no fusion at all or a sutural contact. The bones are strongly held together but they don’t fuse. What these ornthiscians do, the predentary bone holds the bones of the jaw together. It allows them to flex but it holsd them to together. They’ll be imparting a lot offorce on the lower jaw through chewing. They also have the retroverted pubis. It opens up the guts for more intentistines. If you’re an herbivore, you need al onger intentine to hold it for longer time and give more time for breaking down cellulose. Ossified tendons Another thing that ornthiscian dinosaurs have are ossified tendons. Can anyone tell me what a tendon is? It is a form of connected tissue. They connect muscle to bones. Ligament connect muscles to muscles. For example, your Achilles tendon connects your heel to your calf muscles. If your snap it, your foot with dangle. These naimls had ossified tendons. They allow momvenet. If you ossify them, they can become actual support structures. Tis is the tail. The vertical bars are the backbone. Each vertebrate has one. The ornithiscians diversify...there are some in the Triassic, more in the Jurassic, but in the certaceous they really take off. They’re dominant in the northern content. Heterodontosaurus is one of the first ornithisicians. It had well developed teeth that formed a continuous cutting surface. One thing all ornithiscians have...te teeth are set into the jaw relative to the lung. These animals had cheeks too. The cheeks are for chewing. They’re muscular. Cheeks help us manipulate and move food around the oral cavity. These little animals were sexually dimorphic. Sexual dimorphism is...di means two, morph means form. They had two forms. They had a male anatomy and a female anatomy. Many many species of animals are sexually dimorphic. We are dimorphic. We have males and females. Some have tusks, while others don’t. The tusks were used by males to fight for partners. The two main groups of hte ornithisicans are the ones you must remember: stegosaurus beloned to agorup called thyreophora. This is characterized by te evolution of osteoderm. Osteoderm...osteo-derm. Osteo means bone, derm means skin. Bone-skin. This is the formation of bony plates within the dermus of the skin. Alligators and crocodiles also have this. What is the advantage of having bones in your skin? It is for defence against carnivorous dinosaurs. What we seein these guys is that they start off at the top where the. We find the progressive evolution of graeter and greater armoured defences. We see progressive quadrapedal. The stegosaurus had very big hind limbs and very tiny forelimbs. Stegosaurs are an interseitn group. They’re not good chewer. Tehy have weak teeth, but spectacular armour. They had plates running down their backs. What were these plates for? They’re thin and bony. What could they be used for? It could be defence but it could also be thermal regulation or display. The plates are very thin, so they’re not effective armour. They’re thin and delicate. They could be for thermal regulation. Elephants have big trunks and big floppy elephant ears. Is it for hearing? It’s thermal regulation. Elephants are huge. They spend their lives trying to get rid of body heat. The bigger you are, teh more volume you have relative to your skin. For every doubling of your surface area, your volume increases. Elephants are all about staying cool. They’re naked. That helps them lose heat. The big floppy ears are radiators. The plates on stegosaurs perhaps served the same functions. This is not a good idea though. These animals cannot control the plates. Elephants can control their ears. Well, maybe they’re for display? How many people here spend a lot of money on their hair? Disp
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