1Geo 302D: Age of Dinosaurs LAB 4: The vertebrate skeleton Bone is a connective tissue unique to vertebrates. It serves several purposes: - It is a reservoir for chemicals used in metabolic processes, - It provides structural support for soft tissues, - It acts as armor to shield vulnerable body parts, - It is a framework upon which muscles can exert forces to facilitate movement. In addition to being biologically important in the day to day lives of animals, bone is useful to paleontologists because it readily preserves as fossils. Most of the information we use to reconstruct the evolutionary relationships between fossil vertebrates comes from their bones and teeth. Because of this, you must have at least a basic knowledge of the bones of the vertebrate skeleton. You must also be able to identify these elements from figures or on specimens in upcoming quizzes and lecture exams. Terms of orientation: You may notice your TA using, and you will read in this handout, words such as “dorsal”, “lateral”, “medial” and “ventral”. These terms are used to describe the relative relationships in space between anatomical features. You must be able to understand what they mean. - Anterior: towards the front, or head. - Posterior: towards the rear, or behind. - Dorsal: towards the back side. - Ventral: towards the belly side. - Medial: towards the middle, or midline. - Lateral: towards the side, or outside. - Proximal: relatively closer to the body’s center of mass . - Distal: relatively further away from the body’s center of mass. The axial and appendicular skeleton The vertebrate skeleton is easily divided into two distinct parts. These are the axial, and the appendicular, skeletons. The axial skeleton includes the skull, vertebral column, ribs, and sternum. The appendicular skeleton includes the bones of the limbs and the limb girdles that attach the limbs to the rest of the body. Axial skeleton The skull: The skull, or cranium, is an important and complex piece of vertebrate anatomy. It is a complex structure that performs a variety of tasks. These include: - Housing and protecting the delicate brain and sensory organs, - Housing feeding structures such as the jaws and teeth, - Providing attachment points and space for the powerful muscles that close and open the jaw. You are not expected to identify the bones of the skull. You are expected to know and identify the various openings in the skull. A list of openings and their relative positions is given below. You are expected to use the correct tense (singular [sing.] or plural [pl.]) when identifying or talking about all anatomical features.2- Foramen Magnum: the opening in the rear of the skull through which the spinal chord passes to reach the brain. - Naris (pl. nares): the bony external opening for the nostril. Air or water enters the nasal cavity through the naris. In water-breathing vertebrates, water enters and exits the naris the same way. In air-breathing vertebrates, air passes through the naris, down the nasal passages, and enters the mouth or pharynx through the choanae. - Choana (pl. choanae): bony openings in the roof of the mouth (or pharynx in mammals and crocodilians) that communicate with the nares. - Orbit (pl. orbits): The bony socket that houses the eyeball. - Antorbital fenestra (pl. antorbital fenestrae): “Fenestra” means “window”. This particular fenestra is found only in archosaurs (crocodilians, dinosaurs and a few other extinct groups). It is located between the orbit and the naris, on the side of the snout. These fenestrae are found in most dinosaurs (including birds), primitive crocodilians (modern crocs have closed these openings), pterosaurs, and many other extinct archosaurs. Several hypotheses have been put forth to explain the function of these fenestrae. The most popular hypothesis now is that the antorbital fenestra houses a large pneumatic sinus in the side of the face. OrbitAntorbital fenestraNarisMandibular fenestraSupratemporal fenestraInfratemporal fenestraPosttemporal fenestraeForamen magnumPosteriorLateral Openings for jaw muscles: The temporal region of the skull, behind the orbit, is the attachment zone for numerous jaw muscles. Primitively, the braincase lies deep below the outer bones and jaw muscles of the temporal region. As a muscle contracts, it gets shorter, but the volume remains constant. This means that as muscles shorten they also get wider. The jaw muscles of vertebrates originate on the inside of the temporal bones of the skull, between them and the braincase. This means that when a primitive vertebrate flexes its jaw muscles (e.g. during biting) the muscles bulge, putting strain on the bones of the skull and braincase. The earliest vertebrates and tetrapods had a skull that was fully enclosed in bone. This meant that these animals had to keep their jaw muscles small to keep from seriously damaging their skull or brain, or they had to find a way to make more room for muscles. Many tetrapods chose the latter strategy, with different groups evolving fenestrae in the temporal region to allow large muscles to bulge. Keep in mind that lineages can secondarily (after it already evolved) lose a feature, and close over these openings. - Postemporal fenestrae (sing. fenestra): The earliest terrestrial vertebrates have this pair of openings, located on the rear of the skull. Among reptiles these are especially large in turtles, exposing most of the braincase. - Infratemporal fenestrae (sing. fenestra): paired openings in the lower, temporal region. They have evolved independently between reptiles more derived than turtles and in the lineage leading to mammals. In lizards and snakes the lower border of the infratemporal fenestra is lost, exposing the side of the braincase.3- Supratemporal fenestrae (sing. fenestra): paired openings in the upper part of the temporal region, above the infratemporal fenestrae. Among living animals these openings are present in the tuatara, lizards, snakes (where they are secondarily lost), crocodilians, and birds. - Mandibular fenestrae (sing. fenestra): Many ancient animals, as well as modern birds and crocodilians, allow their jaw muscles to invade the space within the lower jaw. The mandibular fenestrae allow these muscles to expand, in much the same manner the temporal