Lesson 5: Birth, Growth, and Reproduction Flashcards
What are amniotic eggs?
A major milestone in tetrapod evolution that developed a little over 312 mil. yrs ago. Prior to this adaptation, all tetropods laid eggs that were similar to those of modern frogs and salamanders and could not retain water. Such eggs would dry out and die if not laid in a wet, humid place. Amniotic eggs are different. They have encapsulating membranes that are watertight.
What are amniotes? Give some examples.
Animals that lay amniotic eggs. Mammals, birds, dinosaurs, and reptiles are all amniotes. Although most extant mammals do not lay eggs, mammalian embryos still have membranes that cover them while in the uterus.
The membranes of amniotic eggs also became adapted to form tough leathery or hard shells. What did this provide the eggs?
Shells improved amniotic eggs’ ability to hold in water and also made the eggs more durable and less vulnerable to small predators.
Although amniotic eggs are watertight, they are not ___. If they were, the eggs would suffocate.
Airtight.
Even hard eggshells are covered with tiny holes that permit gasses to be exchanged between the inside of the egg and the outside world.
What places a limit on how big eggs can be and what is the size of the largest known egg?
The need to breath places a limit on how big eggs can be. (Cube square law)
The largest known dinosaur egg is only half a meter long and most are much smaller.
Recall: Bone ___ has helped paleontologists to better understand dinosaur growth rates and that bone cells are called ___.
Histology and osteons.
What are lines of arrested growth (LAGs)?
As animals grow their bones, they add osteons to their bones’ outer walls. But the rate at which osteons are added is not always the same and varies with changes in growth rates. During seasonal periods, when resources needed for growth are scares, such as during winter or the dry season, growth may slow down. This creates rings inside the bones, analogous to those of a tree trunk.
What can be determined from LAGs?
How long it took a dinosaur to grow to a particular size and at what speed a dinosaur grew.
How quickly did dinos grow? Give examples.
Fast. It is estimated that a Tyrannosaurs rex grew to its adult size in only 20 years. Even large sauropods only took 30 years to fully mature, and they are estimated to have gained an average of one to two pounds every day!
How are bones of younger dinos characterized?
By having high vascularity (many blood vessels) and a texture we call lamellar bone. LAGs formed later, as dinosaurs grew.
What is remodelling (in bones)?
More mature dinosaur bone underwent a process called remodeling, where the old bone cells were replaced by newer bone cells. This kind of bone is called Haversian, or secondary bone (pic in lesson 3).
What is the external fundamental system (EFS)?
Finally, as growth slows and then finally stops, a closely spaced series of LAGs is formed, which is called the external fundamental system (EFS). The presence of an EFS indicates that the dinosaur is skeletally mature and has stopped growing.
What are ontogenetic changes?
Changes in the form of an organism that occur as it matures.
What are some common traits of young animals?
Big eyes, big heads.
The crests of many hadrosaurs were not present in very young individuals, but grew gradually as the dinosaurs reached maturity.
Many baby ankylosaurs hatched with little or no armor and with no tail cubs. Ankylosaur body armor and tail clubs did not grow until later in life.
What are non-isometric ontogenetic changes? Give an example.
Changes in the relative proportions of an animal as it grows, that are not simply changes resulting from a general increase in size. The changes in the relative lengths of the horns and frills of ceratopsians are examples. Another example can be seen in the legs of dinosaurs like Tyrannosaurus, where the tibia was much longer than the femur in juveniles, while in adults the tibia and femur were close to the same length.
