What is this lecture mainly about? According to the professor, what evidence supported the original hypothesis that apatosaurus

Listen to part of a lecture in the biology class.
    Professor:
    We have very good ideas about the various shapes and sizes of dinosaurs from studying their petrified bones, but we’ve had to hypothesize about things like their behaviors and lifespan, because much of that kind of information isn’t preserved the way bones and teeth are or so we thought.
    Now just to review a second, the dinosaur bones we study were preserved and turned to stone millions of years ago through the process of petrifaction. Petrifaction is when all of the original biological material gets replaced with minerals without losing its original shape or details.
    Some petrified dinosaur bones contain almost perfectly preserved micro structures as small as individual cells. And when bones are that well preserved, we can use histology to examine them.
    Histology is the study of biological tissues. And in this case, histology is being used to study petrified bone tissue. To do that, the petrified bones have to be cut into slices so thin that light can pass through them. Then you can examine them under a microscope. And it turns out that the preserved micro structures contain a lot of information, including clues to behavior. In fact, long-standing hypotheses about dinosaur behavior are being proved wrong and new hypotheses about dinosaur behavior are taking their place.
    For example, there is one dinosaur that we know had a high dome-shaped skull composed of thick bone. Since the 1950s, we thought with such a thick skull that males probably butted heads just like big-horn sheep do today, probably when competing for mates, but just a couple of years ago, some university researchers in the United States took a close look at the histological findings in several of these skulls to look for evidence of head butting, like healed cracks or stress fractures. But the analysis clearly showed that there was no sign of that kind of stress to the bones. Instead, what they found were small structures that look like they may have actually anchored a crest to the skull. A crest may be like a rooster comb. So now paleontologists are wondering what the crest might’ve been used for display? Recognition? But certainly not for head butting.
    Now another long-standing hypothesis was that the really big dinosaurs took decades to reach full size. After all, they hatched from fairly small eggs and they had a lot of growing to do. So it’s a fairly logical assumption that it took a long time to reach their giant sizes.
    Then researchers did the histological examination of the apatosaurus bones. Apatosaurus was a species of sauropod, giant plant-eating dinosaur. It had a long neck and a long tail and its full size was about 25 meters long and weighed about 25 tons.
    Now the idea that apatosaurus grew slowly was based on many observations, including its enormous size and the fact that large modern reptiles grow slowly. For example, an apatosaurus had a relatively small mouth and simple teeth. And the plants that lived at that time were not particularly nutritious. Altogether it’s not a recipe for fast growth.
    However, under a microscope, it’s clear that dinosaur bones have growth rings. And by counting them like tree rings, paleontologists can infer how many years of bone has grown. And when comparing the growth rings from several specimens of the same species of dinosaur, paleontologist can figure out growth rates for that species.
    Well, surprisingly, that gain apatosaurus reached its full size in just 8 to 11 years. Just S to 11 years. Can you imagine growing almost 3 meters a year? And the only way information like this can be worked out is through histology, you know?
    And that isn’t enough. Histology has recently revealed another surprise. In Germany, the bones of several small sauropods were found in a quarry. Since sauropods grew to be very large, the discovery of small sauropod bones usually means you found juveniles, young ones. But the histological evidence showed that the bones were from a species of dwarf sauropod—only grew to 6 meters long and matured in just three years. The new hypothesis is that these dinosaurs evolved to be small, because they lived on an island with limited resources.