Dinosaur Hall is a gallery in the Los Angeles County Museum of Natural History. Dinosaurs first appeared more than 230 million years ago and became extinct millions of years before the first humans evolved.
Dinosaurs flourished during the Mesozoic Era, an age which lasted for about 185 million years.
According to the Museum display:
“During the 185 million years of the Mesozoic, also known as the Age of Dinosaurs, our world changed dramatically. When the era began, all land was part of one vast supercontinent. That continent gradually split into pieces, which drifted thousands of miles, changing the face of our planet. Earth’s climate changed and at times was much warmer than today.”
During the Jurassic era of the Mesozoic, a period from 200 to 145 million years ago, the single continent divided into two: Laurasia and Gondwanaland. The first birds appeared during this era. During the Cretaceous, 145 to 65 million years ago, multiple continents were formed.
In looking at the question “What makes a dinosaur a dinosaur?”, the Museum display answers:
“Scientists classify groups of animals based on shared features. Since we know dinosaurs mostly from fossil bones, we classify them mainly based on bone shape. All dinosaurs share a similar structure of some key bones that allows us to identify them as dinosaurs.”
Dinosaurs are reptiles and like other reptiles, they have backbones.
While all dinosaurs share certain defining features, through the process of evolution, different groups develop new features which set them apart from other groups. Cladograms are used to map these features.
Mythology
In the nineteenth century, scientists began to recognize dinosaur fossils as representing an ancient extinct species. However, many people were not sure what to make of these bones and consequently many different stories emerged to explain them. In 1802, for example, fossil footprints of what looked like a giant bird were found on a Massachusetts farm. To explain these tracks, people used the Judeo-Christian story of a flood and the culture hero Noah. Thus, according to the story, the footprints had been left by Noah’s raven, sent out to scout for dry land following the mythical flood. Today, scientists know that the tracks were left by a small, two-legged dinosaur.
In the twenty-first century, there are still some fundamentalist Protestant Christians who refuse to accept scientific findings regarding the age of the Earth. They continue to disseminate the mythology that humans and dinosaurs coexisted, and that one of the factors in the extinction of the dinosaurs was overhunting by humans.
Diversity
At this time, scientists have identified about 700 species of extinct dinosaurs. Since fossils are rare and dinosaur remains only fossilize under specific conditions, it is assumed that there are many dinosaur species for which we have no fossil evidence today. In answering the question “How diverse were the dinosaurs?” the Museum display states:
“One mark of evolutionary success for all forms of life is how diverse and varied a group becomes. Studies of their fossils and their living relatives have shown that dinosaurs were—and still are—surprisingly diverse in size, diet, appearance, and lifestyle. That makes them one of the most successful groups of large animals ever to walk the earth.”
Names
Dinosaurs are given two-part Latinized names by their discoverers. According to the Museum display:
“These names often carry special meaning. Some refer to a distinctive feature of the animal or to where we found the fossil. Others honor those who have contributed to dinosaur science.”
Extinction
About 65 million years ago there was a major extinction in which nearly half of all plant and animal species became extinct. According to the Museum display:
“A giant asteroid hit the Earth at the end of the Age of Dinosaurs. But scientists still disagree whether this was the sole cause of the mass extinction that took place or if there were multiple causes.”