Epidexipteryx, pigeon-size dinosaur. Image from National
Geographic, courtesy of Zhao Chuang and Xing Lida. A model of
Archaeopteryx, on display at the Oxford University Museum of
Natural History. An example of the K-T boundary, appearing here
horizontally at midpoint. Around 245 million years ago the first
dinosaurs appeared. This was during what geologists call the
Triassic Period – the first period of the Mesozoic Era.
Dinosaurs were reptiles and they had been preceded for millions
of years by other reptiles on land. Conditions had allowed the
survival of the larger reptiles, whose size helped them survive.
These first dinosaurs were divided into various species.
Wikipedia, under the heading "Triassic dinosaurs," lists more
than fifty different species. Scientists described them as
having been as much as 10 meters long, including tail. They
looked like big lizards with large and powerful hind legs and
weaker front legs about half as big. On the end of a long neck,
their heads were made light in weight by l