Final answer:
The first definite birds appeared in the fossil record during the Jurassic Period, about 150 million years ago, as evidenced by the fossil record of Archaeopteryx. This was after the mass extinction at the end of the Triassic Period, which allowed dinosaurs to dominate and eventually led to the evolution of birds from theropod dinosaurs.
Step-by-step explanation:
The appearance of the first definite birds in the fossil record marks a significant event in the history of life on Earth. Birds are highly modified diapsids and are part of a group known as archosaurs, which includes dinosaurs. The earliest birds are believed to have evolved from theropod dinosaurs, specifically within the maniraptoran clade. One of the oldest and most well-known fossils of a so-called "dinosaur-bird" is that of Archaeopteryx. The fossil evidence of Archaeopteryx is dated back to the Jurassic Period, around 150 million years ago. This period began after a major mass extinction at the end of the Triassic Period, approximately 200 million years ago, which allowed dinosaurs to become the dominant terrestrial animals and paved the way for avian evolution.
During the Jurassic Period, major groups of mammals were evolving, and flowering plants appeared for the first time. Archaeopteryx not only had dinosaur-like features, such as thecodont teeth but also had avian features like flight feathers, as evidenced in well-preserved fossils. These birds had characteristics such as claws on their wings, a feature that is still found in some modern birds.
Comparatively, the Cambrian Period refers to an earlier time in the fossil record, about 541 to 485 million years ago, which is well before the rise of dinosaurs and birds. To answer the student's question: the first definite birds appear in the fossil record during the Jurassic Period, not the Cambrian or Cretaceous Periods.