Dinosaurs originated before mammals and with mammals. This means that dinosaurs existed alongside the evolution of mammals, but they were not a result of their evolution. Let's break down this answer into headings and bullet points for better understanding:

Dinosaurs and Mammals

- Dinosaurs and mammals were two separate types of animals that coexisted during the Mesozoic era.
- Dinosaurs first appeared around 230 million years ago, while the first mammals appeared around 200 million years ago.
- Dinosaurs were the dominant land animals during the Mesozoic era, but mammals were present and evolving alongside them.

Origins of Dinosaurs

- The origin of dinosaurs is still a topic of debate among scientists, but it is generally believed that they evolved from a group of reptiles known as archosaurs.
- Archosaurs were a diverse group of reptiles that included crocodiles and their relatives as well as pterosaurs (flying reptiles).
- The earliest known dinosaur fossils date back to the Triassic period, around 230 million years ago.

Evolution of Mammals

- Mammals evolved from a group of reptiles known as therapsids.
- Therapsids were once the dominant land animals during the Permian period, but they were mostly wiped out by the end-Permian mass extinction.
- The surviving therapsids gave rise to the first mammals during the Triassic period, around 200 million years ago.

Conclusion

In summary, dinosaurs originated before mammals but coexisted with them. Dinosaurs evolved from a group of reptiles known as archosaurs, while mammals evolved from a group of reptiles known as therapsids. The first dinosaurs appeared around 230 million years ago, while the first mammals appeared around 200 million years ago.