| Disclaimer: this is proving hard from me to explain via text without the use of visualization. Mammals did not evolve from dinosaurs. Mammals are synapsids, a branch of tetrapods. Dinosaurs come from a different branch of tetrapods entirely, called sauropsids. (There is a third branch of tetrapods, amphibia.) Sauropsida is a really diverse branch containing both extinct and extant species. Look at the tree at the bottom of the page here [0]. A fork of interest is Sauria, which splits into lepidosauromorphs (including modern lizards & snakes) and archosauromorphs. Archosauromorphs have many branches as well (one of note being turtles), with one that traces far enough to get to archosaurs [1]. Modern crocodiles come from a branch of archosaurs. Dinosauria are another branch of archosaurs, and birds are descendants of dinosauria. They are the only extant species of dinosauria. The confusion comes because many of these animals co-existed at the same time, some become extinct, others evolved and change and form new branches (but still belong to the parent group). The word 'Dinosaur' itself is also misapplied by the public - it's not all of the animals that lived 65+ million years ago. It's a very specific group of related animals and all of their descendants. TLDR: Mammals didn't evolve from dinosaurs, but from a different branch further up the chain that dinosaurs also belong to. Those mammalian ancestors did co-exist with dinosaurs. Like mammals, modern reptiles didn't evolve from dinosaurs, but from ancestors that co-existed with dinosaurs. Reptiles and dinosaurs are more closely related than mammals and dinosaurs. Birds did evolve from dinosaurs, and are the only living dinosaurs today. [0] https://en.wikipedia.org/wiki/Sauropsida
[1] https://en.wikipedia.org/wiki/Archosaur |